Why do small and big holes (the aperture) in a lens change the depth of field in the image? (if you have big hole you have smaller depth of field).
I wondered if I should ask this on photo.stackexchange.com, but then realize that I want optic based answer which I think will get here.
Answer
The effect, aperture give to the depth of field is caused by the "used part of the lens".
As the a system of lenses can only make a certain point being focused, there is the need of a trick to gain a high depth of field. This is (not only but also) done by the small aperture.
The reduction of the used part of the lens leads to less aberrations for the not perfect focused light paths. You see the Parts (1) and (3) in the image being mapped better focused on the "photo plate" (5). The light paths from (2) are unaffected, because these are perfect focused. As you use a smaller part of the lens, your image will become more dark as is illustrated in the darker background of the "photo plate" on the right.
Source of image: https://en.wikipedia.org/wiki/File:Depth_of_field_illustration.svg
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