What is the standard lens focal length of APS-C format camera?

For APS-C format cameras, the focal length of the standard lens should be about 30-35mm. \x0d\ The reasons are as follows: \x0d\ Standard lenses refer to lenses whose visual angle is approximately close to that of human eyes, which is generally considered as 45 ~ 50. Because the two perspectives are approximately close, the perspective of the scene represented by the standard lens is also close to vision, and the perspective effect is natural, which can realistically reproduce the image of the subject. This is the most basic photographic lens. \x0d\ According to the requirements of this viewing angle, the camera's image sensor has different sizes and the focal length of the standard lens is also different. It is generally believed that the standard lens focal length of a camera is roughly equal to the diagonal length of the camera image sensor. On a full-frame camera, the focal length of the standard lens is about 40-55mm. The image sensor of APS-C camera is smaller than that of full-frame camera. Judging from the diagonal length of the image sensor, the focal length of the standard lens should be about 30-35mm. In addition, the lens focal length of full-frame camera and APS-C camera should reach the same viewing angle, which can usually be calculated by focal length conversion coefficient. Nikon is 1.5 and Canon is 1.6. In this way, the standard lens focal length of APS-C camera is also about 30-35mm.