Perspective, optical not philosophical, may be one of the least understood concepts in photography.
Very simply, perspective is the spatial relationship between elements in your picture. What does that mean? It is the way the various parts of the picture are spaced visually in the image. It is a fixed construct and it depends one only one thing: your distance to the subject. Focal length doesn’t play any role. Aperture doesn’t play any role.
Focal length doesn’t play a role? But wide angle lenses make things look further apart. And telephoto lenses compress things together, you might be saying. Do they?
What the idea of telephoto compression and wide angle expansion are getting at is the idea of apparent perspective. What’s that? Apparent perspective is the way things appear to be spaced in the photo as compared to how they actually are spaced.
If you want to get the same image framed with a wide angle lens and a telephoto lens, you need to be at different distances to your subject. As you move closer with the wide angle lens, those parts of the scene that are closer to you will appear larger because of the physical proximity. Those parts of the scene that are further away, while also getting larger will appear not as large. Conversely, as you move further away with the telephoto lens, everything will appear to get smaller. It has to do with relative distances.
Let’s say to frame a scene at 28mm you need to be 10 feet from the subject. If you have a secondary element 2 feet behind your subject, it is 20% further. That’s quite a large distance. With a 70mm lens, you have to move back to 25 feet to get the same image in the frame (all this assumes a full frame sensor, although the same principles apply for other sensor sizes). How do I come up with 25 feet? Simple, a 70mm lens is 2.5X the focal length of the 28mm lens, so you need to be 2.5X as far from the subject. Now, that secondary element that is 2 feet behind your main subject is only 8% further away. A relatively much shorter distance so the two image elements will appear to be closer together, when actually they are not.
Here’s an example. In the shot below, the image on the left was taken with a 28mm lens at a distance of about 12 to 15 feet from the white dog. By the by, these two are my dogs and it’s a minor miracle that I could get them to sit still for this demonstration. The middle shot is the same image from the same spot taken at 70mm. What do we see? The dogs appear to be closer together. Depth of field is different, but depth of field is not a part of perspective. In the shot on the right, the 28mm picture has been cropped an enlarged to be basically equal to the 70mm shot. What do we see? The apparent distance between the two dogs looks the same as in the actual 70mm photo. The true perspective is the same even though the apparent perspective is different.
Does that put things into philosophical perspective for you?
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