Tamron SP 45mm f/1.8 Di VC USD: Tamron's premium fixed-focal-length 45mm standard lens

摘要

Why do we use 50mm as a standard focal length for 35mm format? The rule is that the length of the diagonal of the frame, of film or sensor, is the focal length with the 'normal' or standard angle of view. But the diagonal of the 35mm frame, and that of a full-frame sensor, is just over 43mm.