In attempting to discern distant details, people will sometimes squint. Why does this help?
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The size of the opening of the "stop" is specified by the f -number or f-stop. The f-number can be defined as,
Where the focal length is f and the diameter of the lens opening is D.
Stepping down a lens to a larger f-number indicates a small opening of the lens and the light rays which are coming through the central part of the lens are only accepted.