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'effective' focal length of zoom lens
I'd like to buy a Canon DSLR but would want to use it, at least initially with my existing EOS 28 - 300mm zoom lens and am confused as to what "effective" zoom range I'd have. If I've correctly understood the posts on this issue, the smaller size of the DSLR sensor (compared to 35mm film) would essentially give my lens the magnification of a 45 - 480mm zoom ... is that correct?.

Are there other issues related to using a zoom lens designed for 35mm film with a DSLR that I should know about?.

Thanks,James..

Comments (33)

I'd like to buy a Canon DSLR but would want to use it, at leastinitially with my existing EOS 28 - 300mm zoom lens and am confusedas to what "effective" zoom range I'd have. If I've correctlyunderstood the posts on this issue, the smaller size of the DSLRsensor (compared to 35mm film) would essentially give my lens themagnification of a 45 - 480mm zoom ... is that correct?.

Yes, that's correct... you won't have any wide-angle with such a lens on a cropped-sensor DSLR..

Best wishesMike..

Comment #1

As stated you loose the the wideangle side of the zoom, while gaining on the telefoto side..

There is also an image quality gain. since lenses perform poorer at the edges than the center, you will pick up some image quality. the reason is that with a smaller image circle you no longer are using the edges, you are useing the sharper center mostly..

As focal length just multiply the crop factor(multiplier) X the lens length in mm and this equals the new focal length in mm. canon on everything but the high end dslrs use a 1.6 multiplier so a 100mm lens becomes a 160mm lens for any practical point of view...

Comment #2

Mike:.

Thanks for the quick reply. On the one hand, I won't mind having that extra 180mm of "effective" focal length for photographing some wildlife but I'm disappointed to learn I'll loose my wide angle capabilities. Why the heck didn't the camera manufacturers just design the sensor to be the same size as the 35mm film it was replacing?..

Comment #3

Imamember wrote:.

Mike:Why the heck didn't the camera manufacturers justdesign the sensor to be the same size as the 35mm film it wasreplacing?.

They did... the 5D, 1DS, etc... those are full frame ... and looking at the price you'll know why they went with smaller sensors for the lower end models...

Comment #4

Thanks for pointing out the "upside" of gains in image quality ... that will help soothe the pain of loosing my wide angle capability...

Comment #5

One additional aspect of a lens on a crop factor digital is that one will have a narrower field of view as compared to the full frame....

A 35-mm/FF camera with a 50-mm lens at f/8 and subject at 10', the field of view in focus is 6.28' near to far.

A Nikon D50 1.5X crop FOV is 3.98' near to far....

Http://www.dofmaster.com/dofjs.html.

Telecorder (Dave)FZee30+RD-S+OlyTC1.7XDee50+Nikon 35mm F2.0D-AF+Nikkor18-70DX+Tam70-300-macro-LD+BIGMA 50-500 EX HSM(Coming soon - Nikon 70-300 VR!)My Image Galleries.

Http://www.nikonians-images.com/...hp?cat=500&ppuser=121399&password=.

Http://Telecorder.smugmug.com/..

Comment #6

Imamember wrote:.

Thanks for pointing out the "upside" of gains in image quality ...that will help soothe the pain of loosing my wide angle capability..

When you get the camera, why not get it with the 18-55 kit lens? It costs very little extra and will give you the wide angle. If you buy them together it's only an extra 50 (UK) for the kit lens..

Best wishesMike..

Comment #7

When DSLR's first came out, it was understood that lets say a 100 prime lens was really a 160 lens on a Canon 1.6 partial sensor crop; that is, a 160 focal length telephoto with a "magnification" factor..

There is a difference between a crop factor and a magnification factor. If you have a software editing program, when you crop, you are removing excess picture around the final image, thus making the final image larger - appearing as though it were magnified..

But magnification is different. Lets take the argument to a bizarre level, this often helps to understand it. Lets say you are out in the wilds of North Dakota, and low and behold a wolf is on a small hill and behind him is the moon. You know to make the moon look really really large you need around a 500 lens to "magnify" the moon. So you pull out of your bag, the 500 and shot the picture - wolf on hill with moon behind him..

You decide on another picture but unfortunately you drop your lens and it smashes the front lens and it is no longer functional. You say what a minute, I have a DSLR with a crop factor of 10 (no such camera exists). You think, if I put my 50mm lens on it with a crop factor of 10, it'll be a 500 lens. So you take the remaining pictures with it. Will the moon by nice and big like in the original 500 prime lens shot? No. Because the image is cropped, framed like a 500 lens, but it is not magnified like a 500 lens.



