Optical Theory: FL and "compression," subjective effect on prints

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So here's something that's been on my mind for a while and I hope some of the optical gurus here can help with. General optical theory as I understand it dictates that, as compared to how the human eye sees things, shorter focal lengths tend to exaggerate subject field depth, while longer focal lengths tend to compress it. For example, I take out my Nikon and take three shots of the same subject, each with a different lens. The first, with a 50mm lens, is a close approximation of how I see things with my eyes. The second, with a 24mm lens, creates a scene with an exagerrated amount of depth, making things essentially seem farther apart than they are in reality. The third, with a 300mm lens, shows compression of the scene, making things appear closer together than they appear to the naked eye.

Now, I'm not terribly knowledgeable about the actual optical physics that makes this happen, but what I'm more interested in is how it affects the appearance of photographs, especially as it relates to large format photography. Correct me if I'm off base, but these are my general observations.

If I put my 24" lens on my 8x10, I get the same kind of subject compression as I would with a 600mm lens on my 35mm. 600mm is 600mm no matter how big an area the negative covers. However, my field of view is a lot bigger, approximate to what I would get with a 100mm lens on a 35mm camera. If I go the other way and put a 150mm or even a 90mm lens on my 8x10, my field of view is enormous, but either lens is still a long lens in 35mm terms.

Now, let's consider the effects this has (or at least I think it has) on the appearance of the final photograph. If I take the same photograph with a 35mm camera and an 8x10, the 35mm cropped to the same aspect ratio as an 8x10, and appropriate lenses on each camera to give the exact same field of view on both cameras, am I wrong in thinking that the final photographs will have a fundamentally different, if not immediately obviously different appearance? For a given field of view, the 8x10 equivalent focal length is roughly 6x that of the 35mm focal length. If I use a 50mm lens on the 35mm camera and a 300mm lens on the 8x10, even if the field of view for both were identical, the 8x10 shot would have a different appearance due to subject field compression in the 8x10 shot, correct? A wide angle shot with a larger format has a different appearance than a similar wide angle shot made with a smaller format due to differences in equivalent focal lengths, correct? Same with long lens situations?

Assuming my assessment is correct, how does this affect how we use the various formats? Has anyone taken or otherwise been aware of any photographs in which this phenomenon is really visible? To me at least, this is something that's really interesting when you really think about it. But then again, if my math is wrong, then I really just have no idea what's going on. Backup or the contrary from the optical gurus or observations from anyone would be greatly appreciated.

-- David Munson (apollo@luxfragilis.com), March 03, 2002


Dave, I think you are confusing yourself here.... Lets take your example.... you set up tripods side by side. You shoot 35mm camera with 50mm lens and 810 camera with 300mm lens. Assuming the aspect ratio of the 35mm is the same as 8x10, then both images would have the exact same image within its borders, no differences.

Now, if you shoot 35mm with a 600mm lens, and 810 with a 600mm lens on it, and you crop the 810 film down to 35mm size, (or use a 35mm back) you once again will have the identical image! But if you compare the 35mm image vs. the full 810 image, the 810 image will be much wider in its view. so to answer your question.....

If I take the same photograph with a 35mm camera and an 8x10, the 35mm cropped to the same aspect ratio as an 8x10, and appropriate lenses on each camera to give the exact same field of view on both cameras, am I wrong in thinking that the final photographs will have a fundamentally different, if not immediately obviously different appearance?

Yes, you are wrong, they will look identical. Whats considered normal on 810 camera is considered long on 35mm! It's all relative to film format.

-- Bill Glickman (bglick@cascadeaccess.com), March 03, 2002.

Hmm....maybe I am confusing myself here. But essentially what I want to know is this: With a larger format, a longer focal length needs to be used in order to get the same field of view as you would get on a smaller format. With the longer focal length, you get more subject compression, despite the same field of view. So how does the longer focal length affect the appearance of the photograph, if at all?

-- David Munson (apollo@luxfragilis.com), March 03, 2002.

To further clarify, I'm not so much interested in why the normal focal length for a given format is the normal focal length, or how it affects what is included within the borders of the photograph, but rather how/if the varying normal focal lengths of different formats affect the appearance of the subject within an otherwise identical field of view.

-- David Munson (apollo@luxfragilis.com), March 03, 2002.

Think about this differently. The focal length has nothing to do with compression or, more properly, perspective. Zilch, zero, nada. That's determined entirely by where you stand. The angle of view that the camera can "see" is determined by the format size combined with the lens focal length. It's that simple.

-- Conrad Hoffman (choffman@rpa.net), March 03, 2002.

