Perspective and the Normal Lens

People have asked about "normal" lenses. I found a lot of opinions and a few concrete answers. Here are a few "expert" answers plus some of my own biased opinions.

We can only see the "correct" perspective of a print, movie or video if the image is viewed exactly reproduced to give the same perspective as seen by the camera. That is if the camera shoots a scene with a certain lens, it must be viewed with an enlargement the same ratio of image size to distance of viewer as it was photographed. If F=camera focal length, I=image height on film, O=image height as projected or printed, then the correct distance to be viewed is D=viewing distance.

(F x O) / I = D

We rarely get a fifteenth row center seat in a theater, view a 24-inch TV set from 32 inches or look at a photograph from the same relative distance that it was shot at. Viewing conditions vary. If a photographer keeps changing his focal lengths, it is very difficult to view his different images with the "correct" perspective. Early in photography there were limitations in the design of lens focal lengths. Cameras only came with "normal" lenses.

Designing wide angle and tele lenses was difficult. Early B movie cameramen often went out with only 2 lenses, a 35 mm for "long" (wider) shots and a 50 mm for close-ups. If you sat in the right place in the theater, you saw a "correct perspective" most of the time. Any extreme angle seemed incorrect to the early audience. Today we have repeatedly seen images shot by extreme wide angle and tele lenses and our brains have learned to perceive them all as "normal". Today the "normal lens" is no longer relevant. Notice how quickly the screen image from the last available seat in a theater, front row side corner, soon looks normal to your brain long before you neck gets stiff.

Early in photography it was decided that a focal length equal to the diagonal of the image formed on the film was "normal". That gives us a 50-mm lens for 35-mm stills, 75-mm lens for 2-1/4 square, 150 mm for 4 x 5 etc. That was well and good if the same size print shot with a normal lens was always viewed at the same distance. But viewing a wide-angle shot close or a tele shot at a distance can also provide a "correct perspective".

Some aspects of vision should be considered. Our brain "sees" a very wide angle of view, over 120 degrees. But our brain can only concentrate at one time on a very small image angle of 1.7 degrees on the fovea. Our brain creates a composite image by scanning scenes with images from the fovea. Concentrate looking at one word and notice the rest of the page becomes blurred. The rest only becomes in focus when we change our attention. Our brain creates the images that we "see". See Vision for Photography.

Cinerama was shot with three cameras and was very real if seen from a center front seat. Today IMAX is the same. They provided a very wide angle of view to the audience much like our field of vision. In this case the audience scans the scene for different areas of concentration, similar to what we do in real life scenes. In a theater, looking at TV or a printed picture we view the images within a limited frame and the image is contained within that frame.

Like much of our "seeing", a lot of interpretation goes on in our brain to make sense of the images that we "see". Native people having never seen pictures before are confused by 2 dimensional images. They haven't learned yet to use the cues of perspective in a 2 dimensional image. Middle age artists discovered reproducing perspective only after years of study and experiment.

Some photo writers said that if we see the same size image in the viewfinder as seen by eye without the camera, that the lens was "normal". They were ignoring differences in viewfinder optics that give different sizes of image to the eye. Camera manufacturers adjust the size of the image to fit a compact economical package on a camera. I personally don't like the larger images in the viewfinder that some manufacturers brag about. For composition I find that I must scan a larger image more than to see it as a whole image than if it is smaller. It's like looking at a picture on the wall standing too close or sitting in the front row of a theater.

Most of us today use whatever lens will record what we want to include in the frame or choose a longer lenses to soften a background or a lens that will make the star look good.