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

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

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.

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

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.

Regards,

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

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.

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

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

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

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.

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

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

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

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.

8. ## Re: Optical Theory: FL and"compression," subjective effect on prints

Hi, David,

The phenomenon of "compression of distant objects" is not a function of lens focal length, although focal length is one parameter in the chain that can bring that phenomenon into play for us. In fact, we are really talking about a special implication of the phenomenon of perspective, one that leads to an optical illusion.

Here is a numerical example that shows how and why this happens.

Imagine that our subject is a pair of telephone poles, each 30' in height, one 500' from the camera and one 132' beyond that. There is an auto near the base of the near pole (this helps our visual system determine "scale".)

To our human vision, the nearer one subtends a vertical angle of about 3.44º and the farther one a vertical angle of about 2.72°. (This assumes our eye at ground level, a close enough approximation for our purposes - we will assume similar geometry for the rest of the example.)

Based on what we know about telephone poles, then (ideally), with the help of the "scale" cue from the automobile, our perceptual system will interpret what we see as something like two 30' telephone poles at distances of about 500' and about 632' - separated by 132' (as they are). This part of the world will "look natural" to us.

Suppose we take a shot of this scene from this same point, and then crop an area including the two poles on the negative and print it so that the near pole is 6.00" high on the print. The far pole will be 4.75" high. It does not matter what focal length lens we use - we must just choose the printing enlargement to make the near pole 3" high on the print. The rest of the story will work the same way regardless of focal length.

We then regard the print from a distance of 18". The image of the near pole will subtend an angle of 19.4° and that of the far pole, 15.3°.

Based on what we know about telephone poles, then (ideally), with the help of the scale cue from the automobile, our perceptual system will treat the image of the near pole as if it were a real 30' pole at a distance of about 90', and the image of the far pole as if it were a real 30' pole at a distance of about 114' - separated by a distance of about 24'. Thus, to the observer, the distance between the two poles seems "compressed" - we expect telephone poles to be further apart than that (like about 132').

In each case, the ratio of the apparent distances to the two poles is 5:6. Thus the apparent distance between them would be about 0.26 times the distance to the near one. That means that if we think the first pole is about 500' away (as it really is), then the distance between them would seem to be about 132' (as it really is); if we think the near pole is 90' away, then the distance between them would seem to be about 24' (which it isn't).

Then important fact here is that it works this way regardless of the focal length of the lens used, so long as in each case the magnification of the print from the negative is chosen to make the image of the near pole the same size (6" high in this specific example) and we view the image from the same distance.

Best regards,

Doug

9. ## Re: Optical Theory: FL and"compression," subjective effect on prints

To my mind it's all angle of view. Draw two 'vees', one tall and skinny, and one fatter. The skinny one is the angle of view of the longer lens. There is a foreground subject. Vary your distance from this subject until it is the same size with both angles of view (lenses). Now consider the background. It is a finite though significant distance away. Extend your 'vees' out to it. With the longer lens you are including a much smaller swath of it (thus magnifying it relative to the foreground subject). There's your compression. The fat 'vee' (shorter lens) is going to include a much greater swath of background, thus minifying it. There's your expansion. I realize that this is a simplified explanation with no actual math and all, so you are all welcome to roll your eyes. :P

10. ## Re: Optical Theory: FL and"compression," subjective effect on prints

To my mind it's all angle of view. Draw two 'vees', one tall and skinny, and one fatter. The skinny one is the angle of view of the longer lens. There is a foreground subject. Vary your distance from this subject until it is the same size with both angles of view (lenses). Now consider the background. It is a finite though significant distance away. Extend your 'vees' out to it. With the longer lens you are including a much smaller swath of it (thus magnifying it relative to the foreground subject). There's your compression. The fat 'vee' (shorter lens) is going to include a much greater swath of background, thus minifying it. There's your expansion. I realize that this is a simplified explanation with no actual math and all, so you are all welcome to roll your eyes. :P

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