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Effective lens focal lengths for Rebel XT lenses
I recently bought a Rebel XT with the kit lens (18-55mm), along with a EF 75-300mm zoom lens. I will also be buying a EF 50mm f/1.8 prime lens. My question is, would I have to multiply all these focal length values by 1.6 (the Rebel XT's field of view crop value) to get is actual effective focal length? So would the equivalent specs for my three lenses in question be:.

29-88mm120-480mm80mm..

Comments (43)

Trale wrote:.

My question is, would I have to multiply all these focal lengthvalues by 1.6 (the Rebel XT's field of view crop value) to get isactual effective focal length?.

Actual and "effective" are two different things. The actual focal lengths are what are marked on the lenses, they do not change, no matter what camera you use them on. To get their 35 mm film equivalence, yes you would have to multiply by 1.6..

So would the equivalent specs for mythree lenses in question be:29-88mm120-480mm80mm.

Roughly yes..

Brian A...

Comment #1

Thanks, just making sure..

So to try to understand why these lenses have a longer effective focal length on my camera than a 35mm film camera, is this simply because the APS-C sensor "crops" the potential field of view from the lens?.

So if I throw the lens on a full-frame camera, and leave it at the exact same physical focal length setting, I would get a wider field of view, right?.

How does this affect the depth of field, vignetting, and barrel distortion?.

For example, with the same focal length setting on the lens, would I get "effectively" more DOF, less vignetting, and less barrel distortion on my Rebel XT than say a Mark II?..

Comment #2

It doesn't make a lick of difference what the effective focal length is. The sooner you stop thinking about it, the better..

Trale wrote:.

Thanks, just making sure..

So to try to understand why these lenses have a longer effectivefocal length on my camera than a 35mm film camera, is this simplybecause the APS-C sensor "crops" the potential field of view from thelens?.

So if I throw the lens on a full-frame camera, and leave it at theexact same physical focal length setting, I would get a wider fieldof view, right?.

How does this affect the depth of field, vignetting, and barreldistortion?.

For example, with the same focal length setting on the lens, would Iget "effectively" more DOF, less vignetting, and less barreldistortion on my Rebel XT than say a Mark II?.

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Comment #3

Trale wrote:.

Thanks, just making sure..

So to try to understand why these lenses have a longer effectivefocal length on my camera than a 35mm film camera, is this simplybecause the APS-C sensor "crops" the potential field of view from thelens?.

Yes..

So if I throw the lens on a full-frame camera, and leave it at theexact same physical focal length setting, I would get a wider fieldof view, right?.

Yes..

How does this affect the depth of field, vignetting, and barreldistortion?.

For example, with the same focal length setting on the lens, would Iget "effectively" more DOF,.

No. DOF is independent of the nature of the crop..

Less vignetting,.

Yes..

And less barreldistortion on my Rebel XT than say a Mark II?.

Hard to say? With most lenses I'm familiar with, the distortion is sorta constant as you go off axis...ie, if the lens has 2% barrel distortion at WA, then it's a constant 2%. If you crop to a smaller size, the absolute error due to the barrel distortion would be less, but relatively it's the same. There may be a few lenses which have less geometric distortion in the center?.

One effect you ignored is CA. It will be better with the cropped sensor..

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #4

Re>would I have to multiply all these focal length values by 1.6 (the Rebel XT's field of view crop value) to get is actual effective focal length?<.

No, but it gets comp[licated and misleading..

Your focal length is your focal length, which seems simple enough..

But some people insist on comparing their new camra lenses to old 35mm camera lenses, even though people with 35mm cameras rarely botheed comparing their lenses to medium format lenses..

Sso, if you feel compelled to compare your lenses to a 35mm camera for whatever reason (if you've shot thousands of 35mmpictures you already have a pretty good idea of what you get in the shot with vvarious 35mm format lenses) multipy by 1.6..

So a 50mm lens used on a Digital Rebel gets you pretty much the same image as an 85mm lens would get you on a Canon film camera..

BAK..

Comment #5

Wide angle lenses distort light and bend the image.

A 50mm on a DSLR is not the identical to an 80mm on a film slr.

The FIELD OF VIEW is the same, because the image the 50mm is projecting is cropped.

The perspective of a 50mm and an 80mm lens are different however, they bend the light in a slightly different way, the 80mm producing less distortion..

This is why longer focal lengths are used for portait applications...

Comment #6

Gooshin wrote:.

Wide angle lenses distort light and bend the image.

Huh!? What is this comment supposed to mean? Please clarify..

A 50mm on a DSLR is not the identical to an 80mm on a film slr.

There's no difference worth worrying about that I know of. Again, what are you getting at, exactly?.

The FIELD OF VIEW is the same, because the image the 50mm isprojecting is cropped.

The perspective of a 50mm and an 80mm lens are different however,they bend the light in a slightly different way, the 80mm producingless distortion..

Nonsense!!.

Perspective is nothing to do with the f-length of the lens, and ALL to do with the distance the lens is from the subject.... (Hmmm... this error used to be a very common... but hardly anyone ever makes it now ... or so I thought!).

Furthermore, 'distortion' as such is not relevant in comparisons of 50mm lenses with 80mm ones, regardless of their use on either FF or cropped sensors..

This is why longer focal lengths are used for portrait applications..

Longer focal lengths are used to to take portraits because long lenses **oblige** the photographer to MOVE BACK to where the perspective is less steep, more compressed. Most people find this viewpoint more flattering to the features than a close view, so it has become a convention in portrait photography of the 'classic' kind..

However, as stated above, it isn't the lens f-length that changes the perspective it is the distance from the subject. Indeed, a wide angle lens would show the same perspective, if used from that same distance...and there would be NO difference whatsoever......

.... other than a wider field of view in the wide angle, of course!.

Gooshin, I am sorry to come down on you in this critical fashion, but I think your comments are misleading and needed correcting.Regards,Baz..

Comment #7

Thanks for clearing that up. Now I see why people are clamoring over the EF 85mm f/1.8 lens, which is about 5x more expensive. I'm sure it's better than the 50mm I was going to buy, but probably not 5x better...

Comment #8

If you think the EF 85 1.8 is expensive take a look at the EF 85 1.2 L .

Trale wrote:.

Thanks for clearing that up. Now I see why people are clamoring overthe EF 85mm f/1.8 lens, which is about 5x more expensive. I'm sureit's better than the 50mm I was going to buy, but probably not 5xbetter..

Some cool cats that can use your helphttp://www.wildlife-sanctuary.org.

Even if you can't donate, please help spread the word...

Comment #9

My buddy has a sigma 10-20 for his canon.

When we mount that sucker on to his filmSLR Rebel at 10mm we both get really big grins on our faces..

At the wide angle side of lens selection, the cropping factor of todays DSLR's is where the real missed opertunities are...

Comment #10

...to compare the price of the body plus lens. What is the difference in price of a 5D+50f1.8 and a 5D+85f1.8? Is it worth it?.

Shoot...

Comment #11

Trale wrote:.

