FX, DX and foc al len gth
The size of the sensor in your camera makes a difference
Nikon’s ‘full-frame’ FX sensor format has the same dimensions as a frame of 35mm film and when focal lengths are described as being standard, wide-angle or telephoto, it’s this format that everyone refers to. However, the same focal length on a smaller-sensor DX camera behaves as if it were half as long again. So, a 28mm wide-angle lens on an FX camera would give the view of a 42mm focal length lens on a DX camera – not really wide-angle at all. Look at this the other way around and to get the equivalent of that 28mm wide-angle view, you’d need 18mm. Hence the term ‘equivalent focal length’ that’s often listed in a lens’s specifications.
A DX sensor has a ‘crop factor’ of x1.5. Multiply the focal length of the lens by this amount to discover the ‘equivalent focal length’ – or the one required to give the same view as with an FX sensor