David, here is a better way to think of it... your description of compression for long lenses and stretch (or exagerated) for near objects with wide angle lenses is very accurate! The question is, what is normal, what is wide and what is long? The answer to that question is dependent on the film formats diaganol.... a normal lens equals the diaganol of the format, a longer lens is greater than the diaganol, etc. So you have the right idea, you just have to change the middle point (normal lens) for each format.

If you take the diaganol of 35mm, 6x7, 4x5, 8x10 and use a lens for each format exactly equal to their diaganol, you will get the identical image, on each piece of film, (correcting for aspect ratio a tad) If you use a lens for each format that is 6x the diaganol, you still will get the identical image on each peice of film. Does this make sense?

-- Bill Glickman (bglick@cascadeaccess.com), March 04, 2002.

Bill, so then the stretch or compression of short and long lenses is entirely relative to the format and not the focal length of the lens? The extreme "compression" of a 600mm lens in 35mm would then have more of an apparent compression than with a 600mm lens on an 8x10? This is starting to make more sense. Thanks.

-- David Munson (apollo@luxfragilis.com), March 04, 2002.

I'm with Conrad on this one. The sense of "compression" is dependent upon perspective, which is dependent only upon the position of the camera. Perspective IS NOT dependent on focal length. So, one will get the same sense of compression from both the 8x10 and the 35mm camera, regardless of the lens used on either camera.

In comparing photos taken with a 300mm on an 8x10 and a 50mm on a 35mm, depth of field can change, which can also give a different sense of "compression". But, by adjusting the aperture on the 8x19 to be smaller (with a longer exposure), the depth of field can be made identical on prints from both cameras. So, even in this case, the sense of compression is the same from both cameras.

By adjusting the depth of field, so long as one adjusts the lenses on both cameras to obtain the same angle of view, then the photos will be identical from both the 35mm and the 8x10. (Let's assume that one is copping the 8x10 to obtain the same length to width ratio as that on the 35mm format.)

-- neil poulsen (neil.fg@att.net), March 04, 2002.

Dave, you wrote......, so then the stretch or compression of short and long lenses is entirely relative to the format and not the focal length of the lens?

Better said, film format defines what lens is long or short as I described above. To alter the amount of stretch or compression from one image to another, you need to alter the camera vs. subject distance. Neil wrote.....I'm with Conrad on this one. The sense of "compression" is dependent upon perspective, which is dependent only upon the position of the camera.

I agree with you and Conrad, but to answer Davids original question, we assumed both hypothetical images were taken in the same spot. His question was about the difference in the final photograph when using a 35mm with normal lens vs. an 810 camera with normal lens, shooting from the same position. He felt the final images would be noticeably different. But in reality they would be near identical. (putting aside, DOF, aspect ratio, movements, etc.)

-- Bill Glickman (bglick@cascadeaccess.com), March 04, 2002.

"If I take the same photograph with a 35mm camera and an 8x10, the 35mm cropped to the same aspect ratio as an 8x10, and appropriate lenses on each camera to give the exact same field of view on both cameras, am I wrong in thinking that the final photographs will have a fundamentally different, if not immediately obviously different appearance?" - Yes, totally wrong.
Perspective has nothing to do with the focal length of the lens used. It's determined only by the camera to subject distance.
A given angle of view will give you the same visual effect on any size of film. It's only depth-of-field that's drastically altered by the format size. You would have to multiply the numerical aperture of the 35mm camera by 8 to get an equivalent DoF on the 10x8 camera. (ie, f/4 on the 35mm camera would become f/32 on the 10x8 ). If you do that, then the pictures will appear exactly the same, excluding the better tonal quality of the 10x8, of course.

-- Pete Andrews (p.l.andrews@bham.ac.uk), March 04, 2002.

Well, so much for that idea. Still not entirely convinced, or at least not entirely understanding of the concepts here, but a trip to the library should cure that. Thanks for all the responses.

-- David Munson (apollo@luxfragilis.com), March 04, 2002.

David, since you shoot several formats, why not do your own tests? Shoot a sheet of 8x10, MF and 35mm. Cut everything down to 35mm size, and the images should be close to identical, given a small variation in what is considered "normal lenses" for each format. To agree with previous posters, it is distance from the lens that makes the difference in perspective, film size that determines what is included at the edges.


-- Doug Paramore (Dougmary@alaweb.com), March 04, 2002.

Begin with Camera & Lens, Ansel Adams, Revised Edition, 1948, 1970, 1976. This edition is prior to his most recent edition in which he worked with an editor. Check page 147 under Visualization With Camera and Lens. You will see a very similar discussion to that which we've had above.