Thanks for clearing that up. Now I see why people are clamoring overthe EF 85mm f/1.8 lens, which is about 5x more expensive. I'm sureit's better than the 50mm I was going to buy, but probably not 5xbetter..

Probably not any better on a crop-sensor camera. 85mm is a short telephoto on a film or full-frame digital camera. Just like a 50mm is on yours. On a crop-sensor camera, 85mm is a medium telephoto, and about the longest commonly used for portraits. Working at the distance required requires a large studio, and can complicate the lighting...

Comment #12

Gooshin wrote:.

Wide angle lenses distort light and bend the image.

A 50mm on a DSLR is not the identical to an 80mm on a film slr.

The FIELD OF VIEW is the same, because the image the 50mm isprojecting is cropped.

The perspective of a 50mm and an 80mm lens are different however,they bend the light in a slightly different way, the 80mm producingless distortion..

This is why longer focal lengths are used for portait applications..

Wrong, wrong, right, wrong, wrong...

Comment #13

Gooshin wrote:.

The perspective of a 50mm and an 80mm lens are different however,they bend the light in a slightly different way, the 80mm producingless distortion..

The perspective is different only if you move the camera position relative to the subject..

The concept of the lens bending light in a different way is completely false. You could try this experiment with a pinhole camera. Take a box with a pinhole positioned 50mm away from the back of the "camera". And another with the pinhole 80mm away from the back..

If you were to record the image on light-sensitive film or paper, the results would show the same effects (or lack of effects) as if actual lenses had been used. But because a pinhole is used, and light travels in straight lines, we know for certain that any differences in the images are NOT due to like bending in a different way. After all, the light never bent, it just passed in a straight line through the pinhole.Regards,Peter..

Comment #14

BAK wrote:.

But some people insist on comparing their new camra lenses to old35mm camera lenses, even though people with 35mm cameras rarelybotheed comparing their lenses to medium format lenses..

If the medium-format format ever was a "consumer" market, it was a very long time ago. By contrast, 35mm was the gold standard for consumer film cameras, as well as for a lot of hobbyist and pro work, for decades..

You can take a 35mm film point-and-shoot camera, and compare absolute focal lengths directly with those of a 35mm SLR lens, and still get useful results..

Try that with digital cameras where sensor sizes vary quite widely among point and shoots, and even vary among consumer/prosumer DSLRs. Having some common point of reference is useful; given that, why shouldn't that common point of reference be "35mm equivalent length on a particular body"?..

Comment #15

Chuxter wrote:.

Trale wrote:.

How does this affect the depth of field, vignetting, and barreldistortion?.

For example, with the same focal length setting on the lens, would Iget "effectively" more DOF,.

No. DOF is independent of the nature of the crop..

No, this is absolutely wrong. The DoF will certainly change. In general, you will get more DoF with a crop camera than with a full frame (FF) camera. This is why compact P&S cameras have such large DoF compared to a DSLR..

Now, for a more detailed answer. For the same "framing" at the same distance, you will get the same DoF as 1.6 times the f-stop on a FF camera. For example, with a 50mm lens at f2 on a 1.6x crop camera, you will have the same FoV and DoF as a 80mm lens at f3.2 on a FF lens (when not near the hyperfocal distance). The larger DoF comes from using a shorter focal length..

Now, for the full answer. You need to understand that the DoF distance given in the DoF calculators is based on printing an 8x10 photo and viewing it at a given distance (usually 10-12 inches). With different sensor sizes, you will have a different circle of confusion (CoC). That is why when you crop a photo, you are changing the CoC, and therefore you are changing the DoF..

Here are some sites that show examples of DoF differences between crop and FF cameras. The DOFMaster website has excellent diagrams that show what happens with a crop sensor vs a FF sensor..

Http://www.dofmaster.com/dof_dslr.htmlhttp://www.bobatkins.com/photography/technical/digitaldof.html.

Summary from the Bob Atkins site:.

1. For an equivalent field of view, the EOS 10D has at least 1.6x MORE depth of field that a 35mm film camera would have - when the focus distance is significantly less then the hyperfocal distance (but the 35mm format need a lens with 1.6x the focal length to give the same view)..

2. Using the same lens on a EOS 10D and a 35mm film body, the 10D image has 1.6x LESS depth of field than the 35mm image would have (but they would be different images of course since the field of view would be different).

3. If you use the same lens on a EOS 10D and a 35mm film body and crop the 35mm image to give the same view as the digital image, the depth of field is IDENTICAL.

4. If you use the same lens on an EOS 10D and a 35mm film body, then shoot from different distances so that the view is the same, the 10D image will have 1.6x MORE DOF then the film image..

5. Close to the hyperfocal distance, the EOS 10D has a much more than 1.6x the DOF of a 35mm film camera. The hyperfocal distance of the EOS 10D is 1.6x less than that of a 35mm film camera when used with a lens giving the same field of view...

Comment #16

Gooshin wrote:.

Wide angle lenses distort light and bend the image.

What are you talking about? Unless you are talking about barrel distortion on cheap lenses or the effect from fisheyes, the "distortion" that you refer to is a function of PERSPECTIVE. By the way, perspective is a function of distance, not focal length..

Http://www.dpreview.com/learn/?/Glossary/Optical/Perspective_01.htm.

A 50mm on a DSLR is not the identical to an 80mm on a film slr.

The FIELD OF VIEW is the same, because the image the 50mm isprojecting is cropped.

This part is true..

The perspective of a 50mm and an 80mm lens are different however,they bend the light in a slightly different way, the 80mm producingless distortion..

No, the perspective is the same. Again, perspective is a function of distance, not focal length. And, if you shoot with the same framing with a 1.6x crop DSLR with the 50mm lens as you do with the 80mm lens on the film/FF SLR, you will shoot from the same distance, so the perspective will be the same..

Not sure what you are talking about with the lens bending light differently..

This is why longer focal lengths are used for portait applications..

Longer focal lengths are used because they allow you to frame portraits at the "correct" distance to give a flatter perspective. I personally like longer lenses, since they have more background blur even with deep DoF. Please note that DoF and background blur are not the same. When you have the same DoF with two different lenses, the longer focal length lens will have more background blur..

P.S. This is a beginners forum. Please do not post incorrect information here, since it can really screw them up...

Comment #17

Trale wrote:.

Thanks for clearing that up. Now I see why people are clamoring overthe EF 85mm f/1.8 lens, which is about 5x more expensive. I'm sureit's better than the 50mm I was going to buy, but probably not 5xbetter..

I would not put much into what Gooshin is saying. He does not seem to understand much of the technical aspects of focal lengths, perspective, FoV or DoF..

A 50mm lens on a 1.6x crop DSLR will give the same FoV as an 80mm lens on a FF DSLR. Perspective is a function of subject distance, not focal length. If you frame items the same (with the 50mm on the 1.6x and the 80 on the FF), the perspective will be the same..