The moon will look like a 50mm lens shot of the wolf and moon in background, then cropped in Picasa, to resemble the field of view a 500 lens. The moon and wolf will appear closer, but the moon won't be that wonderful full moon of the original 500 prime shot.l..

Comment #8

Rsn48 wrote:.

Will the moon by nice and big like in the original 500 prime lens shot?.

Yes, it will be, in the final print, because in the printing process, you will adjust the images from the two cameras to fit the paper size. The perspectives will also be the same..

Where the 10x crop camera may suffer is in image quality. The printouts from the two cameras' images may have the same size. But if the sensors have similar pixel sizes, the camera with the larger sensor will have much more megapixel resolution. And if they have the same megapixel count, the camera with the larger sensor will have a high-ISO / low-light noise advantage..

There also may be depth of field differences between the 50mm and 500mm lens and between the small and large sensors. That wouldn't affect the size of the image of the Moon, but it might affect the artistic merit of the photo...

Comment #9

Rsn48 wrote:.

When DSLR's first came out, it was understood that lets say a 100prime lens was really a 160 lens on a Canon 1.6 partial sensor crop;that is, a 160 focal length telephoto with a "magnification" factor..

There is a difference between a crop factor and a magnificationfactor. If you have a software editing program, when you crop, youare removing excess picture around the final image, thus making thefinal image larger - appearing as though it were magnified..

But magnification is different. Lets take the argument to a bizarrelevel, this often helps to understand it. Lets say you are out inthe wilds of North Dakota, and low and behold a wolf is on a smallhill and behind him is the moon. You know to make the moon lookreally really large you need around a 500 lens to "magnify" the moon.So you pull out of your bag, the 500 and shot the picture - wolf onhill with moon behind him..

You decide on another picture but unfortunately you drop your lensand it smashes the front lens and it is no longer functional. Yousay what a minute, I have a DSLR with a crop factor of 10 (no suchcamera exists). You think, if I put my 50mm lens on it with a cropfactor of 10, it'll be a 500 lens. So you take the remainingpictures with it. Will the moon by nice and big like in the original500 prime lens shot? No. Because the image is cropped, framed likea 500 lens, but it is not magnified like a 500 lens.



The moon will look like a 50mm lens shot of the wolf and moon inbackground, then cropped in Picasa, to resemble the field of view a500 lens. The moon and wolf will appear closer, but the moon won'tbe that wonderful full moon of the original 500 prime shot.l.

Sorry, the image will be the same, as long as your 10x crop camera has the same resolution as the first one, or you resize to match. Caveats: DOF will be much greater with the 50mm, and image quality greater with the 500mm. The apparent size of the moon in relation to the wolf will be the same..

I do agree in part: "crop factor" is better than "magnification factor," and "equivalent focal length" is preferable to "effective focal length."..

Comment #10

Imamember wrote:.

I'd like to buy a Canon DSLR but would want to use it, at leastinitially with my existing EOS 28 - 300mm zoom lens and am confusedas to what "effective" zoom range I'd have. If I've correctlyunderstood the posts on this issue, the smaller size of the DSLRsensor (compared to 35mm film) would essentially give my lens themagnification of a 45 - 480mm zoom ... is that correct?.

Are there other issues related to using a zoom lens designed for 35mmfilm with a DSLR that I should know about?.

Thanks,James.

Get it with the 18-55IS to use in addition to your 28-300. The only other issue I can think of is that current DSLR's are much higher resolution than typical film, making lens defects more apparent. On the other hand, you'll only be using the center of the image, so it's probably a wash...

Comment #11

Rsn48 wrote:.

The moon and wolf will appear closer, but the moon won'tbe that wonderful full moon of the original 500 prime shot.l.

Assuming you are correct. Let's assume that the wolf is silhouetted against the sky and we can see the moon AND some stars. The shot with the 500mm shows the wolf, the moon and the stars..

With the 50mm and the 10x crop, we get the picture of the wolf. But hang on a minute, the moon is going to be smaller this time, and the stars too. So we get to see more stars in the sky, which were previously hidden behind the body of the wolf. Can you explain how the changed lens has allowed us to see objects hidden behind the wolf?.

Well, simply it can't. light travels in straight lines and no matter which lens we use, we get the same wolf, the same moon and the same stars.Regards,Peter..

Comment #12

I think the comparison I made previously regarding hidden stars may be somewhat back-to-front..

