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

The single word explanation of why one cannot focus everything is ‘aberration’. Aberration is all of the little errors that happen to screw up the image. As u/whyisthesky mentioned, geometry is one of the biggest issues. Your camera lens is a one size fits all arrangement and it is not matched to the shape of your object. You can get a perfect image, at least geometrically, if the shape of your lens “matches” the shape of the object. (You won’t ever get a perfect image, but you hopefully get the idea.)

Color is another common aberration because each color of light moving from the object to the camera will have a different focal position. You can see this in some images because some colors will be out of focus.

All of the aberrations in the object/camera system add up to an image where you have to focus as best you can and usually that means you focus on a particular part of the object, e.g. the face of a person instead their entire body.