As for the reason the 85 f1.8 is more expensive then the 50 f1.8, that is because the 50mm lens is easier to build, and the build quality and AF of the 85 f1.8 is much, much better. Both are optically good, but most 50mm lenses are optically good, since it is very easy to design and manufacture an optically good 50mm prime lens. Pick and hold both lens, and you will definitely know why one is more expensive. Whether the price difference is worth it is a different question, and that is based on your needs/wants...

Comment #18

Actually, Dave, you are absolutely wrong...primarily because you didn't understand what I was saying. I said that if you take a picture and subsequently crop it in PP, the DOF is not affected. You can also crop it IN CAMERA, by using a smaller sensor. That is in effect what going from a FF sensor to a 1.5 crop sensor does...it just crops the image!.

Go read Bob Atkins' list. In #1 you will find the key to our disagreement. He said, "For an equivalent field of view..."!.

I never said anything about having an equivalent field of view. I was trying to say, "With an identical FL..." the DOF is not affected by the nature of the crop..

This was a comment to a long thread. Perhaps I should have used more words?.

Dave_s93 wrote:.

Chuxter wrote:.

Trale wrote:.

How does this affect the depth of field, vignetting, and barreldistortion?.

For example, with the same focal length setting on the lens, would Iget "effectively" more DOF,.

No. DOF is independent of the nature of the crop..

No, this is absolutely wrong. The DoF will certainly change. Ingeneral, you will get more DoF with a crop camera than with a fullframe (FF) camera. This is why compact P&S cameras have such largeDoF compared to a DSLR..

Now, for a more detailed answer. For the same "framing" at the samedistance, you will get the same DoF as 1.6 times the f-stop on a FFcamera. For example, with a 50mm lens at f2 on a 1.6x crop camera,you will have the same FoV and DoF as a 80mm lens at f3.2 on a FFlens (when not near the hyperfocal distance). The larger DoF comesfrom using a shorter focal length..

Now, for the full answer. You need to understand that the DoFdistance given in the DoF calculators is based on printing an 8x10photo and viewing it at a given distance (usually 10-12 inches).With different sensor sizes, you will have a different circle ofconfusion (CoC). That is why when you crop a photo, you are changingthe CoC, and therefore you are changing the DoF..

Here are some sites that show examples of DoF differences betweencrop and FF cameras. The DOFMaster website has excellent diagramsthat show what happens with a crop sensor vs a FF sensor..

Http://www.dofmaster.com/dof_dslr.htmlhttp://www.bobatkins.com/photography/technical/digitaldof.html.

Summary from the Bob Atkins site:1. For an equivalent field of view, the EOS 10D has at least 1.6xMORE depth of field that a 35mm film camera would have - when thefocus distance is significantly less then the hyperfocal distance(but the 35mm format need a lens with 1.6x the focal length to givethe same view).2. Using the same lens on a EOS 10D and a 35mm film body, the 10Dimage has 1.6x LESS depth of field than the 35mm image would have(but they would be different images of course since the field of viewwould be different)3. If you use the same lens on a EOS 10D and a 35mm film body andcrop the 35mm image to give the same view as the digital image, thedepth of field is IDENTICAL4. If you use the same lens on an EOS 10D and a 35mm film body, thenshoot from different distances so that the view is the same, the 10Dimage will have 1.6x MORE DOF then the film image.5. Close to the hyperfocal distance, the EOS 10D has a much more than1.6x the DOF of a 35mm film camera.



Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #19

I realize that young people have difficulty with History. But....

The term "Medium Format" was not used when cameras with big pieces of film were used by "consumers", ie, nobody went to the drugstore and bought a "medium format" Brownie. They were just cameras..

"Medium Format" came into popular use after little 35mm cameras appeared. Even then, it was more common to have someone describe a MF camera as a "two & one-quarter by three & one-quarter" camera. At that juncture, we didn't think of ALL medium-sized film as being sorta the same..

35mm film may have had a longer run than 2.25" film, but not by much. It's like with you kids, that if you were not alive when something was popular, then it didn't happen. .

Tom_N wrote:.

BAK wrote:.

But some people insist on comparing their new camra lenses to old35mm camera lenses, even though people with 35mm cameras rarelybotheed comparing their lenses to medium format lenses..

If the medium-format format ever was a "consumer" market, it was avery long time ago. By contrast, 35mm was the gold standard forconsumer film cameras, as well as for a lot of hobbyist and pro work,for decades..

You can take a 35mm film point-and-shoot camera, and compare absolutefocal lengths directly with those of a 35mm SLR lens, and still getuseful results..

Try that with digital cameras where sensor sizes vary quite widelyamong point and shoots, and even vary among consumer/prosumer DSLRs.Having some common point of reference is useful; given that, whyshouldn't that common point of reference be "35mm equivalent lengthon a particular body"?.

Because 24mm x 36mm 35mm film was NOT that unique? Do you have a clue how many different film formats there are? I did a quick Google/Wikipedia search and counted over 210! In contrast, there are about 20 digital formats. During the entire lifetime of film photography (even with hundreds of different formats), nobody thought it was necessary or important to pick one format and describe all others in reference to that "standard"!.

That it's necessary today speaks volumes about the knowledge of the typical customer. It's confusing to reference FOV and ignore everthing else, especially to anyone who has a clue about optics and photography..

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #20

Chuxter wrote:.

Actually, Dave, you are absolutely wrong...primarily because youdidn't understand what I was saying. I said that if you take apicture and subsequently crop it in PP, the DOF is not affected. Youcan also crop it IN CAMERA, by using a smaller sensor. That is ineffect what going from a FF sensor to a 1.5 crop sensor does...itjust crops the image!.

No, you do not understand. The CoC is dependent on the final image size. When you crop an image (either via a crop sensor or in PP) you are changing the CoC. The CoC determines the DoF..

Go read Bob Atkins' list. In #1 you will find the key to ourdisagreement. He said, "For an equivalent field of view..."!.

I never said anything about having an equivalent field of view. I wastrying to say, "With an identical FL..." the DOF is not affected bythe nature of the crop..

Go read #2 & #3. With an equivalent focal length and subject distance there is a different DoF (#2). When you crop the FF picture in PP (#3), you will get the same DoF..

Dave_s93 wrote:.

Summary from the Bob Atkins site:1. For an equivalent field of view, the EOS 10D has at least 1.6xMORE depth of field that a 35mm film camera would have - when thefocus distance is significantly less then the hyperfocal distance(but the 35mm format need a lens with 1.6x the focal length to givethe same view).2. Using the same lens on a EOS 10D and a 35mm film body, the 10Dimage has 1.6x LESS depth of field than the 35mm image would have(but they would be different images of course since the field of viewwould be different)3. If you use the same lens on a EOS 10D and a 35mm film body andcrop the 35mm image to give the same view as the digital image, thedepth of field is IDENTICAL4. If you use the same lens on an EOS 10D and a 35mm film body, thenshoot from different distances so that the view is the same, the 10Dimage will have 1.6x MORE DOF then the film image.5. Close to the hyperfocal distance, the EOS 10D has a much more than1.6x the DOF of a 35mm film camera. The hyperfocal distance of theEOS 10D is 1.6x less than that of a 35mm film camera when used with alens giving the same field of view...