But to be clear about one thing, the only way that the wolf can be larger and the moon smaller is to walk towards the wolf. Changing the viewing position is the ONLY way to get the relative sizes to alter. The lens makes no difference.Regards,Peter..

Comment #13

Sherwoodpete wrote:.

Changing the viewing position is the ONLY way to get the relative sizes toalter. The lens makes no difference..

The perspective changes, how "close the moon would appear to be" would change in the 50mm 10x crop image versus the true 500mm image..

Http://www.luminous-landscape.com/tutorials/dof2.shtml.

Good Day,Roonal.

'Money doesn't buy happiness, but it makes for an extravagant depression' by golf tournament sportscaster..

Comment #14

Roonal wrote:.

Sherwoodpete wrote:.

Changing the viewing position is the ONLY way to get the relative sizes toalter. The lens makes no difference..

The perspective changes, how "close the moon would appear to be"would change in the 50mm 10x crop image versus the true 500mm image..

I'm not with you here..

Firstly perspective - this depends on only one thing, the viewing position. If the camera is moved, the perspective changes. If remaining in the same position, perspective is identical..

Secondly, how "close the moon would appear to be" - sorry, I don't know what is meant by this..

Regards,Peter..

Comment #15

Greg Nut wrote:.

On the other hand, you'll only be using the center of the image, so it'sprobably a wash..

Yes..

The idea that a lens is somehow "improved" in image quality, by cropping off the softer periphery of the image, and using only the sharper centre, is quite wrong..

The remaining image is smaller in area, will need more enlargement to fill a screen or print, and that enlargement will reduce quality by simple magnification..

In the end the effects pretty much cancel out, and the quality of the cropped lens remains very close to what it was.Regards,Baz..

Comment #16

Looked thru one of my photography books which has example photos and saw where I was mistaken..

Good Day,Roonal.

'Money doesn't buy happiness, but it makes for an extravagant depression' by golf tournament sportscaster..

Comment #17

Telecorder wrote:.

One additional aspect of a lens on a crop factor digital is that onewill have a narrower field of view as compared to the full frame....

True. But that is what a longer "effective focal length" implies - a narrower field of view. I think you are thinking of Depth of Field because of your statement below..

A 35-mm/FF camera with a 50-mm lens at f/8 and subject at 10', the.

Field of view in focus is 6.28' near to far a Nikon D50 1.5X crop FOV is 3.98' near to far....

Http://www.dofmaster.com/dofjs.html.

Yes, you will have less DoF in this situation, but you will also get a different framing. If you keep the framing the same by using a shorter focal length or longer shooting distance, then the DoF in the FF/35mm will be thinner. See this link for a better explanation..

Http://www.dofmaster.com/dof_dslr.html.

Hope this helps...

Comment #18

I think you're right, but a number of folks here have dissented from your opinion and i'm uncertain as to who's right. can we clarify who's right here?..

Comment #19

Bugzie wrote:.

I think you're right, but a number of folks here have dissented fromyour opinion and i'm uncertain as to who's right. can we clarifywho's right here?.

I'd suggest reading this Luminous Landscape article:.

"OK, firstly, when you put a 300mm lens on a D60 you do NOT get a 480mm lens  it is still a 300mm lens. What has changed is the format of the camera  from 36x24mm film to a 22.7x15mm chip.".

"This is the crucial point  its a different format therefore the same lenses behave differently. In this case the format is smaller so the angle of view is also smaller. Therefore it behaves in similar, but not identical, fashion to a longer focal length lens.".

Http://www.luminous-landscape.com/...ials/understanding-series/dslr-mag.shtml.

In a nutshell, the sensor is smaller, so it makes the lens seem longer, even though it really isn't...

Comment #20

Dave_s93 wrote:True. But that is what a longer "effective focal length" implies - anarrower field of view. I think you are thinking of Depth of Field.

Busted... you're correct, I meant to say Depth of Field/acceptable focus....

Re: Dissenting opinion - not a magnification factorbugziei think you're right, but a number of folks here have dissented from youropinion and i'm uncertain as to who's right. can we clarify who's right here?.

Bugzie -.

Magnification implies that the amount of image data, when magnified, is spread over a larger area; In a crop factor camera, the data is not 'magnified' by the sensor or the lens; the full field of view is cropped so that the image effectively occupies a larger portion of the smaller sensor if the image/field of view from the same lens is projected onto a full frame/35-mm film plane, the image LxW remains the same..

