All about crop factor
Most photographers are used to the 35 mm. cine-film and the frame/image size of 24 x 36 mm. Since cameras are produced with other frame sizes the focal length of a lens may change, although the focal length on the lens body still refers to the 'old' 35 mm. size.
Nikkor lenses made for film cameras and/or for digital cameras are marked with a focal length based on the 35 mm. frame size. It may sound strange, but also DX-lenses - specially made for digital cameras with a DX-sensor - are marked with lens data referring to the 24 x 36 mm. frame size! The viewing angle of these lenses is - however - differing from those of the lenses made for 35mm. photography.
Let's take a so-called standard lens - made for 35 mm. photography - which has a focal length of 50 mm. as an example. Mounted on a 35mm.-film camera this lens will have a viewing angle of 46°. When mounted on a APS-camera the viewing angle will be around 36°48'. And on a DX-camera the viewing angle will shrink to around 31°30'. When mounted on a Nikon 1-series camera of that 46° only 17° remains!
Why?? It all has to do with the exit pupil aka Ramsden disc.
Each lens will produce a circular image. That circular image has to cover/include the image frame. A film frame size of 24 x 36 mm. has a diagonal length of 43.26 mm. So, to cover the entire film frame a particular lens has to produce/project a circular image with a diameter of at least 43.26 mm. This projection is called the exit pupil or Ramsden* disc. If the frame size is larger vignetting will occur. If the frame size is smaller the viewing angle will decrease. The factor of decrease is called 'crop factor'.
When we take 35mm. frame size = 1, FX = 1, APS = 0.8, DX = 0.65 and CX = 0.37. The other way round: the focal length - referring to an 35 mm. equivalent - will increase by a factor of: FX = 1, APS = 1.35, DX = 1.525 and CX = 2.7.
A standard lens with a focal length of 50 mm. will give a viewing angle equaling a lens with a focal length on an FX camera as 50 mm., on an APS camera as 67.5 mm., on a DX camera as 76.3 mm. and on a CX camera as 135 mm.
For those using a tele-lens this 'longer' focal length may be an advantage, for those loving wide-angles it is a pity.
* named after the English instrument-maker Jesse Ramsden (1735-1800)