Why are all the shots suitable for portrait shooting fixed focus? Like 85 or 50.

Portrait is one of the subjects who take the most photos, and the portrait head must contain several elements: first, the aperture should be large, so that the depth of field can be controlled at will; Second, we should have a proper focal length (85mm, 135mm and 200mm, etc. ) Ensure that the viewing angle and perspective of the lens can conform to the habits of human eyes, instead of exaggerating like a wide-angle lens. A lens that basically meets these two requirements can be called a "portrait head". However, what we usually call a "portrait head" is a fixed-focus lens. This is because, compared with the zoom lens, the aperture of the fixed focus lens can be made larger and the optical quality is higher. For example, the maximum aperture of a lens with a focal length of 85mm can reach F 1.2 or F 1.4, while a lens with a focal length of1.8 or F2.0 can reach a lens with a focal length of 200mm, which is impossible for a zoom lens. In addition, photographers can choose lenses with different focal lengths according to different shooting themes, which is very convenient. Generally speaking, on Quan Huafu digital SLR, 85mm lens is suitable for taking full-length photos, and 135mm lens is suitable for taking half-length photos. For more digital SLR with APS-C size photosensitive elements, the 50mm pan-tilt is also suitable for shooting portraits (because the equivalent focal length needs to be multiplied by the coefficient of 1.5 or 1.6, the viewing angle of the 50mm lens is the same as that of 85438+0.6. So, isn't the zoom lens suitable for portraits? It doesn't have to be like this. Lenses like Canon's EF 24-70mm2.8L USM or EF 70-200mm2.8L IS USM (XBIS) have a large aperture (the maximum aperture is F2.8) and the focal length also covers 50mm, 85mm, 135mm and 200mm, so they are also suitable for taking portraits. However, taking portraits with such a lens is just "killing the chicken with an ox knife", so if you particularly like portrait shooting, you might as well buy a "portrait head" separately.