Eg - an image of 8-arc-seconds x 10-arc-seconds will occupy the same L x W whether it's projected onto a full frame or crop sensor it'll just occupy a larger relative portion of the crop sensor making it appear larger....

While the illustration is for a Pany, the sketch can be used to illustrate the circumstances. Assume the center sketch is a full frame sensor and the bottom sketch is a crop sensorNote in the following that the lens element on the lower sketch hasn't moved, the image is onto a smaller portion of the sensor much as it would be onto a crop sensor.

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

Clear as mud, eh?.

Telecorder (Dave)My Image Galleries.

Http://www.nikonians-images.com/...hp?cat=500&ppuser=121399&password=.

Http://Telecorder.smugmug.com/..

Comment #21

As the article says, a 300 lens is not a 420 lens, it won't magnify like real 420 lens. The reason I set up my illustration as I did was to deliberately introduce a second element into the picture, something between one part of the picture and the lens; that something was the wolf..

If you crop a 50mm picture of the moon, you can pull it in to make it look as big as the moon with a 500 lens, BUT, you can't do that with the wolf in between the lens and the moon. The 500 has "flattened" the perspective and pulled the moon in to be quite large against the silhouette of the wolf. You just can't do that with my mythical 10X crop camera using a 50mm lens..

But for those that want to experiment; grab your camera and probably your tripod and head to your local cemetery on a full moon night. Hopefully there will be a taller tombstone which will work just fine. Now with your camera fairly low to the ground and lets say at least 40 feet from the tombstone and a long lens, preferably using your 100 - 400 with a TC 1.4 and shot the top one quarter of the tombstone against the full moon..

Now take another shot with a significantly less focal length, lets say roughly at 100mm and shoot the same shot. Now download the images into your computer. Now with photoshop, elements, picasa, etc try cropping your 100mm shot to duplicatethe image on your 400mm shot using a 1.4 TC and see if you can do it. Let us know how you did...

Comment #22

Below is a quote from 'The Photographer's Handbook 3rd Edition' by John Hedgecoe. Published 1992.

Page 105Section Title: "Magnification with Lenses".

"Image magnification varies with focal length. You might argue that everything could be shot with a wide-angle lens and the required picture area enlarged from the negative through the enlarger. To show just how different the result would be, the picture, below right, is from a whole negative taken with a 500 mm lens. The one, below left, is enlarged from the center of the 40 mm lens negative. Immediately you can see that the depth of field is much shallower with the long focus lens. The degree of enlargement given to the 40 mm negative has also substantially increased it's graininess, destroying fine detail.



Note: Here's the applicable part.

... even though perspective and scale remain exactly the same in each of the pictures.".

Good Day,Roonal.

'Money doesn't buy happiness, but it makes for an extravagant depression' by golf tournament sportscaster..

Comment #23

Here are my example images taken at a variety of focal lengths. Unfortunately I don't have either a wolf or a moon in my pictures..

Here is a shot taken with an 18mm lens:.

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

And from the same spot, a shot with a 300mm lens:.

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

My intention here was to capture the "No boating" signs, on opposite sides of a small lake.See the next post for cropped versions...

Comment #24

Sherwoodpete wrote:.

My intention here was to capture the "No Boating" signs, on oppositesides of a small lake..

Here are cropped versions at a variety of focal lengths:18mm:.

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

55mm:.

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

70mm:.

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

300mm:.

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

As can be seen, the difference in these shots is in the depth of field, which is shallow with the longer lens. But the images are otherwise virtually identical.There is a slight variation as these were hand-held, no tripod.Larger versions in the gallery here:http://www.pbase.com/peter_k/lens_perspectiveRegards,Peter..

Comment #25

You haven't duplicated what I am talking about. If you just "shoot the moon", you can pull the moon in and make it look like the identical size to the moon shot with a longer tele lens..

But I added a subject between the moon and the lens - the wolf. It is this relationship of sensor receiving the image, distance to the wolf, distance to the moon as flatten and magnified by the long tele lens..

Your picture is just of the "moon" for lack of a better word. You need to add the wolf...

Comment #26

Rsn48 wrote:.

You haven't duplicated what I am talking about. If you just "shootthe moon", you can pull the moon in and make it look like theidentical size to the moon shot with a longer tele lens..

But I added a subject between the moon and the lens - the wolf. Itis this relationship of sensor receiving the image, distance to thewolf, distance to the moon as flatten and magnified by the long telelens..

Your picture is just of the "moon" for lack of a better word. Youneed to add the wolf..

