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Focal length definition...
Ok, can someone please give me a simple definition for focal length? I am still very green when it come to photography and I am reading a lot about exposure settings lately and it seems that the rule is that the exposure time should be at least 1/2 x focal length of the lens. Can someone explain this to me in layman's terms? Thanks for any and all help in advance...

Comments (39)

I'll answer first what you want to know and then what you asked .

Shutterman wrote:.

Ok, can someone please give me a simple definition for focal length?.

Not important for the question. It's a number printed on the lens..

I am still very green when it come to photography and I am reading alot about exposure settings lately and it seems that the rule is thatthe exposure time should be at least 1/2 x focal length of the lens..

Either you got it wrong, or they gave the advice wrong..

Can someone explain this to me in layman's terms? Thanks for any andall help in advance..

Well .

If you ever looked through binoculars or a scope, you noticed that the image "moves" because your hands shake. The exact same phenomenon happens with the camera, and the blur resulted is named "due to camera shake"..

In order to prevent camera shake, a rule of the thumb was devised for 35mm photography (that is the common cameras and slrs):Don't shoot slower than the reciprocal of the focal length..

So, if you are shooting handheld a 50mm lens, don't go slower than 1/50 of a second. 1/60 is ok, 1/40 is not. Again, this is an empirical rule..

Now comes the concept of 35mm equivalent. Since the digital sensor is usually smaller than the film surface, the lens "feel" longer. You get that equivalent focal lenght by multiplying the crop factor with the lens focal length..

In other words, on a Canon 40D, a 50mm lens equivalates a 50x1.6= 80mm. So with the lens on that camera, don't shoot handheld slower than 1/80..

Of course, the rule implies you are trying to keep the camera steady  No shooting from the hip..

Now, the definition you requested. The focal lenght of a lens is the distance from the center of the lens to the focal point. See, it does not help.http://en.wikipedia.org/wiki/Focal_length.

Just look what's written on the lens and try not to shoot slower than 1/equivalent focal length. Better, test your hands. See how slow can you go..

/d/n..

Comment #1

What about for a telephoto lens? Are you saying that say on a 18-200mm lens that I would not want to shoot slower than 28.8 of a second or 1/30 handheld? So, I am assuming that the rule would apply to whatever focal distance you were at the time per photo on a telephoto lens. So, even on a 18-200mm lens if I am at 70mm then I do no want to shoot slower than 1/112 or 1/125 since you cannot get a 1/112 shutterspeed and 1/100 would be too slow. I think I am getting it now. I just have one question. How do you know what mm focal point you are at on a telephoto or zoom lens? For example, the lens is labled 18, 35, 50, 70, 100, 135, and 200 but what about everywhere in between. Let's say you are somewhere between 50-70mm.

Does that make sense? I just want to make sure that I am understanding this correctly. Thanks for the help...

Comment #2

Joke, right?.

Or just a newcomer who thinks that 1/112 actually means something?.

BAK..

Comment #3

Things get more complicated than the unknowledgable can help you with..

Focal length is the "length" of the lens, calculated using complicatged formulas that take into account modern lens design technology. Short focal lengths are wide andl lenses and long focal lengths are telephoto lenses, but the actual numbers that short and long would define deopend on the camera and the size of film or sensor..

With a camera like any Nikon digital single lens reflex except the new D3, and any Canon with Rebel in the name or the 20D, 30D and 40D, anything around 10-20mm focal length is wide angle, and anything over 50 is telephoto, with the amout of telephotoness increasing as the numbrs climb..

The shuttr soeed fraction rule is, first of all, only a guideline, varies between people, and varies tremendously depending on the camera..

For a 35mm film camera or a so-called full frame digital SLR, like the Canon 1Ds or Nikon D3, set your shutter speed number to match the focal length, at a minimum..

So, 1/50 of a second for a 50mm lens..

1/30 for a 35mm lens..

1/125th for a 135mm lens..

For a camera with a smaller sensor, double the shutter speed compared to the focal length..

Digital Rebel or most Nikons, for an 18mm lens, set the camera at at least 1/50th of a second. For a 50mm lens, set the shutter at, at least, 1/100..

Remember, these are minimums, require you to be really still, etc. And othger kinds of cameras have different formulas..

As for physical length compared to focal length - some designs, especially for wide anglesw, create lenses that are longer in actual physical millimeters than the focal length..

