All lenses must be focused to present a sharp image. At the point of focus, the light rays emanating from any point on the subject are brought back together again as a single point in the resulting image—or as close as possible to a single point. Rays of light (photons) leave the subject in many directions. A few of them strike the lens. The wider the lens aperture, the more of them the lens can collect. Then the job of the curved surface of the lens is to nudge the photons in a new direction. The curve of the lens varies the nudging for the different angles at which the photons left the subject. The problem is that once the lens nudges the photons, they keep traveling straight. The photons from any given point will converge on a point ...

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