My "wolf" is the sign on the nearest edge of the lake. My "moon" is the sign on the opposite shore. There is quite some distance between the two..

Of course I would be extremely interested in any example pictures you or anyone else might have.Regards,Peter..

Comment #27

Sherwoodpete is absolutely correct. The only way to change the relationship between a near object and a far object is to change the camera position. If the camera position relative to the two objects does not change, the relative size of the two objects will not, and cannot change. This is true regardless of the combaintion of lens focal length, film size, sensor size, aperture, you name it..

The quote that Roonal gives below provides good documentation for this. But it is also simple common sense, when you think about it a bit: The camera can only record an image of the light rays that enter it the camera cannot reach out and change the path the light takes to get to it. At a given distance, both the moon and the wolf will subtend a certain arc of your (or the camera's) vision. Unless you move, or the wolf moves, or the moon moves, that arc will always be the same, and it won't change regardless of what gear you are able to fish out of your camera bag! The camera will record the relative size of the moon and wolf based on the actual arcs that each represents; that's all it can do..

Now, other points have been correctly made that there may be differences in image quality based on the combination of lens, sensor size, sesnor resolution, vulnerablity to noise, etc. But the size relationships will not change unless the relative position of the moon, wolf and camera changes..

Dave.

Http://www.pbase.com/dsjtecserv..

Comment #28

Rsn48 wrote:.

You haven't duplicated what I am talking about. If you just "shootthe moon", you can pull the moon in and make it look like theidentical size to the moon shot with a longer tele lens..

But I added a subject between the moon and the lens - the wolf. Itis this relationship of sensor receiving the image, distance to thewolf, distance to the moon as flatten and magnified by the long telelens..

Your picture is just of the "moon" for lack of a better word. Youneed to add the wolf..

Perspective is dependent on subject distance, not focal length. if you shoot the moon and wolf from the same position with a 500mm lens on a FF or a 50mm lens cropped (either in camera or after) to 500mm, you will get the same composition (image quality is another subject)..

So when you "pull the moon in and make it look like the identical size to the moon shot with a longer tele lens" the wolf will be in the same relative position in both the cropped 50mm and the 500mm..

Here is a link from this site explaining perspective:http://www.dpreview.com/learn/?/Glossary/Optical/Perspective_01.htm.

As for your statement about the perspective being flattened with a telephoto lens, this is actually due to subject distance, not focal length. I've talked about this in a previous post, here it is posted again if you want the reason:.

Http://forums.dpreview.com/...forums/read.asp?forum=1029&message=23900401.

Yep. Longer lens make you shoot from farther away, and the compression is caused by the background distance and subject distance being relatively similar with longer lenses. For example, if you were to take a shot of a person standing 5 ft away and 5ft in front of a chair (chair is 10ft from camera) with the 50mm lens, you would need to shoot from 13.5 ft away with the 135mm lens to frame the person the same. This will cause you to be 18.5 ft from the chair. The chair is now only 1.37x further than the person with the 135mm lens vs 2x further with the 50mm lens...

Comment #29

Barrie Davis wrote:.

Greg Nut wrote:.

On the other hand, you'll only be using the center of the image, so it'sprobably a wash..

Yes..

The idea that a lens is somehow "improved" in image quality, bycropping off the softer periphery of the image, and using only thesharper centre, is quite wrong..

The remaining image is smaller in area, will need more enlargement tofill a screen or print, and that enlargement will reduce quality bysimple magnification..

In the end the effects pretty much cancel out, and the quality of thecropped lens remains very close to what it was.Regards,Baz.

Hi Baz, vignetting comes into IQ of course. Wider apertures are more likely to allow the sensor to 'see' parts of the lens body than smaller apertures are. Likewise, less cropped sensors are more likely to 'see' parts of the lens body than more cropped sensors are. This mechanical vignetting is a large contributor to overall vignetting, which degrades IQ towards the edges of the image..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #30

Myth: Higher crop factor camera (smaller image sensor) means greater magnification for the same focal length lens..

Reason: Magnification has a specific meaning in photography. It means the size (eg in mm) of the subject projected onto the image sensor plane (or film plane) compared to the actual size of the subject itself (eg in mm). The size of the sensor is irrelevant as it doesn't affect the size of the projection. 1:1 magnification (true macro) means that an object that is x mm wide will be projected at a size of the same x mm at the sensor plane. The crop factor of the camera will then determine how much of the sensor/frame is filled by that subject..