Comment #21

Dave_s93 wrote:.

Chuxter wrote:.

Actually, Dave, you are absolutely wrong...primarily because youdidn't understand what I was saying. I said that if you take apicture and subsequently crop it in PP, the DOF is not affected. Youcan also crop it IN CAMERA, by using a smaller sensor. That is ineffect what going from a FF sensor to a 1.5 crop sensor does...itjust crops the image!.

No, you do not understand. The CoC is dependent on the final imagesize. When you crop an image (either via a crop sensor or in PP) youare changing the CoC. The CoC determines the DoF..

Close. Cropping the image doesn't change it. Resampling it in PP or using a sensor with different pixel density does. Smaller pixels = smaller CoC..

Back in the days when DOF scales were on lenses, they were assuming some average film grain. Even there, it's really different for ISO 50 film than it is for ISO 800. With digital, crop-sensor cameras almost always have smaller pixels than full-frame, which is why the DOF changes...

Comment #22

35mm has completely dominated the film world for 50 years, which is longer than most people have been alive. Even today, 24x36 is still relevant as full-frame DSLR's use it. P&S use many different size sensors. While 1.5-1.6 are by far the most common crop-ratios on DSLR's, there are also 2.0, 1.3, and 1.0..

The only alternative would be to measure in degrees. Even that would be confusing as many lenses can be used with different size sensors. You can't take the angle in degrees and multiply it to correct for sensor size, like you can with focal length (it would require trigonometry)..

To me, labeling a lens with it's actual focal length and multiplying by 1.5 (I use Nikon) is plenty simple. It's also easy to think of any result less than 50 wide angle, and any longer telephoto...

Comment #23

Greg Nut wrote:.

Dave_s93 wrote:.

No, you do not understand. The CoC is dependent on the final imagesize. When you crop an image (either via a crop sensor or in PP) youare changing the CoC. The CoC determines the DoF..

Close. Cropping the image doesn't change it. Resampling it in PP orusing a sensor with different pixel density does. Smaller pixels =smaller CoC..

No. This is wrong. Pixel density has nothing to do with DoF (as it is currently defined with DoF calculators). I am surprised by the number of people who respond with incorrect information. DoF is defined as the "acceptable sharpness" on a 8x10 print viewed at 25cm (10 inches). There are many ways to calculate the CoC based on this definition*, but it generally yields a value of .025mm - .035mm for the CoC when using 35mm film..

And if you think that I am making this up about cropping/crop sensor/film format size impacting DoF, see the following quotes from the links:.

Some quotes:http://ronbigelow.com/articles/depth-of-field-1/depth-of-field-1.htm.

"Sensor/Film Size: The size of the sensor or film also has an affect on the DOF. For instance, most DSLRs have sub-full-frame sensors. A typical sub-full-frame sensor might have dimensions of around 22.5mm x 15mm. Such a sensor would have a diagonal of 27.04mm. The ratio of the diagonal of the print to the diagonal of the sensor is 12. Thus, the CoC is 12 times smaller than 0.25mm.

This smaller CoC reduces the DOF.".

Http://www.cjcom.net/articles/hyperfoc.htm.

"The question is often asked whether the depth of field (DOF) for a 35mm camera lens remains the same when it is used on a digital camera with a smaller APS sized sensor. It would seem that it shouldn't change just because a smaller part of the image circle is used, but in reality it is different. This is because DOF calculations are based upon three factors: the degree of enlargement of the final image, the viewing distance, and the smallest discernible point at that viewing distance.".

Http://photo.net/learn/optics/dofdigital/.

"So the bottom line - and all you really need to know - is that DOF is inversely proportional to format size. Note that format size is inversely proportional to the "digital multiplier". The higher the "digital multiplier", the smaller the format and thus the greater the depth of field. Note also that now you can see one of the reasons large format camera users need tilts and swings to get adequate depth of field. With an 8x10 camera you have about 8.5 times LESS depth of field than you do with 35mm for the same image. This also explains why consumer digicams, some of which have sensors 1/6 the size of 35mm film, have such a large depth of field and one of the reasons why it's almost impossible to get blurred backgrounds when using them.".

Back in the days when DOF scales were on lenses, they were assumingsome average film grain. Even there, it's really different for ISO50 film than it is for ISO 800. With digital, crop-sensor camerasalmost always have smaller pixels than full-frame, which is why theDOF changes..

Film grain has nothing to do with DoF. The reason crop sensors have more DoF than FF sensors is due to the shorter focal lengths used to get the same framing. In actuality, crop sensors have smaller CoC, so the depth of field is SHALLOWER on crop sensors when calculating DoF based on subject distance and focal length. With a crop sensor, you either step back (longer subject distance) or use a shorter focal length. This is why you have more DoF..

Table for CoC - notice crop sensors have smaller CoC:http://www.dofmaster.com/digital_coc.html.

Here is a DoF calculator, notice that you get MORE DoF on a crop camera at the same settings:http://www.dofmaster.com/dofjs.html.

*Standard conventions for CoC.

1) 1/1730 of the size of the diagonal of the final image (CoC = .025mm with 35mm film).

2) 5 lines per mm (.2mm) on the final image (CoC = .029 with 35mm film and an 8x10 print).

3) 4 lines per mm (.25mm) on the final image (CoC = .035mm with 35mm film and an 8x10 print)..

Comment #24

Dave_s93 wrote:.

Chuxter wrote:.

Actually, Dave, you are absolutely wrong...primarily because youdidn't understand what I was saying. I said that if you take apicture and subsequently crop it in PP, the DOF is not affected. Youcan also crop it IN CAMERA, by using a smaller sensor. That is ineffect what going from a FF sensor to a 1.5 crop sensor does...itjust crops the image!.

No, you do not understand..

Actually, I do..

The CoC is dependent on the final imagesize..

Huh? What does CoC have to do with it. When I cropped the image in-camera by substituting a physically smaller sensor, the CoC doesn't change...at least if the sensors have higher resolution that the CoC...ie, the CoC is largly lens dependent..

When you crop an image (either via a crop sensor or in PP) youare changing the CoC..

Please explain to us how and why this happens..

The CoC determines the DoF..

The CoC is ONE parameter that determines the DOF..

Go read Bob Atkins' list. In #1 you will find the key to ourdisagreement. He said, "For an equivalent field of view..."!.

I never said anything about having an equivalent field of view. I wastrying to say, "With an identical FL..." the DOF is not affected bythe nature of the crop..

Go read #2 & #3. With an equivalent focal length and subjectdistance there is a different DoF (#2). When you crop the FF picturein PP (#3), you will get the same DoF..

Dave, you keep returning to the "equivalent focal length" argument. Let me state again, "when cropping in-camera or later in PP, the focal length is not involved.".

Bob is quite correct. You are quite correct. But both of you are talking about something different that I am talking about..

