eli5 Why do camera lenses need to focus on something? Why can’t they just render an image in which everything is clear?

Question

Or maybe only some types of lenses work like that?

In: 281

Answer 1

For things that a far away, the rays of light entering a lens are all almost exactly parallel, no matter how far away the object sending the rays. The lens bends those rays and puts them together to form an image behind the lens.

For objects close to the lens, those rays are not all parallel. If you are standing looking at a nearby tree, rays from the top of the tree come into your eye from above and the rays from the bottom of the tree come in from below. For nearer trees, that spread is larger than for trees farther away. Because the rays enter the lens of your eye with different spreads, the lens can’t create images of the nearer tree and fartherer tree at the same place and one has to be fuzzy.