Perspective distortion — does it matter what format you shoot?

<p>While we can achieve the same 24mm full frame field of view using a 18 mm lens on an APS-C camera, 12 mm lens on m4/3, and 10 mm on a 1-in. sensor camera (Nikon 1 system), do we get less perspective distortion with a 24 mm lens on a full frame camera? In other words if you want to do environmental portraits with minimal perspective distortion, would you be better off shooting full frame with a 24 mm lens rather than a 12 mm lens on a m4/3 camera? Thank you for your inputs. </p>

<p>Perspective depends entirely on subject distance. The lens or the format don't make any difference. </p>

<p>Hecktor is correct. I shoot many different formats and have noticed the same thing. Format does have an effect on DoF though.</p>

<p>Kent in SD</p>

<p>It matters only in the trivial sense that a large-format view camera will have much more available to the user in camera/lens movement corrections than you can get with any smaller format shift-and-tilt lens. Without corrections, no real difference.</p>

<p>To qualify Kent's statement: format has an effect on DoF if you consider aperture in terms relative to the focal length used (i.e. f-stop); it has no effect if you keep the same absolute aperture size.<br />

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And no, format doesn't matter for perspective. To put it another way, there's no such thing as "perspective distortion". What we tend to think of in those terms is the result of viewing the image from a distance other than the distance between the lens nodal point an the sensor. If you shoot a full frame image using a 24mm lens, make a 36cm x 24cm print of the result (10x enlargement from full frame), then put your nose close to the image such that you're viewing it at 24cm (again, 10x enlargement, this time of focal length) from the centre, the result won't look distorted at all. I've got a 40"x30" print made of a shot taken with a 24mm lens and, from about 27" away, it looks pretty good and very immersive (except that you can see the pixels - this is one reason I now have a D800). From the other side of the room, it's distorted.<br />

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The same is true of all focal lengths, it's just that it's harder to notice that a shot with a 500mm lens (say) looks "flatter" than it should from a normal viewing distance - though there are those who say models shot with long lenses look artificially two-dimensional.</p>

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<p>"...it has no effect if you keep the same absolute aperture size."</p>

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<p>- Sorry Andrew, but that's not correct. Try entering a 35mm lens at f/2 into a depth-of-field calculator, and then a 70mm lens at f/4, both with the same subject distance. You'll find that you have to stop the 70mm lens down to f/8 to achieve the same DoF. This is because depth-of-field varies inversely with the square of the focal length, but directly in proportion to the F-number. In this case, doubling the focal length reduces the DoF by a factor of 4, so we have to multiply the F-number by 4 to keep DoF the same. Of course if you go changing the format and C-o-C as well, then the physical aperture size does stay the same for a similar DoF.</p>

<p>However it's completely true that perspective depends only on subject distance and not the lens. If we were to split hairs though, we'd find that it's near impossible to keep the exact same field of view and subject distance when we change lenses and formats. So there is a <em>teensy tiny</em> change of either perspective or framing with format, even if the aspect ratio between formats is exactly the same. This isn't normally noticeable unless the subject distance is very close, but at macro distances changing formats and focal lengths can result in quite different-looking pictures.</p>

<p>Anyhoo! It's probably best to think of changing lenses or formats as simply cropping a larger or smaller area of view out of the scene in front of you and your camera. That way it's easy to see that perspective (the relative sizes of objects with distance) will stay the same unless you move your point of view.</p>

<p>There are two good web articles on perspective <a href="http://www.patricktaylor.com/1988">here</a> and <a href="http://www.learnmyshot.com/Telephoto-Lens-Perspective-Compression-and-the-Angle-of-View">here</a>.</p>

<p>"To put it another way, there's no such thing as "perspective distortion"."</p>

<p>According to Wikipedia (if you trust such a source) it does! https://en.wikipedia.org/wiki/Perspective_distortion_%28photography%29</p>

<p>RJ: I'm specifically talking about the circumstances where sensor size is changing and the focal length is being adjusted to compensate, since the thread was about changing formats. You're quite right that if you take a photo with a different field of view (by changing the camera position or lens focal length on the same sensor) then the aperture relationship differs. I tend to consider these cases as secondary in depth of field discussions, because the depth of field difference is frankly less important than that you've taken an entirely different photograph. (The possibly exception to my dismissal of these situations is maintaining approximate depth of field by scaling subject-to-camera distance with focal length and maintaining relative aperture.)<br />

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And to clarify, physical aperture is, in theory, <i>exactly</i> the same for the same DoF (in case the "similar" gets picked up on) - it's the cone of confusion in front of the lens that's causing the depth of field, not what happens with the sensor. As for getting an exact match in the real world, it ought to be possible, with the proviso that the nodal point might change with focal distance. Of course, not many lenses have exactly their stated focal length, and there are manufacturing tolerances involved, so I appreciate that theory and practice aren't entirely aligned.<br />

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Simon: I simply object to calling it "distortion" as if it were a lens aberration. Perspective distortion is the result of looking at the image from the wrong place - it's your fault, not the lens's.</p>

<p>Hey guys, this has totally confused me again! Won't an 18mm lens look like an 18mm lens, regardless of format (except for cropping the edges on smaller format), and doesn't it have a different look (perspective) from a 24mm lens? So if you want the 24mm "look" with a wider field of view, the OP is correct in suggesting you will do better with a larger format? The image will look the same but cropped down, if the 24mm lens is used on the smaller format?</p>

<h2>"Won't an 18mm lens look like an 18mm lens, regardless of format (except for cropping the edges on smaller format), and doesn't it have a different look (perspective) from a 24mm lens?"</h2>

<p>Not necessarily. Wide angle shots exhibit anamorphic distortion. So a 24 mm lens on FX will have this distortion, but a 24mm lens on DX will have very little - hence a different look.</p>

<p>Oops - turned my back. In case this helps... the crop is all-important. It's true that the middle of a 16mm shot looks like a 24mm shot, but (compared with a full 16mm print) it's also a smaller area of the total imaage. If you print a 30" x 20" 16mm shot with a DX camera and look at it from 20" away, it won't look distorted. If you take the same shot with an FX camera, you're now too far away to look at the middle DX crop (which would now be only 20" x 13.3"); move your head closer to get the DX view to look right and the edges of the FX frame will look right as well. Alternatively, blow up the DX crop of the FX image to the original 30"x20" size and you need to move your head back by the same scale factor if it's not to appear distorted - which takes it back to 20".<br />

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That may have been more confusing than helpful, but I hope if you wave a wide-angle print around in the air for a minute or two it will make sense!</p>