Lots of telephoto designs are shorter in real millimeters than in "optical" focal length..

And lots of zoom lenses change focal legth without getting shorter or longer...

Comment #4

There are three things to consider when choosing your minimum focal length..

Firstly, camera shake..

That's where this rule of thumb originates. If you have a 35mm sensor or film or very close to that size, then the reciprocal of the focal length is a guide to the slowest shutter speed you can use without significant blur from camera shake. For a 17mm focal setting, shoot no slower than 1/15 or 1/20 of a second to avoid camera shake. For a 200mm setting, shoot no slower than 1/200 second to avoid blur from camera shake..

If you're using a smaller digital sensor, such as the 18mm DX format, then I would add 50% to the time for the shutter is open. So I would make it 1/320 second for a 200mm lens, because I know it's really 1.6 times the focal length and that's the nearest setting..

What if your camera is using image stabilisation? Did you hear it can let you shoot three or four times as slow? These can work by using gyroscopes, like Abrams tanks, which wiped out vastly greater numbers of Iraqi tanks during the first US Gulf War because they could keep their barrels steady while moving. So for camera shake, they really do allow three or four times more exposure time. Again, this is a rule of thumb and depends greatly on the circumstances..

Secondly, subject motion..

If people are moving normally (not rapidly), then 1/125 is a rule of thumb for capturing sharp images. When standing still, it's 1/60. This is for ambient light such as daylight, because if you are shooting in the dark and all your exposure will come from a flash, which lasts for maybe as little as 1/10,000 of a second, then it really isn't going to matter how long your shutter is open for..

I shot a wedding recently with a camera using an 18mm sensor and with a 70-200 lens on it. I set my camera to f2.8 for the entire ceremony. I wasn't using flash so I had the camera set to auto ISO, with a minimum shutter speed of 1/125 and a default of ISO 100, and a maximum of ISO 1600. This meant the camera would change the ISO setting as needed for exposure, while not letting the shutter speed fall below 1/125 of a second. This was in Aperture Priority Mode, and I had set Manual Mode to be at 1/70 second or something like that, because 1/60 second is the rule of thumb for freezing people who are standing still and I could do faster with very low ISO. This meant I could use A mode for the processional and recessional and M mode for the rest, and auto ISO would take care of exposure..

Thirdly, proximity. Something closer to the camera is going to more through more of the viewfinder than something far away. That's why these rules are only a guideline. It also depends on the weight and centre of gravity of the camera, your own energy and hold, and whatever other obvious factors you care to consider...

Comment #5

You got the calculation right. However, keep in mind this is just a rule of a thumb, a guideline, not a law..

It's a good ideea to test your hands every now and then and see how steady you can shoot. Except that, most people choose to be on the safe side and shoot faster than the rule says. Same with the actual focal length - guess on the safe side .

/d/n..

Comment #6

Interesting. Thanks for the help guys. I did not realize photography took into account so many factors and calculations. It is going to take a long time to get used to remembering and using all of this information. For now, taking pictures for me takes quite a bit of time because I am learning all of this. I have to stop myself and take into account things like we are discussing in this thread and then change settings quite frequently whereas others will take the same photo and make it look easy.

Thanks for the help...

Comment #7

Hi,.

You mentioned an 18-200 mm lens a while ago. If that's 18 to 200 mm in 35 mm film terms then the old rule says at least 1/18th second at the wide end and at least 1/200th of a second at the tele end. Simple as that..

Only it isn't because you may not have a shutter that uses those speeds so go a bit faster to(say) 1/25th second and 1/250th second. Even they won't work if you've been at the whisky recently or worse . In practice you have to find out what you can hold still and learn the technique which is to breath in and out slowly and squeeze the shutter at the pause between going from in to out..

In practice, you need all the help you can get at the 200 mm end and so a tripod or some support, even jamming it against a tree, wall or a chair (or anything) helps. there's no cast iron and guaranteed rule. Especially if you've IS and think it will cover everything, because, in my experience it doesn't and makes people lazy..

BTW, the focal length is roughly the point where a simple lens in the air focuses parallel rays to a point. Borrow reading glasses from someone, focus the sun on a piece of paper and the lens to paper distance is roughly the focal length as the suns rays are pretty well parallel. Zooms mess simplicity up as they are lenses designed to give the appearance of a range of focal lengths but we won't go into that here..

