# Exactly why does length of an object seem to distort as we change prespective?

Suppose we take photos of a object from varying distances (all of which we can see the edge of an object), then one would see that in each photo the dimensions of the objects are distorted from each other. Why does this happen?

• How can one see all of an object from a single perspective/viewpoint? At best one can see about half of it, with the other half facing away from one's viewpoint. Apr 18, 2022 at 7:03
• Sorry, you are correct. I misspoke. I meant an edge of the object @MichaelC Apr 18, 2022 at 7:05
• Does this answer your question? What is the difference between perspective distortion and barrel or pincushion distortion? What your question describes is differences in perspective, which is explained in the largest part of the accepted answer. Apr 18, 2022 at 7:10
• Apr 18, 2022 at 7:28

Your question revolves around the fact that we see the world using a combination of eyes and brain. From infancy we learn that objects that are close can block our view of more distant objects. We also learn that objects appear to retreat seem to diminish in size. All this and more contributes to how we judge distances.

If we gaze out a glazed window at a vista, we can draw the outlines of objects on the glass with wax pencil. Such a sketch reveals the “human perspective”. When we use a camera and take a picture of this same vista, the resulting image may or may not replicate the “human perspective”.

To exactly replicate this view, we replace the position of our eyes with the camera. We then make and display an image of this vista. If we view this image from a distance equal to the focal length of the taking lens multiped by the degree of enlargement, the image we are viewing will match the “human perspective. As an example, we mount a 100mm lens and take this picture. We display this image taken with a 35mm size camera enlarged to 8 by 12 inches. To accomplish we must enlarge the original camera image 8 ½ X.

This degree of magnification is the amount of enlargement required to display the cameras 24mm by 36mm image as an 8 x 12 inch picture. To view and perceive this picture to be the “human perspective”, we view from a distance equal to the focal length multiplied by the degree of enlargement. In other words.100mm taking lens times 8 ½ magnification = viewing distance of 850mm (about 1 yard). Such a lash-up replicates the “human perspective”.

You also need to know, most images we make need not be viewed from a distance of focal length times magnification; most time we look OK. Some images however will look weird. As an example, we know what we look like based on tiny facial differences like nose size compared to ear size. If we make a head shot and view the resulting image at a distance that utterly violates the above viewing distance math, you will see a greatly distorted face.

Again, most pictures look OK even if the perspective is awful. Some shots of familiar things look awful when viewed from the wrong viewing distance.

Some of the many guidelines on this subject:

• light comes from above • objects are normally not viewed from below • faces are seen (and recognized) upright • closer objects can block the view of more distant objects, but not vice versa • figures (i.e., foreground objects) tend to have convex borders