Adams begins by defining perspective by saying that, "True perspective depends upon the distance of the subject from the lens." But, a few sentences down, he makes the statement, "A 2-inch lens, an 8-inch lens, and a 24-inch lens mounted on the same camera at the same 'position' relative to the subject will give identical perspective."

It's this second description to which I adhere. It isn't just distance from subject, it's the relative position. Lift the camera 20-feet, or move it 20-feet to either side, and although the distance might be the "same", one can achieve completely different perspectives. It's not just "distance" that's the determining factor of perspective. Actually, moving the camera in this way changes ALL distances from the camera to each point in the photograph.

-- neil poulsen (neil.fg@att.net), March 04, 2002.

The reference I found most helpful to clarify this was Stroebel's textbook. The notion of compression of space is entirely a function of relative magnifications of foreground and background objects. Magnification (with a given focal length) is a function of distance i.e., magnification increases dramatically as an object comes closer to the lens. With a wide angle lens, nearby objects are rendered at dramatically larger sizes than distant objects, which provides the 'illusion' of depth in the scene. In contrast, a long focal length (which is typically used from a distance) is operating far enough from foreground and background object that there will be no such exaggeration of 'space'. Since magnification (its worth thinking of distances as multiples of focal length since it is intuitive to think of magnifications in this way) is relative to format, the 50mm on 35mm will provide a similar look to a 300mm on 8x10.

Set up 2 little objects of the same size on a scale and focus your 8x10 on it. Now move one object along the scale and measure the change in image size on the ground glass - you will see the dramatic reduction in magnification as an object moves away from the lens. At a distance between 2 times the focal length and 1 time the focal length, the magnification will increase from 1X to infinity (your bellows draw of course will get inordinately huge). But it will give you a good idea of how relative magnifications (which of course is reliant on relative distances) can alter subjective depth tremendously - in other words, it is where you place the camera which determines the relative distances and therefore the relative magnifications. A wide angle lens is typically used up close to a foreground object (exagerrating its size i.e., the ratio of foreground object distance and background object distance can be 5 times focal length and 30 times focal length respectively). A 300mm on 35mm will compress things a lot (because both foreground and background are far away e.g., 25 times and 30 times focal length, and relative magnifications are nearly equal). As stated above, true perspective is a function of position and not focal length (if you crop off the wide angle, you will get the identical framing as from a longer lens).

In other words, a 50mm lens on 35mm will provide an angle of view similar to a 300mm on 8x10 - the 300mm magnifies things more of course but it magnifies everything equally and maintains the relative sizes. The relative magnifications provided by these lenses on these formats will be pretty close i.e., the illusion of space will be identical.

Cheers, DJ

-- N Dhananjay (dhananjay-nayakankuppam@uiowa.edu), March 04, 2002.

Perhaps this is an over simplification, but it helps to think of it this way. Perspective and depth are determined by the relative distance between the camera and the subject/forground/background. Where you stand controls the perspective. The choice of lenses controls how much of the "general area" is recorded on film, and can be thought of as in-camera cropping. Finally, larger film sizes use longer "normal" lenses because the image must be magnified more to cover the piece of film.

-- Steve Gangi (sgangi@hotmail.com), March 04, 2002.

Now it makes sense! Just one of those things I was having a bit of trouble figuring out myself. Thanks much to everyone.

-- David Munson (apollo@luxfragilis.com), March 04, 2002.

A spanner in the works: I agree fully with the analysis of how perspective doesn't change with different focal lengths, but believe strongly that that analysis misses the point.

Go and look at some photos without thinking too hard. The compression of distance by long focal lengths is obvious, universal, and requires a dedicated, pig-headed effort of will to avoid seeing. But, crucially, it is a result of the psychology of viewing a print, not the physics of making a negative.

When people look at a photograph they make subconscious assumptions about how the scale of objects on the paper translates to the scale of the photographed objects in the real world. Much of the time, they imagine themselves to be looking through a window of the same dimensions as the photograph, and *that*'s why wide angle views look so sweeping, and long lens shots look so compressed. The viewer assumes that a normal lens must have been used, and constructs a false, overly dramatic model of the scene in their head.

This is easiest to observe if you make a large print of a wide angle shot. From a 'normal' viewing distance, or from across the room, you get the usual wide angle effect of sweeping drama. Get close though, and the view starts to appear more and more normal, especially if you close one eye to restrict your brain's depth cues. Once you get close enough that the image takes up the same field of view as did the original scene, the photograph no longer looks wide-angle at all.

So my answer to David's original question would be that yes, long lenses do compress 'perspective', if you use 'perspective' in a more figurative sense than the purely geometric. When you move between formats you change the definition of 'long', but the same effect pertains nevertheless.

-- Struan Gray (struan.gray@sljus.lu.se), March 05, 2002.

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