If you want to find out how well or badly you hold the lens, then get something like EXIF Image Viewer (free from http://home.pacbell.net/michal_k/)and use it to sort your pictures by shutter speed, then view them in that order and you'll soon see where the image starts getting blurred. You can improve on it by the breathing in and out and pause technique, panning for sports like motor racing and generally jamming the camera against something if you don't use a tripod..

If you've a dSLR with a moving mirror then mirror slap will further confuse the issue but looking at the pictures with EXIF Image Viewer should tell you all you need to know. Most people when younger than this old git can manage one second with the standard lens but not when there's a mirror bouncing about in the camera. Even so or second should be easy for most people, once they've sorted out the technique..

Other points to worry you are that the shutter speed being high means a wide aperture and that means shallow DoF or else a higher ISO setting and that means noise..

Hope this helps, David..

Comment #8

David Hughes wrote:.

If that's 18 to 200 mm in 35 mm film terms.

18-200mm is 18-200mm no matter how big the recording device behind it...

Comment #9

Seriously, don't look so far into it. It is just a "rule of thumb". My wife has super steady hands and has hand-held a 1/2 sec exposure time shot at around 40mm equivalent and it turned out just fine. Me on the other hand, I find it hard to keep my shot steady at slower shutter speeds unless I purposefully concentrate on being still... Holding my breath at the time of shooting, tucking my arms in to the side of my body, finding something to lean against, etc...

Comment #10

Martin Caie wrote:.

18-200mm is 18-200mm no matter how big the recording device behind it..

Hi,.

Lots of people quote straight focal lengths and others quote them in 35 mm film terms and you've no way of knowing which at times: look at so-called crop lenses. It gets confusing at times and so I ask or say what it is..

Regards, David..

Comment #11

I was browsing the 350d/400d galleries, the "focal equivalent lengths' were noted in the shooting information for each image. I found myself wanting to convert that to what the actual focal length would be on my 400d, to make it easier to make notes for future shooting opportunities. So I did a quick chart to use while I browse the galleries, thought I would share it..

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

Comment #12

Cool. Thanks for the chart. That saved me some work...

Comment #13

These charts and conversions are nonsense. You cannot create an image that looks like it was taken at 200mm by setting your lens to 130mm and cropping it..

DX lenses do not have different measurements to others, because the measurements are exactly the same..

There is no such thing as "equivalent focal length". Or "equivalent aperture". Or "equivalent shutter speed"...

Comment #14

Martin Caie wrote:.

There is no such thing as "equivalent focal length". Or "equivalentaperture". Or "equivalent shutter speed"..

There is "equivalent field of view" .

You are technically right, of course. But there is a difference between technically and practically. Tecnically, there is no miles per hour for speed. Neither kph, to be correct. But meters per second is not a very practical way of measuring car speed .

/d/n..

Comment #15

Martin Caie wrote:.

These charts and conversions are nonsense. You cannot create an imagethat looks like it was taken at 200mm by setting your lens to 130mmand cropping it..

The geometry (or perspective or whatever term you prefer) of a cropped image is *exactly* the same as the geometry of an uncropped image taken with a correspondingly longer lens, of the same subject from the same viewpoint. The depth of field can be made the same, if wished, by adjusting the aperture - assuming your lens is capable of the necessary aperture of course..

If you also need to maintain the same shutter speed then the ISO speed must be altered to get a correct exposure, and the deeper you look into this the more issues of image quality enter into it. But while that is a fertile topic for debate on a technical forum, it goes way beyond what is normally understood by 'equivalent'. The key point is that the crop factor calculation does work and does result in 'equivalent' images...

Comment #16

Martin Caie wrote:.

These charts and conversions are nonsense. You cannot create an imagethat looks like it was taken at 200mm by setting your lens to 130mmand cropping it..

DX lenses do not have different measurements to others, because themeasurements are exactly the same..

There is no such thing as "equivalent focal length". Or "equivalentaperture". Or "equivalent shutter speed"..

Being new to dslr, lots of things confuse me. If what you say is correct, then what do the staff here at dpreview mean when they provide shooting info for an image as follows (from 1st image in 400d gallery):.

"Canon EOS 400D Review Samples (1 of 46), 88 mm equiv, ISO 100, 1/160 sec, F6.3, +0.0 EV, EF-S 18-55 mm".