I am talking about taking a specific 50mm lens (adjusted to f:4.0 and focused at 10m) and mounting it on two different cameras. First, let's put it on a new D3 with a 24mm x 36mm sensor. Second, let's put it on a Oly E510 with a 13.5mm x 18mm sensor. I claim that the DoF of that specific 50mm lens is unchanged as it moves from a large sensor camera to a small sensor camera. That is in essence, "cropping in-camera". There are a few cameras that allow us to electronically vary the number of pixels and aspect ratio recorded.

The "equivalent FL" argument simply does not apply in these examples..

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #25

Greg Nut wrote:.

Dave_s93 wrote:.

Chuxter wrote:.

Actually, Dave, you are absolutely wrong...primarily because youdidn't understand what I was saying. I said that if you take apicture and subsequently crop it in PP, the DOF is not affected. Youcan also crop it IN CAMERA, by using a smaller sensor. That is ineffect what going from a FF sensor to a 1.5 crop sensor does...itjust crops the image!.

No, you do not understand. The CoC is dependent on the final imagesize. When you crop an image (either via a crop sensor or in PP) youare changing the CoC. The CoC determines the DoF..

Close. Cropping the image doesn't change it. Resampling it in PP orusing a sensor with different pixel density does. Smaller pixels =smaller CoC..

Many current camera models have lenses that can not approach the resolution of the sensors. In these cases, the DOF is relatively independent of the sensor size..

Back in the days when DOF scales were on lenses, they were assumingsome average film grain. Even there, it's really different for ISO50 film than it is for ISO 800. With digital, crop-sensor camerasalmost always have smaller pixels than full-frame, which is why theDOF changes..

Yes, but they also have shorter FL lenses...ACTUAL FL. You are comparing two completely different animals..

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #26

Dave_s93 wrote:.

Greg Nut wrote:.

Dave_s93 wrote:.

No, you do not understand. The CoC is dependent on the final imagesize. When you crop an image (either via a crop sensor or in PP) youare changing the CoC. The CoC determines the DoF..

Close. Cropping the image doesn't change it. Resampling it in PP orusing a sensor with different pixel density does. Smaller pixels =smaller CoC..

No. This is wrong. Pixel density has nothing to do with DoF (as itis currently defined with DoF calculators). I am surprised by thenumber of people who respond with incorrect information. DoF isdefined as the "acceptable sharpness" on a 8x10 print viewed at 25cm(10 inches). There are many ways to calculate the CoC based on thisdefinition*, but it generally yields a value of .025mm - .035mm forthe CoC when using 35mm film..

Finally! I agree with you about something. .

One of the factors that is causing us a a group (could we be called a "circle of confusion"?) is that there are two definitions of CoC. One is the simple to understand version associated with optics. It is simply the optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source. The other abbortion is the CoC as used in photography. There is no single definition of this! There are however, many different groups that think THEIR favorite definition is the standard..

A good article is here:.

Http://en.wikipedia.org/wiki/Circle_of_confusion.

The authors sorta agree that the formula for CoC involves:.

Visual acuityViewing conditionsEnlargement ratio from the original image.

Note that NONE of the three variables in the CoC formula involve ANYTHING associated with a camera or a lens!.

The article goes on to say that there are some general assumptions made to simplify the problem of calculating the CoC. First, the viewing distance is assumed to be 25 cm, since it is generally accepted as the closest comfortable viewing distance for most people. Next, it is assumed that a person with good vision can distinguish an image resolution of 5 line pairs per millimeter (lp/mm). Next it is assumed that a comfortable viewing distance is also one at which the angle of view is approximately 60 at a distance of 25 cm, this corresponds to about 30 cm. Finally, since the final image size is not usually known at the time of taking a photograph, it is common to assume a standard size such as 25 cm width, along with a conventional final-image CoC of 0.2 mm, which is 1/1250 of the image width. Conventions in terms of the diagonal measure are also commonly used.



There is a LOT of wiggle room in these above assumptions..

The article continues to say that using the so-called Zeiss formula the circle of confusion is sometimes calculated as d/1730 where d is the diagonal measure of the original image (the camera format). For full-frame 35 mm format (24 mm 36 mm, 43 mm diagonal) this comes out to be 0.024 mm. A more widely used CoC is d/1500, or 0.029 mm for full-frame 35 mm format, which corresponds to resolving 5 lines per millimeter on a print of 30 cm diagonal. Values of 0.030 mm and 0.033 mm are also common for full-frame 35 mm format. For practical purposes, d/1730, a final-image CoC of 0.2 mm, and d/1500 give very similar results..

Note that all these calculations can be made w/o reference to the lens used! IOW, the resolving power of the lens is not a factor in computing the CoC of a camera! Even if the lens has a big CoC (according to optical measurements) the CoC of the camera system can be small if the format is tiny!.

I have been arguing CoC asuming that it was a measured optical parameter. Others have been using one of the above formulas that ignore optics. Sigh....

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #27

Trale wrote:.

I recently bought a Rebel XT with the kit lens (18-55mm), along witha EF 75-300mm zoom lens. I will also be buying a EF 50mm f/1.8 primelens. My question is, would I have to multiply all these focal lengthvalues by 1.6 (the Rebel XT's field of view crop value) to get isactual effective focal length? So would the equivalent specs for mythree lenses in question be:.

29-88mm120-480mm80mm.

You got the answer you needed in your first two posts. To expand very slightly. you do not have to do anything but some people find it useful to convert figures to a 35mm equivalent. Despite all the squabling in later posts 35mm was by far the most commercially succesful standard in the second half of the 20th century and different sensor sizes for P & S cameras make it a sensible reference point for comparison..

But once you have a DSLR and one lens there is really no need for equivalance figures. You just get a wider or more telephoto lens by reference to the numbers on your first lens..

You do not even need to remember that the crop factor will affect Depth of Field. All you need is to make sure you print off some figures for YOUR DSLR/crop factor..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #28

Chuxter wrote:.

Huh? What does CoC have to do with it. When I cropped the imagein-camera by substituting a physically smaller sensor, the CoCdoesn't change...at least if the sensors have higher resolution thatthe CoC...ie, the CoC is largly lens dependent..

No, the CoC is dependent on sensor/format size and is not dependent on the lens (for DoF purposes)..

When you crop an image (either via a crop sensor or in PP) youare changing the CoC..

Please explain to us how and why this happens..

I'll let others explain this..

Some quotes:http://ronbigelow.com/articles/depth-of-field-1/depth-of-field-1.htm.

"Sensor/Film Size: The size of the sensor or film also has an affect on the DOF. For instance, most DSLRs have sub-full-frame sensors. A typical sub-full-frame sensor might have dimensions of around 22.5mm x 15mm. Such a sensor would have a diagonal of 27.04mm. The ratio of the diagonal of the print to the diagonal of the sensor is 12. Thus, the CoC is 12 times smaller than 0.25mm.

This smaller CoC reduces the DOF.".

Http://www.cjcom.net/articles/hyperfoc.htm.

