A lens with a focal length approximately equal to the diagonal length of the photosensitive material is a standard lens.

It is correct to judge that a lens with a focal length equal to the diagonal length of photosensitive material is a standard lens.

Focal length is a measure of light convergence or divergence in an optical system, which refers to the distance from the optical center of the lens to the focal point of light convergence when parallel light is incident. It is also the distance from the center of the lens to the imaging plane such as the backplane or CCD in the camera. An optical system with a short focal length has a better ability to collect light than an optical system with a long focal length. Simply put, the focal length is the distance from the focal point to the center of the mirror.

Standard lens, a general term for lenses with a viewing angle of about 50 degrees, and a photographic lens with a focal length approximately equal to the diagonal length of the photographed picture. Its viewing angle is generally 45 ~ 50. 35mm lens with a focal length of 40-60mm, 6 * 6cm lens with a focal length of 75-80mm, and 4 * 5in lens with a focal length of 120- 150mm. A standard lens usually refers to a photographic lens with a focal length of 40 to 55 mm, and the perspective effect of the scene represented by the standard lens is close to vision.