For those of us trying to learn, doesn't this info mean that the 18-55 lens was at maximum zoom (55mm x 1.6) for the shot that was taken?.

I don't dispute your statement, but this is something I would like to have clear in my head as I continue to learn and base practice shots on the images in the galleries here. Thanks..

Beepclick...

Comment #17

Beepclick wrote:.

I don't dispute your statement,.

You may not have the confidence to do that yet - but I do. He's talking complete nonsense (and not for the first time, I might add)...

Comment #18

Steve Balcombe wrote:.

Beepclick wrote:.

I don't dispute your statement,.

You may not have the confidence to do that yet - but I do. He'stalking complete nonsense (and not for the first time, I might add)..

Martin wrote:.

"These charts and conversions are nonsense. You cannot create an image that looks like it was taken at 200mm by setting your lens to 130mm and cropping it.".

"DX lenses do not have different measurements to others, because the measurements are exactly the same.".

"There is no such thing as "equivalent focal length". Or "equivalent aperture". Or "equivalent shutter speed.'"***********************.

What he means by this is that to resolve detail in an image actually requires a lens capable of doing so. Meaning that no matter how much you crop a 50mm lens, it will still not be able to resolve the detail that a 200mm lens will be able to..

A 200mm lens, on a dSLR is never a 300mm lens, no matter what the crop. 200mm is 200mm, no matter what the camera..

To make matters More confusing, the little digicams, with their "focal lenght equivalents" are actually miniature lenses that DO resolve more detail. In other words a digicam that boast 400mm at it's top zoom, does indeed do that because it's using a miniature telephoto lens. In other words, it is equivalent in resolving power....

Dave..

Comment #19

Chato wrote:.

What he means by this is that to resolve detail in an image actuallyrequires a lens capable of doing so. Meaning that no matter how muchyou crop a 50mm lens, it will still not be able to resolve the detailthat a 200mm lens will be able to..

He did *not* say that..

In addition, it was I who said in my other post that if you want to go beyond the image geometry/perspective to consider image quality, then the two sensor sizes do introduce a difference. It is not limited to resolving power, it also affects noise, and the onset of diffraction, and more..

A 200mm lens, on a dSLR is never a 300mm lens, no matter what thecrop. 200mm is 200mm, no matter what the camera..

And I did *not* say that the lens changes it's focal length. Of course it doesn't..

You have apparently read neither Martin Caie's post nor mine...

Comment #20

Steve Balcombe wrote:.

Chato wrote:.

What he means by this is that to resolve detail in an image actuallyrequires a lens capable of doing so. Meaning that no matter how muchyou crop a 50mm lens, it will still not be able to resolve the detailthat a 200mm lens will be able to..

He did *not* say that..

In addition, it was I who said in my other post that if you want togo beyond the image geometry/perspective to consider image quality,then the two sensor sizes do introduce a difference. It is notlimited to resolving power, it also affects noise, and the onset ofdiffraction, and more..

A 200mm lens, on a dSLR is never a 300mm lens, no matter what thecrop. 200mm is 200mm, no matter what the camera..

And I did *not* say that the lens changes it's focal length. Of courseit doesn't..

You have apparently read neither Martin Caie's post nor mine..

But THIS discussion is based on the OP, who is asking questions. It does not go back in history to various disputes that you have with others..

In that sense the OP is asking, "what is it with this focal lenght business,?" to unfairly paraphrase him. If I mischaracterised your dispute - My apologies. On the other hand, I've also tried to answer the OP's question in a way that corrresponds to reality..

Dave..

Comment #21

Chato wrote:.

But THIS discussion is based on the OP, who is asking questions. Itdoes not go back in history to various disputes that you have withothers..

I agree, and everything I said, except for one passing comment, was in direct response to the discussion..

In that sense the OP is asking, "what is it with this focal lenghtbusiness,?" to unfairly paraphrase him..

Well, yes, but specifically in relation to it's impact on minimum shutter speed. And that in turn has led, quite properly, to an explanation of equivalent focal length. I can't imagine that Martin Caie's post - the one I responded to - has helped the OP's understanding very much..

What is rather odd is that his first post in this forum is useful, detailed, and correct in every respect as far as I am qualified to judge - and mentions the need to compensate for the crop factor!..

Comment #22

Steve Balcombe wrote:.

Chato wrote:.

But THIS discussion is based on the OP, who is asking questions. Itdoes not go back in history to various disputes that you have withothers..