"The question is often asked whether the depth of field (DOF) for a 35mm camera lens remains the same when it is used on a digital camera with a smaller APS sized sensor. It would seem that it shouldn't change just because a smaller part of the image circle is used, but in reality it is different. This is because DOF calculations are based upon three factors: the degree of enlargement of the final image, the viewing distance, and the smallest discernible point at that viewing distance.".

Http://photo.net/learn/optics/dofdigital/.

"So the bottom line - and all you really need to know - is that DOF is inversely proportional to format size. Note that format size is inversely proportional to the "digital multiplier". The higher the "digital multiplier", the smaller the format and thus the greater the depth of field. Note also that now you can see one of the reasons large format camera users need tilts and swings to get adequate depth of field. With an 8x10 camera you have about 8.5 times LESS depth of field than you do with 35mm for the same image. This also explains why consumer digicams, some of which have sensors 1/6 the size of 35mm film, have such a large depth of field and one of the reasons why it's almost impossible to get blurred backgrounds when using them.".

The CoC determines the DoF..

The CoC is ONE parameter that determines the DOF..

True..

[snip].

Go read #2 & #3. With an equivalent focal length and subjectdistance there is a different DoF (#2). When you crop the FF picturein PP (#3), you will get the same DoF..

Dave, you keep returning to the "equivalent focal length" argument.Let me state again, "when cropping in-camera or later in PP, thefocal length is not involved.".

Bad choice of words on my part. I should have said "with the SAME focal length and subject distance, there is a different DoF (#2). When you crop the FF picture in PP (#3), you will get the same DoF..

Bob is quite correct. You are quite correct. But both of you aretalking about something different that I am talking about..

I am talking about taking a specific 50mm lens (adjusted to f:4.0 andfocused at 10m) and mounting it on two different cameras. First,let's put it on a new D3 with a 24mm x 36mm sensor. Second, let's putit on a Oly E510 with a 13.5mm x 18mm sensor. I claim that the DoF ofthat specific 50mm lens is unchanged as it moves from a large sensorcamera to a small sensor camera. That is in essence, "croppingin-camera". There are a few cameras that allow us to electronicallyvary the number of pixels and aspect ratio recorded.

The "equivalent FL" argument simply does notapply in these examples..

No. There is a different DoF. The Oly (2x crop) will have LESS DoF (#2), unless you crop the Nikon D3 (FF) photo to the same FoV (#3)..

From the following calculator:http://www.dofmaster.com/dofjs.html.

Oly 51050mm, f4, 10m subject distance => 5.06m DoF.

Nikon D350mm, f4, 10m subject distance => 12.4m DoF..

Comment #29

Dave, reading this thread has been like reliving a bad experience that I had been trying to forget. I've had two or three arguments with Chuxter, and each time the pattern has been:.

1) Chuxter gets something bizarrely wrong (bizarrely considering he is actually quite an experienced photographer)..

2) I post a rebuttal..

3) Charlie digs his heels in..

4) I explain further (probably haven't lost patience yet)..

5) Charlie starts twisting my words, but even more characteristically he twists his own words. That is, he will argue that he said one thing when in fact he said something quite different..

6) I get completely exasperated and have to take a number of deep breaths to avoid a significant breach of forum rules..

7) Charlie tells me not to discuss advanced topics on the Beginners forum!.

Any of that sound familiar? I think you are up to about number (5)..

It does give me strength to realise I am not alone...

Comment #30

Sherwoodpete wrote:.

An interesting article on Depth of Field here:http://www.wrotniak.net/photo/tech/dof.html.

Thanks, that's interesting. I keep thinking that all these experts are missing something. When Andrezj says:.

"In other words, if your camera is focused at d, acceptable circle of confusion will be achieved for subjects ranging in distance from d1 to d2, or from d”d1 to d+”d2..

The value of c should be set to the 1/1440 of the frame diagonal: for example, 0.03 mm for 35 mm cameras, 0.0061 mm for ones with 1.8" sensors (Olympus C-series from C-2000Z to C-7070WZ) 0.0077 mm for 2/3" ones (E-10/E-20), or 0.015 mm for the Four Thirds system, and so forth.".

He seems to be thinking that cameras have a CoC CONTROL! How can I SET my CoC to 1/1440th of the frame diagonal? Perhaps "set" was the wrong word?.

Next, that 1/1440 is a SWAG. Other experts use different numbers...like 1/1500 (5 lines/mm on 30 cm print), 1/1720 (Kodak) and 1/1730 (Zeiss). With multiple SWAGs to choose from, how can we trust one result?.

Also, he seems to ignore the issue that on some (perhaps many) digicams, the lens is incapable of matching the resolution of the sensor. Or that neither the lens or the sensor is capable of a resolution better than the CoC! What is the effect of camera hardware that does not satisfy the CoC requirement of one of the many possible formulas?.

All this seems like pseudo-science to me..

I think the CoC is CONTROLLED by the lens. There is a different equation for CoC that obeys optical laws. It is not a SWAG...it is absolute..

As an example, consider a hypothetical camera/sensor that has several formats: 1:1, 3:4, 2:3, and 9:16. These 4 formats are all possible within a sensor size of 16mm x 24mm. The 1:1 format is 16mm x 16mm, the 3:4 format is13.5mm x 18mm, the 2:3 format is 16mm x 24mm, and the 9:16 format is 13.5mm x 24mm. The number of pixels of course varies with each of these formats, but the pixel pitch is constant..

According to these photography experts, each of these formats has a different CoC, in spite of using the same sensor and lens. Think about this for a minute....

Suppose I took a picture with the largest (area) of these formats, the 2:3 format and produce 4 prints of "standard" size (according to the photography experts)...12" x 18". I then cropped 3 of these 4 prints to the 1:1, 3:4, and 9:16 aspect ratios, making them 12" x 12", 10" x 13.5", and 10" x 18". Putting all 4 prints on the wall and viewing them from the same "standard" distance (ie, 10"), I contend that all 4 pix will have the same DoF. How could it not be the case???.

This thought experiment points out the error of the "experts", they vary the print size and/or viewing distance such that all size/shape formats subtend the same angle. It seems logical, but it's not..

At best, DoF is subjective. All attempts to quantify it run into the same obstacle: How to define what is "suitably sharp"? The term CoC is an attempt to define this. It is an ESTIMATE of the required sharpness needed to make OOF images appear sharp at the STANDARD size and viewing distance, assuming an observer with STANDARD eyesight..

How does Circle of Confusion differ from the Blur Circle? A Blur Circle is created at the sensor when a point source is OOF...the more OOF, the larger the Blur Circle. This part is NOT pseudo science. But even this is subjective, as the circle produced does not have a sharp perimeter, but rather is a blurry, round spot. The issue here is to decide where on the blurry edge to measure the diameter? A common answer is the smallest circle that can contain 90% of the optical energy. Others use the half-way point up/down the slopes:.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

The difference in BC and CoC is that the BC is an artifact of an actual image and can be measured quantatatively...it is a blurry spot produced by an OOF adjustment of a lens when imaging a point light source. In contrast, the CoC is a hypothetical minimum observable spot, measured at the sensor, but observed on a print of "standard" size from a "standard" distance" by a "standard" observer. The spot is often not even produced by a lens ("what lens?")...it's just a spot that is so small that it can't be seen by a human eye under the control conditions. That spot is then translated back to the sensor. The assumption is that if an OOF condition produces a Blur Circle of that diameter, it will be invisible under the control conditions and thus will appear sharp or in focus..

