What does this mark on Sony a7 camera mean?

All SLR cameras have this logo. This mark is the plane position of the photosensitive element, which is the so-called focal plane.

Digital camera is the position of CCD or CMOS, and film camera is the position of film. This mark is used for precise focusing, mainly to measure the distance from the camera to the subject when taking macro photography. The method is to measure the distance from this mark to the subject with a tape measure, and then adjust it to the corresponding distance on the lens distance mark. At present, autofocus lenses can generally focus accurately. In addition, in order to achieve fast autofocus, the deflection angle of the lens barrel from the nearest distance to infinity is very small, and the distance scale is very short (some even do not exist), so manual focusing is not allowed. In addition, even if manual focusing is performed, there is a focusing instruction in the viewfinder, so it is basically not used. Unless you use a macro lens, because macro shooting needs accurate focusing, the distance scale of the macro lens is generally long, and manual focusing can also be accurate when shooting with a tripod.

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