I agree, and everything I said, except for one passing comment, wasin direct response to the discussion..

I wasn't posting to attack you. And Rereading this thread, I can see why you're annoyed. My apologies..

In that sense the OP is asking, "what is it with this focal lenghtbusiness,?" to unfairly paraphrase him..

Well, yes, but specifically in relation to it's impact on minimumshutter speed. And that in turn has led, quite properly, to anexplanation of equivalent focal length. I can't imagine that MartinCaie's post - the one I responded to - has helped the OP'sunderstanding very much..

Probably not....

What is rather odd is that his first post in this forum is useful,detailed, and correct in every respect as far as I am qualified tojudge - and mentions the need to compensate for the crop factor!.

Personally, I'm not a fan of the "rule of thumb," but kept my mouth shut anyway. It's being talked about as if it's a one size fits all option. Whereas the best thing thee OP can do is go out and spend a few hours shooting at various speeds and let him find out what His rule of thumb is. Even that isn't necessarily definitive. I bought the Tamron 28-105, 2.8 lens and it took a month of constant shooting before I "grew" into it. And I'll neever grow into Sigmas 50-500.



Dave..

Comment #23

Chato wrote:.

I wasn't posting to attack you. And Rereading this thread, I can seewhy you're annoyed. My apologies..

Cool..

Personally, I'm not a fan of the "rule of thumb," but kept my mouthshut anyway. It's being talked about as if it's a one size fits alloption..

It's better than nothing, and I think it is very helpful for people to understand that using (say) a 200 mm lens on a crop camera is significantly not the same as using a 200 mm lens on a 35 mm camera. Multiplying by the crop factor is at worst equally bad, and hopefully equally good as the original rule of thumb..

I can definitely hand-hold my 40D better than my older and more familiar 400D, due to the weight I believe and perhaps the size as well. That's the same sensor size and the same pair of hands producing different results. There will be even more variation between different people, and different shooting circumstances. But that's why it's only a rule of thumb. It's like one handful of lawn seed per square yard - never mind that handfuls vary, and so do lawns, and so do estimated square yards. It doesn't matter, it's still good advice..

Oddly enough, the crop factor adjustment is possibly the most accurate part of this. However bad I am with a 200 mm lens on a film camera, I will be almost exactly 1.6x worse if I put the same lens on the 40D!..

Comment #24

Steve Balcombe wrote:.

Martin Caie wrote:.

These charts and conversions are nonsense. You cannot create an imagethat looks like it was taken at 200mm by setting your lens to 130mmand cropping it..

The geometry (or perspective or whatever term you prefer) of acropped image is *exactly* the same as the geometry of an uncroppedimage taken with a correspondingly longer lens, of the same subjectfrom the same viewpoint..

Yes, but you're not actually going to have the same perspective because you will adjust your position according to how things look in the viewfinder...

Comment #25

Martin Caie wrote:.

Steve Balcombe wrote:.

Martin Caie wrote:.

These charts and conversions are nonsense. You cannot create an imagethat looks like it was taken at 200mm by setting your lens to 130mmand cropping it..

The geometry (or perspective or whatever term you prefer) of acropped image is *exactly* the same as the geometry of an uncroppedimage taken with a correspondingly longer lens, of the same subjectfrom the same viewpoint..

Yes, but you're not actually going to have the same perspectivebecause you will adjust your position according to how things look inthe viewfinder..

Do people do that? Adjust position I mean. Using a fixed focal length then yes..

But for many people when using a zoom lens, the whole point is they can achieve the framing they want by adjusting the focal length. And all without leaving their seat.Regards,Peter..

Comment #26

Yes, certainly. Your position affects depth of field (and perspective), as does your focal length...

Comment #27

Martin Caie wrote:.

Steve Balcombe wrote:.

Martin Caie wrote:.

These charts and conversions are nonsense. You cannot create an imagethat looks like it was taken at 200mm by setting your lens to 130mmand cropping it..

The geometry (or perspective or whatever term you prefer) of acropped image is *exactly* the same as the geometry of an uncroppedimage taken with a correspondingly longer lens, of the same subjectfrom the same viewpoint..

Yes, but you're not actually going to have the same perspectivebecause you will adjust your position according to how things look inthe viewfinder..

No, that's not what 'equivalent focal length' means. Let me restate it:.

