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When shooting different scenes with an ordinary camera, it is necessary to adjust the lens to get a clear image on the negative, which is called "focusing" in photography terminology. This operation is actually not to adjust the focal length of the convex lens, but actually to adjust the distance from the film to the optical center of the lens, that is, the image distance. In order to control the exposure properly, the "aperture" needs to be adjusted, and the exposure time can also be controlled by changing the shutter.