Mar 5th 2017, 06:14 AM
Join Date: Jun 2016
Consider a digital camera
Now select a point in the scene being photographed.
If every "light ray" from that point that passes through the lens is focused onto the same pixel, then that point can be said to be at "infinity".
If if light rays (from that point) that pass through the bottom part of the lens are focused onto different pixels to the light rays (from that same point) that pass through the top part of the lens, then the image will be fuzzy and the point is too close to be considered at infinity.
This is one reason that phone camera lenses are so small, "infinity" is closer for a small diameter lens than for a large diameter lens (of the same focal length).
A big lens is needed to get lots of light in, but if you have a very sensitive detector and/or a very bright scene, then a small lens will (generally) be able to focus on closer objects.
In old school photography, one would close up the aperture, effectively only using the central part of the lens, when the scene is bright and thereby get a much crisper image with a much deeper field of focus.
It is instructive to think of the pin hole camera.
Here you don't have any lens at all, but you can still produce a surprisingly good image (of a very bright scene) because the aperture is so small.