A 200 mm lens used on a 1.6x crop factor camera has the same field of view as a 320 mm lens used on a "full frame" camera. We say that the 200 mm lens has an "equivalent focal length" (or "focal length equivalent" if you prefer) of 320 mm on the smaller sensor camera..

That is the comparison which is being made - using "equivalent" equipment to take the *same* photograph..

(Edit: Note that the perspective also stays the same because the viewpoint (subject distance) remains the same. It is not a property of the equipment as such. The field of view is a property of the equipment; the perspective is a property of the viewpoint.).

There are many other valid and useful comparisons which can be made between the same/different lenses on the same/different sensor sizes, but the above is the only one which explains the meaning of "focal length equivalent"...

Comment #28

Steve Balcombe wrote:.

A 200 mm lens used on a 1.6x crop factor camera has the same field ofview as a 320 mm lens used on a "full frame" camera..

Not when you're trying to create the same photo..

"1.6x crop factor" is another misnomer...

Comment #29

Martin Caie wrote:.

Steve Balcombe wrote:.

A 200 mm lens used on a 1.6x crop factor camera has the same field ofview as a 320 mm lens used on a "full frame" camera..

Not when you're trying to create the same photo..

Yes it does, you are completely wrong on this. Please don't tell me I have to go out and take sample shots to prove it, I've got more pressing things to do..

"1.6x crop factor" is another misnomer..

Well, yes it is, but it is the accepted colloquial term and I don't mind using it..

To me it falls into the same category as "100% crop" - nonsensical term but it's an uphill battle to get people to say "1:1" or "actual pixels". I've more or less given up, apart from the occasional rant...

Comment #30

Martin Caie wrote:.

Yes, certainly. Your position affects depth of field (andperspective), as does your focal length..

Correct. But let's say I have a full-frame camera with a 300mm lens, and an APS-C camera with a 200mm lens. Without changing position, I can take a shot which has the same framing and same perspective with each camera.The depth of field may be different, depending on the aperture..

But I would see no reason to get up and walk backwards or forwards with either camera, since they give me identical framing of the subject.Regards,Peter..

Comment #31

The only way to get the same shot with each camera is to have the same lens and crop the image from the camera with the larger sensor...

Comment #32

Shutterman wrote:.

Interesting. Thanks for the help guys. I did not realize photographytook into account so many factors and calculations. It is going totake a long time to get used to remembering and using all of thisinformation. For now, taking pictures for me takes quite a bit oftime because I am learning all of this. I have to stop myself andtake into account things like we are discussing in this thread andthen change settings quite frequently whereas others will take thesame photo and make it look easy.

Thanks for the help..

Amidst all the discussion and debate, always remember to have fun and shoot with good composition! - Q: Is photography 90% math? and 10% art? A: Always an opinion - and I'd like to to say I enjoy the art, and for me that ratio is nowhere correct.

"A photographer is as good as where he or she can be - and at what time they can be there" - Myself.

~ The Real dj Tru ~.

'Image quality is not the product of the machine, but of the person who directs the machine.' - Ansel A...

Comment #33

Martin Caie wrote:.

The only way to get the same shot with each camera is to have thesame lens and crop the image from the camera with the larger sensor..

I don't understand why you keep saying this, you are completely wrong and it is easy to prove. Just point the camera at any convenient subject and take two shots at different focal lengths. Take them into Photoshop and resize the longer focal length image so you can overlay it on the wider image. You will find that they are, in geometry, *identical*..

If you wish, you can take it one stage further by increasing the aperture (and, optionally, ISO speed) when you take the shorter focal length shot, so that the depth of field also matches. The image quality, of course, will never match, but at modest print sizes that may not be apparent..

Zooming does not change the perspective, and in that sense it is the same as cropping. This is basic beginner stuff Martin...

Comment #34

Steve Balcombe wrote:.

Martin Caie wrote:.

The only way to get the same shot with each camera is to have thesame lens and crop the image from the camera with the larger sensor..

I don't understand why you keep saying this, you are completely wrongand it is easy to prove. Just point the camera at any convenientsubject and take two shots at different focal lengths. Take them intoPhotoshop and resize the longer focal length image so you can overlayit on the wider image. You will find that they are, in geometry,*identical*..

If you wish, you can take it one stage further by increasing theaperture (and, optionally, ISO speed) when you take the shorter focallength shot, so that the depth of field also matches. The imagequality, of course, will never match, but at modest print sizes thatmay not be apparent..

