Equivalent Focal Length
The full-frame focal length that would give the same angle of view as a given lens on a smaller format.
Formula / rule
f_equiv = f × crop factor
Measured in
mm
In depth
Equivalence exists purely as a translation device, because photographers learned their angles of view on 35 mm. It is accurate for framing and slightly misleading everywhere else: perspective is set by camera position, not focal length, and depth of field and light gathering need the aperture converted too before any comparison is fair.
Examples in practice
A 25 mm Micro Four Thirds lens is quoted as 50 mm equivalent, framing identically to a full-frame standard prime.