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

[robc]

David,

The way you describe your thinking is almost correct except you need to think about it in terms of the object size on the film. Forget about angle of view for this. For a 300mm lens to give the same object size on film as a 50mm lens, regardless of film format, the 300mm lens needs to be further away from the subject to compensate for the additional magnification of the 300mm lens. It is the altering of distance from the subject which alters the perspective and relative sizes of different parts of the subject at different distances from the lens.

This is very well illustrated in AA's "The Negative" in the chapter on "artificial light photography". There is an image of two pianists (fig: 7-14 in my edition) and a longer lens was used to keep the relative sizes of the two pianists fairly equal on film. Had a shorter lens been used the camera would need to be closer to the subjects thereby making the nearer pianist look much bigger than the more distant pianist.

So the selection of lense focal length is a very important tool in adjusting the relative importance of different parts of the subject according to your viewpoint.

[C. D. Keth]

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.

Ding ding ding! Correct!

Say we're talking contact prints from 8x10 negs for simplicity. For reference, that is a 12.8 inch diagonal, which is also going to be considered the standard viewing distance for these prints. Now, if you take a negative with a 12.8 inch focal length, the perepective will look realistic to the eye when viewed from 12.8 inches from the print. If a shorter focal length is used but the viewing distance remains constant, the perspective will be exagerated. If, however, the viewing distance is always equal to the focal length in this situation, the perspective will always appear normal.

[Leonard Evens]

This sort of thing comes up regularly, and it is a source of much confusion and difference of opinion.

First, let me address the issue of perspective and point of view and focal length. It is true that geometric perspective, in the sense of what lines up with what and where vanishing points are, depends just on the position of the lens. But when you make a photographic print, it gets a bit more complicated. The relevant question is whether or not things will look the same to you when you look at the print as they would if your eye had been where the lens was. That will be the case only if you view the print from the appropriate position. Suppose, for example, that you take a picture using 6 x 7 (actually about 57 x 70 mm) format with a 90 mm lens. Suppose also that the subject is distant enough that you can consider it at infinity for all practical purposes. Then the film will be just about 90 mm from the lens. Suppose you make an 8 x 10 print from the negative by enlarging about 3.6 times. You should view the print from 3.6 x 90 mm ~ 320 mm for your eye to see what it would see by looking directly at the scene. Suppose instead, you make an 8 x 10 contact print using a 320 mm lens. Again your eye should be at 320 mm from the print for proper viewing. So, taking into account the final print, the perspective obtained with a 90 mm lens and 6 x 7 format is equivalent to a 320 mm lens and 8 x 10 format.

Now what happens if you use a lens that is much shorter than the normal focal length for the format. If you make an 8 x 10 print, you are unlikely to put your eye at the proper position for viewing the print given the focal length of the lens. You will likely view the print from what is a normal distance, i.e., about 300 mm, and you will be too far away. That will create some apparent distortions. Objects will look too small, and three dimensional objects at the edges of the field won't look quite right. S milarly, if the focal length of the lens is longer that the normal focal length, you are going to be viewing t he print from too close up. That will create an apparent contraction of distances and the feeling of compression.

That probably explains most of the reason why you see compression in such a print, but there may also be more to it than that. For example, with a long lens, the subject is magnified more than would be the case with a normal lens, and that by itself may exaggerate the sense of compression.

[C. D. Keth]

That probably explains most of the reason why you see compression in such a print, but there may also be more to it than that. For example, with a long lens, the subject is magnified more than would be the case with a normal lens, and that by itself may exaggerate the sense of compression.

To add to this:

Two objects equally spaced in depth from the lens will look closer when you are farther away than when you are close. Since for a given framing a long lens will put the lens far away from the subject, this also adds to the compressed space feeling.

[Jeffrey Sipress]

Conrad and Steve have said it all. It's really quite simple. Perspective, which is generally defined as the relationship among objects in a scene, is only determined by the camera-to-subject distance. Once you establish that to your liking, you then select a lens that gives you the angle of view you want. Photographic Craft 101, but it's amazing how many well known and longtime photographers don't know that.

[C. D. Keth]

Conrad and Steve have said it all. It's really quite simple. Perspective, which is generally defined as the relationship among objects in a scene, is only determined by the camera-to-subject distance. Once you establish that to your liking, you then select a lens that gives you the angle of view you want. Photographic Craft 101, but it's amazing how many well known and longtime photographers don't know that.

It's also surprising how many people teahing photography and film can't seem to explain it as well and concisely as you just did in two sentences. Nicely done.

Now go to a cinematography forum and explain to them that film format has no effect on depth of field. I think I answer that every other week.

[Alan Davenport]

The apparent compression of depth with long vs. short lenses, is a function not of the lens, but of the distance from which we view the image. A "normal" lens is so-called because its view subtends roughly the same angular view as will the resulting 8x10 print, when the print is viewed from the so-called standard distance.

With a lens that is substantially shorter or longer than normal, because we tend to view the final image at that same "normal" distance, the angular relationships between near and far subjects is distorted. But if you view a print so that it (the entire print) subtends the same angle to your eye, as did the subject to the camera, then the relationships will appear correct. This ignores binocular vision effects, which tend to compensate for the difference in viewing distance.

[Dominique Labrosse]

David,

Here is a tutorial that hopefully will clarify the answer for you...

http://www.montizambert.com/educatio...s/articles.htm

Regards,
Dominique