To find this out, multiply your lens's focal length by 1.5 if you're using an APS-C Nikon, Fujifilm or Sony camera, 1.6 if it's an APS-C Canon camera, and 2.0 if it's a Micro Four Thirds camera.

(Longest Focal Length) / (Shortest Focal Length) = Zoom Multiple Your 14-42mm lens would be advertised as a "3X zoom". The zoom multiple tells you little about how much effective reach you have ...

To see why that is, consider the following example. A variable focal length lens with a fixed aperture will have a variable focal ratio. For a 2:1 zoom lens adjustable from 10 to 20 mm focal length ...

The focal length f of a converging lens is defined as the distance between the optical centre, C, and the principal focus, F. Focal length is measured in m, cm or mm.