In practical terms: All that aside, using a lens of the same FL at the same distance from the subject with a more cropped camera will result in the subject filling more of the captured frame. It's common to state what FL lens would be needed on a FF camera to provide the same framing from the same shooting distance as the lens in question on the cropped body. The issue of magnification is only one of terminology..

Myth: f-stop is the same as aperture..

Reason: The aperture is the size (diameter) of the iris/opening between the aperture blades that lets light through the lens. f-stop or f/ratio is a ratio between the focal length of the lens and the size of the aperture..

In practical terms: f-stop is easy to set and know, whereas aperture has to be calculated, so aperture isn't generally used. However, image sensor pixel density aside, depth of field is largely determined by focus distance and aperture (not f-stop). Larger focus distance mens larger DOF. Larger aperture means smaller DOF. For a given FL, increasing the f-stop reduces the aperture size and increases DOF. Likewise, for a given f-stop, increasing FL increases the size of the aperture and decreases DOF.



Myth: When shooting from the same position, changing the lens FL or changing the camera crop factor change the image perspective..

Reason: Perspective is the relationship between the elements of the image, not how the image is framed. When shooting from the same spot, the elements of the image are exactly the same with respect to each other as each path from the lens to any part of the scene can't change..

In practical terms: A longer lens from the same position won't foreshorten the perspective when compared with using a wider angle lens on a cropped body from the same position. If the f-stop isn't changed, DOF will change, with the longer lens resulting in a shallower DOF. However, the relative sizes of elements in the scene won't change, even if the framing of the scene changes from this same position because the difference in FL doesn't match the change in crop size. Paths of light from the parts of the scene to the camera can't alter when shooting from the same position and change perspective..

Myth: More cropped cameras with the same subject framing don't provide greater DOF for the same f-stop..

Reason: To keep the subject framing the same while changing the camera crop size, you either change the FL while keeping the camera in the same place, or move the camera while keeping the lens FL the same. Keeping the camera in the same place not only retains subject framing, but also retains the perspective in the scene. The shorter FL you need on the more cropped camera for the same framing as the longer lens on the les cropped camera means that for a constant f-stop, the longer lens has a larger aperture, meaning a shallower DOF on the less cropped body. If the same FL lens is used and subject framing is retained by backing up with the more cropped body (which also changes the perspective), the increased focus distance means a greater DOF for the more cropped body..

In practical terms: It's important to specify f-stop and framing (and to some extent perspective) conditions when stating whether DOF is different for different crop cameras..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #31

Bugzie wrote:.

I think you're right, but a number of folks here have dissented fromyour opinion and i'm uncertain as to who's right. can we clarifywho's right here?.

There's confusion as people assume way too much without thinking logically about what really happens, then they perpetuate incorrect information. Soem have got it right here and some have got it wrong..

Let's take the example of the wolf with moon behind taken with a FF camera and 500mm lens vs 10x crop camera with 50mm lens. If you shoot from exactly the same spot, the perspective has to be the same, ie the elements of the image will be exactly the same with respect to each other. Furthermore, as the crop factor (10x) exactly complements the FL change (1/10), the framing of the scene will also be the same. The moon will appear to be the same size with repsect to the wolf in both cases as the paths of light from the moon and wolf have to travel exactly the same to the camera that's at the exact same location. Simple laws of physics. The lens doesn't make that light bend on it's way to the camera, so the front of the lens sees the same two dimensional scene in both cases.



If the same f-stop is used with both lenses (500mm on FF and 50mm on 10x crop), the depth of field will be very different. Remember, we're shooting from the same location, so distance doesn't change to make any different to DOF. However, the longer lens means the aperture opening in the lens is much bigger than it is for the shorter lens using the same f-stop. As the size of the aperture opening is the other main determinant of DOF (along with focus distance, with some effect from image sensor pixel density that I'll ignore here), the longer lens in this scenario will mean a much shallower DOF than with the shorter lens. None of that changes the image perspective as some have suggested..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #32

John down under wrote:.

There's confusion as people assume way too much without thinkinglogically about what really happens, then they perpetuate incorrectinformation. Soem have got it right here and some have got it wrong..

One of the problems is placing too much value on logical thinking. I'm trained to think logically - I develop computer software. But there is a well-known expression Garbage In - Garbage Out. So if the assumptions are wrong, all the logic in the world leads nowhere useful..

The best answer to many issues is to suspend thinking for a while, go out and performs some actual tests. That way you can be at least be sure that any subsequent thinking is based on actual facts.Regards,Peter..

Comment #33

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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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