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I'm trying to learn how camera lenses work, and have gotten stuck. Say that we have an optical system consisting of a single lens with focal length f, and we want to take pictures of objects which are infinitely far away. The light rays will in other words be parallel, and when these rays pass through the lens they will be focused at a single point at a distance f away from the lens. But, how can we then take pictures of such objects if all of their rays are focused at a single point?

I'm not questioning the laws of optics since obviously they work, but I can't figure out where the error in my thinking is. Could someone please shed some light on this (pun intended)?

EDIT: For those who are interested, this question is closely related (although not exactly the same).

Community
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gablin
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3 Answers3

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It is not the case that all the rays will focus at the same point. Rays with different directions will focus at different points. You are probably thinking about rays parallel the the optical axis all converging on axis, at a distance equal to the focal length from the lens.

Dr Chuck
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Rays from the scene do not converge to a single point. Only rays coming from a single point in the "infinitely distant" scenery converge to a single point in the focal plane. Rays from a different point in the scenery converge to a different point in the focal plane. And all the different rays from all the different points in the scenery converge to their respective points in the focal plane, thus reconstructing the scenery in a 2-dimensional image.

enter image description here

The focal plane is generally curved, unless the lens is specifically designed. You can find some nice drawings of this here.

mpv
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In the configuration you described (Infinite/focal plane conjugation) the lens acts as an angular position to linear position mapping system. Each parallel ray bundle at a given angle $\alpha$ with the optical axis converge to a position $y$ away from the optical axis in the focal plane and:

$y = \alpha.f$

where $f$ is the focal length of the lens. The size of your image (zoom) is then determined by the focal length.

Anael
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