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You’re probably suspecting that I’m going to claim that field compression so obvious in images taken with long lenses doesn’t really exist. You’re right!

With all respect, perhaps the single most common misconception in professional imaging ranks is that shooting with a telephoto lens somehow compresses the subject. This tends to add a great deal of weight to the argument that we don’t really know much about about what we do (perhaps due to the esoteric and intimidating nature of optical science), because this issue is both fundamental to our craft and frankly, one of the simpler topics to comprehend.

Of course, the first course of business is to establish our definitions. From my post on “Crop Factor Sensors:”

“Telephoto” most appropriately refers to lenses that are physically shorter than their focal length. The correct term for “long” lenses (those exceeding the focal distance of the normal-itself an optical science term not widely understood, which in the 35mm format is about 50mm)-is “narrow angle lens.” This is according to Dr. Rudolf Kingslake, former Director of Optical Design for Kodak, Emeritus Professor of the University of Rochester: Optics in Photography, published by the International Society for Optical Engineering, 1992, pp. 49-50.

In fact, it is Dr. Kingslake’s explanation of the issue at hand that I learned from. However, since we are on the subject of books, another publication I use for reference is Applied Photographic Optics, Second Edition, by Sidney F. Ray. The subtitle is “Lenses and Optical Systems for Photography, Film, Video, and Electronic Imaging.” It’s a sort of “Myth Buster’s Bible” for photographers-extremely comprehensive and saturated with excellent diagrams.

The apparent “compression” of long lens images and the apparent foreground magnification of wide angle lenses is actually a product of improper viewing distance.

In short, a photograph is simply a “point-projection” of a three dimensional scene. When the image was captured, the point at which it was captured from (the “center of perspective”) was irrevocably established. When an image is viewed, it should be viewed from a distance that represents that “center of perspective.” If it is viewed too closely or from too far, the subtended angles in the image will be inaccurate, and the image will appear to be distorted (either “compressed” by an exaggerated magnification of the background, or “decompressed” by an exaggerated magnification of the foreground).

Here’s an abbreviated explanation:

If an image is taken of two tall columns from point A, but the image is viewed from a distance represented by point B, the angular difference of the two columns will obviously be different (and easily calculated, thanks to the recreational discipline of trigonometry). It is easy to see that the angle from point A will be smaller than the angle from point B, so viewing from point B will produce an apparent magnification of the red column (the angle will be inaccurately greater between the two than it is from the accurate center of perspective).

This works in the reverse as well. If the image is captured at point B and viewed from a distance representing point A, the angular difference of the columns will be inaccurately portrayed as smaller-in other words, with an inappropriate magnification of the foreground. This is the situation when a wide angle lens is used and the image is viewed from too far a distance.

Now, to cover myself from erudite criticism I’m going to state that this is an aggressively distilled description of what is actually going on, and that there are many other geometrical optical effects that play a part (though not a cardinal part) in making, for example, wide angle images appear bizarre (e.g. Elliptical Distortion).

So…how do we resolve this matter? By viewing the image from the correct viewing distance! Again, nothing in optics can be too simple, and we must distinguish between monocular and binocular viewing (viewing with one eye or both). For our practice, even though it is distinguished from the camera’s perspective, we’ll address binocular viewing since it is by far the most common practice.

The correct viewing distance for an uncropped full frame image is calculated by multiplying the focal length of the lens (or for a zoom lens, it’s actual focal length at capture) by the enlargement ratio. For a compliant (uncropped, full frame) print, the enlargement ratio is a linear product. A 30″ print of a 36mm (1.42″) capture is a linear enlargement of 30/1.42, or 21.2. Thus, if the image was captured with a 50mm lens (2″), multiplying 21.2 x 2 calculates a viewing distance of ~42″.

The only remaining question at this point: “Is this important?”

The answer, in Dr. Kingslake’s own words:

“By far, the most important rule for correct perspective in photography is that the final print must be viewed from approximately its correct center of perspective, so that the angle subtended at the eye by the various images in the picture will be the same as the subtense angles of the original objects at the camera lens.”

In the world of art, the effects of displacing perspective can be harnessed with great effect and little criticism. In the forensic world, not knowing how to produce an accurate, defensible artifact can land you, and the people depending upon your performance, in a world of hurt. If you are unfamiliar with even the basics, but are promoting your services for use in litigation, you might reconsider…

L