The effective diameter (y ǒǒǒǒǒ u j ाा ng) is also called "diameter" or "maximum diameter". It refers to the ratio of the beam diameter of the front mirror (also known as the lens diameter) to the focal length when each lens is fully opened. It represents the light absorption capacity of the maximum aperture of the lens. If the focal length of a lens is 4 and the beam diameter of the front mirror is 1, this means that the focal length is 4 times longer than the beam diameter. It is generally called the f coefficient, and f represents the focal length.
The formula for calculating the reciprocal F coefficient (aperture coefficient) of the effective aperture is as follows: F coefficient=focal length/beam diameter. According to the inverse square ratio between the illuminance of the focus plane and the focal length of the lens, the smaller the F coefficient of a lens, the greater the light absorption capacity of the lens. Since each lens has only one effective aperture, when we talk about the comparison of effective aperture, we are talking about more than two different lenses.
Another meaning of effective caliber is "T" caliber, which is called "T" effective caliber. In the past, the value given by the F system aperture was only the ratio of the focal length of the lens to the diameter of the entrance pupil, without taking into account the structural differences of various lens groups manufactured by different manufacturers, the number of lenses forming the lens group, the difference of optical materials used for grinding lenses, and the coating antireflection film technology quality The size of the lens transmittance ratio caused by factors such as high and low, compared with the influence of plane illuminance E, leads to shooting objects under the same illuminance. Although the aperture with the same F coefficient is used camera lens The exposure of the picture is inconsistent, which affects the quality of the work. The values and calibration methods of the effective aperture of the T system on the lens are no different from those of the F system. [1] 