My biggest issue with the DOF experts, is that their assumptions seem wrong. Is it logical to assume that a Minox camera that uses 9.5mm film will produce prints that are the same size as a 35mm camera? Is it logical to assume that a 35mm camera will produce prints the same size as an 8" x 10" view camera? A Minox might make a decent 3" x 4" print...an 8" x 10" view camera will produce a stunning 8' print! Most people hold 3" x 4" prints about 10" away and stand across the room to view a 8' print. Why make the standard observers hold the reference prints so close? I just don't get the whole process....

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #31

IMac, therefore iAm wrote:.

It doesn't make a lick of difference what the effective focal lengthis. The sooner you stop thinking about it, the better..

For someone new to SLR shooting with no historical context, I would agree..

For someone with years of experience shooting film-based SLRs using different interchangeable lenses, then, yes, doing the mental math to obtain the familiar FOV is important. I've shot pure digital since 1999, but to this day I STILL think in terms of 1x per 50mm for my DSLRs..

For me, abandoning the intimate familiarity of the 1x/50mm rule of thumb for my crop sensor camera would be like converting all (American) football yardage to meters. Can you just imagine watching the Super Bowl with all the play-by-play given in meters???.

Http://www.pbase.com/digirob..

Comment #32

Steve Balcombe wrote:.

Any of that sound familiar? I think you are up to about number (5)..

It does give me strength to realise I am not alone..

Are you sure you are not also thinking about BAK also?.

Http://forums.dpreview.com/...forums/read.asp?forum=1002&message=25629890http://forums.dpreview.com/...forums/read.asp?forum=1002&message=25711395.

Hey, you just thought I'd give you a smile...

Comment #33

Chuxter wrote:.

Suppose I took a picture with the largest (area) of these formats,the 2:3 format and produce 4 prints of "standard" size (according tothe photography experts)...12" x 18". I then cropped 3 of these 4prints to the 1:1, 3:4, and 9:16 aspect ratios, making them 12" x12", 10" x 13.5", and 10" x 18". Putting all 4 prints on the wall andviewing them from the same "standard" distance (ie, 10"), I contendthat all 4 pix will have the same DoF. How could it not be the case???.

The DoF scales are all based on an 8x10 print. If you print at a different size, you will need to make an adjustment. It is not practical to have multiple DoF scales for every different print size. If you need to have it that exact, you should be able to calculate it yourself if you understand the principle..

Http://www.cambridgeincolour.com/tutorials/depth-of-field.htm.

"When does the circle of confusion become perceptible to our eyes? An acceptably sharp circle of confusion is loosely defined as one which would go unnoticed when enlarged to a standard 8x10 inch print, and observed from a standard viewing distance of about 1 foot."[snip].

"A different maximum circle of confusion also applies for each print size and viewing distance combination."..

Comment #34

Been there. Done thatGot the T shirt!.

Steve Balcombe wrote:.

Dave, reading this thread has been like reliving a bad experiencethat I had been trying to forget. I've had two or three argumentswith Chuxter, and each time the pattern has been:.

1) Chuxter gets something bizarrely wrong (bizarrely considering heis actually quite an experienced photographer)..

It is normally something so outrageous that you feel you have to respond for the benefit of others who might otherwise be seriously misled..

2) I post a rebuttal..

3) Charlie digs his heels in..

4) I explain further (probably haven't lost patience yet)..

5) Charlie starts twisting my words, but even more characteristicallyhe twists his own words. That is, he will argue that he said onething when in fact he said something quite different..

Yes. You get all sorts of smoke and mirrors to cloud the issue! He never answers a point directly but introduces all sorts of sub issues to explain why he wasn't wrong while moving the goal posts. I think the strategy is to bluster until he has exceeded your attention span and will to live. You will note an excessive reliance by him on that well known suspect source Wikipedia..

6) I get completely exasperated and have to take a number of deepbreaths to avoid a significant breach of forum rules..

7) Charlie tells me not to discuss advanced topics on the Beginnersforum!.

Any of that sound familiar? I think you are up to about number (5)..

It does give me strength to realise I am not alone..

I simply ignore him now. It is not worth the energy expended to argue with him. I used to worry that large numbers were being misinformed by his posts but I have decided that is his and their problem and not mine..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #35

Dave_s93 wrote:.

Steve Balcombe wrote:.

Any of that sound familiar? I think you are up to about number (5)..

It does give me strength to realise I am not alone..

Are you sure you are not also thinking about BAK also?.

Http://forums.dpreview.com/...forums/read.asp?forum=1002&message=25629890http://forums.dpreview.com/...forums/read.asp?forum=1002&message=25711395.

Hey, you just thought I'd give you a smile..

I'm not saying Chuxter has no competition ..

Comment #36

Chris Elliott wrote:.

I simply ignore him now. It is not worth the energy expended to arguewith him. I used to worry that large numbers were being misinformedby his posts but I have decided that is his and their problem andnot mine..

I'm sure you will have noticed that the more experienced posters tend to ignore incorrect posts and instead give a correct reply to the original question. That way they avoid getting into arguments. I've tried to use that tactic a bit more lately, but I am under no illusions about my inability to stay out of trouble ..

Comment #37

Dave_s93 wrote:.

"A different maximum circle of confusion also applies for each printsize and viewing distance combination.".

Wrong!! ... Well, if not "completely wrong"... let's say it's 3/4 wrong..

(I tell you folks... This show is gonna run and run!)Regards,Baz..

Comment #38

Barrie Davis wrote:.

Dave_s93 wrote:.

"A different maximum circle of confusion also applies for each printsize and viewing distance combination.".

Wrong!! ... Well, if not "completely wrong"... let's say it's 3/4 wrong..

I think you'll need to explain that. I'm not going to get into the argument per se (it's crowded enough already), but I will make the observation that Sean McHugh's web site which the quote was taken from is one of the best out there. He is a fine photographer with a very good technical understanding - he's not often wrong..

(I tell you folks... This show is gonna run and run!)..

Comment #39

Steve Balcombe wrote:.

Barrie Davis wrote:.

Dave_s93 wrote:.

"A different maximum circle of confusion also applies for each printsize and viewing distance combination.".

Wrong!! ... Well, if not "completely wrong"... let's say it's 3/4 wrong..

I think you'll need to explain that..

Okay. It isn't complicated..

It's just that the ONE circle of confusion......

(here I'm talking about the one CoC that applies for the assumed 10x8" at reading distance [whatever that CoC happens to be for the format in use]).

... will serve equally well for ANY size of print viewed at a distance where said print subtends the same angles to the viewer as if it WERE a 10x8" at normal reading distance..

Or, put another way....

