[Photography] Focal length



The focal length of a lens is the distance between its center and its principal focus. Since most of camera lenses are convex, which converge light, one easy way to determine the focal length is to take the lens and put it in front of a parallel beam (for example under the sun), while on the other side, place a paper and vary the distance between the paper and lens to the point when the image light is smallest (shown in the figure above).



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