When parallel light rays enter the lens parallel to the optical axis, the distance along the optical axis from the lens\' second principal point (rear principal point) to the focal point is called the focal length. In simpler terms, the focal length of a lens is the distance along the optical axis from the lens\' second principal point to the film plane when the lens is focused at infinity.
When parallel light rays enter the lens parallel to the optical axis, the distance along the optical axis from the lens\' second principal point (rear principal point) to the focal poin ...
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In simpler terms, the focal length of a lens is the distance along the optical axis from the lens\' second principal point to the film plane when the lens is focused at infinity.