Consider a pinhole camera with a flat, centered, film pointed directly at the center of the face of a cube. Only the front face of the cube is visible, and each side of the cube in the projected image has the same length.
Suppose the camera is on a track which is perpendicular to the line formed between the camera and the cube.
As the camera is slid to the left several things happen:
- the projection moves to the left
- a new face comes into view
- of the edges making up the front face, the one furthest away appears smallest while the closest edge appears longest
If instead, the camera was rotated:
- part of the film would be closer to the cube and part would be further
- the projected edge on the far part of the film would appear largest.
- no other face of the cube would become visible
- the cube's position on the film would not change
In short, as we rotate and translate the camera, the image changes in different ways.
Once we take a photograph, we can crop the photo. Cropping doesn't affect what the cube looks like, but it can affect where on the film, (with respect to the film's center) the cube lies. We still have several cues about the relationship between the camera and the object but we lost one cue. Is it still possible to identify which point was directly in front of the camera?