Zooming does not change the perspective, and in that sense it is thesame as cropping. This is basic beginner stuff Martin..

Steve you are right. Martin you are wrong. It is amazing how this focal length/crop factor/FOV/perspective thing comes up again and again, with the same old misconceptions flying around.tim..

Comment #35

Martin Caie wrote:.

These charts and conversions are nonsense. You cannot create an imagethat looks like it was taken at 200mm by setting your lens to 130mmand cropping it..

DX lenses do not have different measurements to others, because themeasurements are exactly the same..

There is no such thing as "equivalent focal length". Or "equivalentaperture". Or "equivalent shutter speed"..

Please get a clue. How can cropping NOT give you a different field of view?.

The field of view (what you see in the viewfinder) at 200 mm focal length with 35 mm film or full frame digital is what it is. Put that lens on a cropped sensor camera and what you see in the viewfinder will be 200mm x crop factor. So with a crop factor of 1.5 using the same lens will give you the same field of view as a 300mm focal length lens on 35 mm..

I have lenses from my film SLR that I can use on my 4/3 camera. I get a 50 mm field of view with a 50 mm lens on the film SLR. I put the same lens on my 4/3 camera and I get a field of view that would require the use of a 100 mm lens on the film camera. You are wrong about the DX lenses. Put one on a 35 mm film or full frame digital camera and look through through the viewfinder. And then try to say that conversions are nonsense.



The focal length of a lens is always the same. People keep saying this like everyone else doesn't have the focal length stated on their lenses. I mean, duh, no kidding the focal length doesn't change when you put the lens on a different camera. But change the sensor size and you will change the field of view given by that lens. We use a crop factor to determine how the cropped sensor field of view will differ from using that same lense on full frame..

On my digicam, the lens actual focal length is 63 mm at the long end. To get the same field of view, the same image in my viewfinder, the same photo, I use a lens of 200 mm focal length on my 4/3 camera and a lens of 400 mm on my 35 mm camera..

You cannot create an imagethat looks like it was taken at 200mm by setting your lens to 130mmand cropping it..

This is just nonsensical. You can do it by cropping the image coming through the lens by a factor of 1.5..

Take a 200 mm photo with a 1.5 crop sensor camera and on your computer monitor, crop it by a factor of 1.5. You will then see what the photo would have looked like if you used a 300mm lense on a full frame camera. This is a fact you cannot disprove. 200mm x 1.5 =300mm. You want me to believe the answer is 200..

The chart is accuratte and just the tiniest amount of knowledge of optics and the math involved is enough to prove it..

Here's something for you to try using your eyes as the lens. Cut a square out of a piece of cardboard. Cut a larger square out of another piece of cardboard. These cutout areas will represent different sized sensors. Now, one at a time, put each piece of cardboard the same distance from your eyes and notice what you see in the cutout. Are they the same view? Of course not.

Yet do the same thing with a camera and you say it won't work?..

Comment #36

Martin Caie wrote:.

The only way to get the same shot with each camera is to have thesame lens and crop the image from the camera with the larger sensor..

It really depends what qualities you seek to preserve..

For example, cropping the image as you describe results in a loss of pixels and hence the images will not be the same.But if the aim is to preserve perspective and framing, either method will do..

Regards,Peter..

Comment #37

Sherwoodpete wrote:.

Cropping the image as you describe results in a loss ofpixels and hence the images will not be the same..

No, the pixels you crop are on a different part of the sensor. It has no bearing on the pixels on the inside. No pixels are lost from the part of the image that is being compared...

Comment #38

Zadam wrote:.

On the other hand, I find it hard to keep my shot steady at slowershutter speeds unless I purposefully concentrate on being still...Holding my breath at the time of shooting, tucking my arms in to theside of my body, finding something to lean against, etc..

Holding your breath might be one of the very worst things you can do if you want to keep still. Any marksman will tell you that it's much better to gently squeeze the trigger of a gun when you have exhaled, and before you inhale another breath. When you try to hold your breath, your abdominal muscles tremble ever so slightly, which is bound to introduce a little bit of movement which you are after all trying to eliminate..

So the best thing to do is:1. Compose your shot2. Breathe out3. Gently press the shutter release rather than stabbing at it4. Breathe in.

Amyaka Ms. Miyagi..

Comment #39

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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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