Angle of view vs. Perspective

It's easy to get a bit rusty on optics and theory from time to time so a quick review of two simple but often-confused aspects may not hurt:

We all know that things of the same size appear smaller the farther they are from our eyes. In figure 4 the fence posts are all the same size in reality but not in the picture. Figure 4 is taken from the perspective of the point in space at which the film frame within the camera was located at that instant.

Of the three shots shown in Figure 5:

• For the first I stood 20 feet from the crossbeam (with an 80 mm focal length)
• For the second I walked toward the crossbeam and stopped at 10 feet (35 mm used to show more of the scene
• For the third I walked forward toward the crossbeam again and stopped at 5 feet (also 35 mm).

The reason we know that the above details are true is because of the change in relative sizes of objects when compressed from three dimensions into the two dimensions of a picture*. Notice how the crossbeam and picnic tables grow in size in comparizon to the background trees. Notice how the crossbeam stays roughly the same size in the 20 feet and 10 feet shots but the picnic tables in front of it change dramatically in size. Notice how the amount of yellow tree under the crossbeam dramatically changes in each of the three shots. These are changes in perspective. If I had stayed at 20 feet but moved to left or right my perspective would also have changed.

Figure 6 takes the opposite tack. Here I stay at the original 20 feet but double then re-double the focal length. Notice how the amount of yellow tree showing under the crossbeam remains exactly the same from 80 mm to 160 mm. When looking in a given direction from any given position in the world, the perspective remains the same regardless of the lens or focal length used (perhaps barring the distortions of fish-eyes!).

The red rectangle inside the 80 mm shot shows how it can be cropped to achieve exactly the same picture as the 160 mm shot; ditto from 160 mm to 320 mm. There is no such relationship between the three shots of figure 5. Which leads us to the moral of the angle of view vs. perspective story:

Even with a zoom lens you still have to walk closer in or farther back to leverage perspective.

From time to time you will hear of a long-time prime lens user who throws in the towel, replaces his primes with zooms, uses them for a time, then switches back to primes. He complains that the zooms made him lazy, so that he didn't get as good compositions as he used to with primes. The zooms were not the problem. The real problem is that they didn't come with decals with the above warning plastered all over them.