Grades 6–8 · Arts & Music
Why does moving a camera change the sizes of near and far objects?
A person near a camera can look much larger than an equally tall person farther away. The nearby person occupies a larger angle in the camera's view. In a simple camera model, light rays map that angle onto the image, giving image height proportional to object height and focal length, divided by distance.
In the diagramMoving back changes relative image sizesCamera distances: 1 m and 2 mCamera distances: 5 m and 6 mimage-height ratio 2 : 1image-height ratio 6 : 5Equal real objects; each displayed pair is scaled to the same near-object height.
For equal objects at distances of 1 and 2 meters, the nearer image is twice as tall as the farther one. Now move the camera straight back 4 meters. The distances become 5 and 6 meters, so the image-height ratio becomes 6 to 5. The original one-meter separation is a much smaller fraction of their new distances.
Both objects become smaller in the frame, but their relative sizes are more alike. Enlarging or cropping the second image can restore the nearer object's displayed height. It cannot restore the first view's 2-to-1 ratio because the changed viewpoint has already changed the geometric relationship.
Changing focal length while staying at one camera position scales both image heights together in this model. Moving the camera changes their distance ratio and therefore their perspective relationship. Photographers often change the lens and position together, making the causes easy to confuse. The two examples isolate position: backing away changes relative size even when the objects themselves remain unchanged.
About this explanation
Starting knowledge: Understand that equal objects can form different image sizes at different distances and compare simple ratios.
Sources
- Kenji Hata and Silvio Savarese, Stanford University — CS231A Course Notes 1: Camera Models