What does the header of a SLR camera mean?

Material is a photography term, which means "standard lens". The so-called standard lens refers to a fixed-focus lens with an angle of view similar to that of the human eye (monocular, non-rotating). The general focal length is about 50mm. This type of lens is small in size, has a large aperture, usually has mature technology, and has huge sales volume. For example, Canon's 50mm/F1.8 is a famous cheap lens.

The angle of view of the lens depends on the camera's frame (for traditional cameras, it is the size of the film), so the focal lengths of the headers of cameras with different formats are different. The focal length of the 35mm camera is about 50mm (anything from 40mm to 60mm can be counted). The focal length of the 120 camera is about 75mm, and the focal length of the APS camera is about 35mm. DSLR cameras have different 35mm equivalent coefficients, and the corresponding focal lengths are also different. Generally speaking, the actual focal length of a non-full-frame SLR digital camera needs to be multiplied by a factor of 1.6. Therefore, for a digital SLR like the Canon 40D, the focal length of the lens should actually be 50/1.6=31mm. However, there is usually no need to investigate so carefully. It can be generally believed that a lens of about 50mm can be called a "head" for ordinary cameras.