When discussing crop factor, it’s important to note that 35mm is used as the ‘standard’, this is the base factor from which we will work. In the photography world this is a piece of 135 (35mm) film, in the digital world this is “full-frame”, as a full-frame sensor is the same size as a piece of 35mm film.

Not all cameras are full frame, and there are also different size crop sensors, this is where crop factor comes in. A full frame camera has a crop factor of 1.1x (no crop at all), so a 50mm lens would produce an image with an accurate field of view. If you were to mount the same lens on a camera with a APS-C sensor with a crop factor of 1.6x, we need to multiply the lenses focal length by the crop factor to give us the “equivalent” field of view, in this instance the image would appear like it has been made with an 80mm lens (50 x 1.6 = 80).

Remember, the lens's actual focal length and aperture are unchanged, it’s just how it appears on a smaller surface area.