35 mm equivalent focal length

In photography, the 35 mm equivalent focal length is a measure of the angle of view for a particular combination of a camera lens and film or image sensor size.

The term is popular because in the early years of digital photography, most photographers experienced with interchangeable lenses were most familiar with the 35 mm film format.

Two lens-sensor combinations with the same 35 mm equivalent focal length are expected to have the same angle of view.

Most commonly, the 35 mm equivalent focal length is based on equal diagonal angle of view.

[3] For example, a 50mm f/2 lens on a 2× crop factor Micro Four Thirds camera would be equivalent to a 100 mm (= 2×50 mm) f/4 (= f/(2×2)) lens on a full-frame digital SLR in terms of field of view, depth of field, total light gathered,[4] and diffraction effects.