It is exceptional, not routine, that we need to apply a different CoC, because mostly people DO adjust their viewing distance to suit the size of print they are looking at.....

So it is that 3/4 of the time we do NOT need a different CoC, because the viewing conditions are NOT different enough to require one..

Wrotniak's statement does not make that clear..

In fact, it is the principle point that is being missed, here the one-size-fits-all kind of DoF principle that allows us to work with some sort of standardisation in Depth of Field tables and scales in the first case......

.... even though they are not engraved on lenses anymore, dammit!!.

See what I'm driving at, Steve? Regards,Baz..

Comment #40

Barrie Davis wrote:.

Steve Balcombe wrote:.

Barrie Davis wrote:.

Dave_s93 wrote:.

"A different maximum circle of confusion also applies for each printsize and viewing distance combination.".

Wrong!! ... Well, if not "completely wrong"... let's say it's 3/4 wrong..

I think you'll need to explain that..

Okay. It isn't complicated..

It's just that the ONE circle of confusion......

(here I'm talking about the one CoC that applies for the assumed10x8" at reading distance [whatever that CoC happens to be for theformat in use]).

... will serve equally well for ANY size of print viewed at adistance where said print subtends the same angles to the viewer asif it WERE a 10x8" at normal reading distance..

Agree completely. I produce exhibition/display graphics in the course of my work so I am more than usually familiar with this issue..

If Sean McHugh had said "each print size and viewing distance *ratio*" that would have been more strictly accurate - and I believe that is what he meant..

Or, put another way....

It is exceptional, not routine, that we need to apply a differentCoC, because mostly people DO adjust their viewing distance to suitthe size of print they are looking at.....

Yes, also agreed. But that doesn't make Sean McHugh's words wrong, it simply means he was referring to a less common (but not *un*common) situation..

An everyday example is the case of a double page spread in a magazine. They tend to be viewed just as closely as a single page or even a half page, despite being significantly more enlarged..

So it is that 3/4 of the time we do NOT need a different CoC, becausethe viewing conditions are NOT different enough to require one..

Obviously you have plucked '3/4' out if the air but let's use it. That means 1/4 of the time we do need a different CoC. So it does merit consideration...

Comment #41

Steve, let me say that while we have had technical disagreements, I do respect your opinions..

Consider what it looks like from the other side....

Steve Balcombe wrote:.

Dave, reading this thread has been like reliving a bad experiencethat I had been trying to forget. I've had two or three argumentswith Chuxter, and each time the pattern has been:.

1) Chuxter gets something bizarrely wrong (bizarrely considering heis actually quite an experienced photographer)..

Often it looks to me as if YOU are bizarrely wrong, considering that you normally get things very right..

2) I post a rebuttal..

3) Charlie digs his heels in..

When I think I'm right, I don't normally admit that I'm wrong. I think that applies to most people..

4) I explain further (probably haven't lost patience yet)..

We both seem to be patient. I know that often it takes days for a different idea to gestate..

5) Charlie starts twisting my words, but even more characteristicallyhe twists his own words. That is, he will argue that he said onething when in fact he said something quite different..

This I think happens when some of my ideas start to make sense. Your ego starts to work to protect itself. This is a VERY difficult time..

6) I get completely exasperated and have to take a number of deepbreaths to avoid a significant breach of forum rules..

7) Charlie tells me not to discuss advanced topics on the Beginnersforum!.

I thought that you asked if I thought that you should go away? I answered that the thought had crossed my mind, but I decided not to say that. That's a bit like the psychologist asking me if I have ever thought about suicide. Of course I have thought about suicide. Even the psychologist has thought about suicide. We can't have a conversation about suicide unless we think about it. But because we all think about suicide doesn't mean that we have made plans to kill ourselves.

So I thought about suggesting that you not post on the Beginners Forum, but REJECTED that idea. But when you asked if I had thought about it, I said "Yes"..

But often I don't remember correctly, so I went back and researched our discussion. It was on 27-28 Sept. The thread was entitled, "Setting focus won't affect field angle?" started by CedrikOs. He asked, "If setting the focus is changing the distance between the lens and the CCD to bring the object's image in the CCD plan, then the vision angle should change slightly when moving the focus." Your approach was to assume he had a dSLR and that his lens on that dSLR worked like your Canon EF 100/2.8 Macro lens worked...ie, that as it was focused, it also ZOOMED..

However, the OP had a "compact camera"...he never did tell us which one. Your response did NOT answer the question he asked, which was about the simple, small geometry change as a simple lens is focused and how it affects the FOV..

Regarding your contention that I asked you to get off the BQF, here is our actual exchange:.

Charlie said in previous post....

I also think your assumption that the OP had a dSLR was presumptious.He never gave you any reason to think that..

Steve said....

True, and very remiss of me. I should never darken the doors of thisforum again..

Charlie said...Actually, that was what I was thinking last night..

Charlie said in previous post....

You are NOT much help teaching beginners about photography! KISS....

Steve said....

That is a real jerk remark to finish with..

Note that I ignored that last remark. I also admitted that I was unaware that ANY prime lens zoomed as it focused and thanked you for that knowledge..

Jim Cockfield answered the OP's question correctly and comprehensively:.

Http://forums.dpreview.com/...forums/read.asp?forum=1002&message=24989883.

Your approach was to be mildly miffed that Jim didn't mention all YOUR lenses! You replied by listing all the lenses you own and (I think) stating that YOUR 100mm macro lens had the variable FL feature before the lens that Jim mentioned. I didn't say you were a jerk for reacting this way...but I thought it!.

SO, in conclusion, I did NOT ask you to leave the BQF...I only thought about it. .

I DID ask you to keep your answers simple. The OP's question was simple in this case and you gave a VERY esoteric answer and didn't supply much information for why you gave that answer..

Steve, I think much of the cause of our occasional "discussions" is language. Although we nominally speak the same one, in practice, my explanations are not understood. Perhaps it's age too? I'm older and have a different way of writing..

We both can be jerks on occasion. .

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #42

In general, I agree that the standard DoF charts work well for most circumstances, however, I was responding to chuxter's statement about DoF from different size prints cropped from the same photo with the SAME viewing distance. In this case, he wanted the same viewing distance for the smaller print as the bigger print..

Just to be nit picky, though..

Barrie Davis wrote:.

It's just that the ONE circle of confusion......

(here I'm talking about the one CoC that applies for the assumed10x8" at reading distance [whatever that CoC happens to be for theformat in use]).

Well, there are actually quite a few CoC conventions for an 8x10 at 25cm. They range between .025 to .035 for 35mm film - see my previous post..

... will serve equally well for ANY size of print viewed at adistance where said print subtends the same angles to the viewer asif it WERE a 10x8" at normal reading distance..

I believe the quote was "A different maximum circle of confusion also applies for each print size AND viewing distance COMBINATION." You are right that for larger prints, people generally change the viewing distance to compensate. However, if you have a larger print and wanted the same viewing distance (or the same print at a different viewing distance), you need to use a different CoC...

Comment #43

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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