relative aperture

Definition: Indication of how bright is the projected image of a lens. * Measured as the ratio of the diameter of the light beam entering the lens (the entrance pupil) to the focal length of that system. * Symbol: f/ followed by the value: e.g. f/2 means that the diameter of the entrance pupil is equal to half the focal length. Watch out for relative focal length, however Focal length is not what it seems, especially if you use a lens designed for one system (e.g. 35mm photography) on another (a digitial camera). As the digital camera has a sensor chip that is smaller than the format the lens is designed for, the focal length is effectively increased. But the relative aperture must stay the same. Think about it. When you crop an image you don't change the exposure: a CCD or CMOS chip using a smaller part of the image projected by the lens than is normal is just the same -- it is sampling from the centre of the usual image.

Related Terms: f/number

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