The mathematics is easy. A lens power in dioptres is 1 divided by its focal length in metres. So a 300mm lens is 1/0.3 = 3.3 dioptres.
Originally Posted by Tempest09
If you have two lenses and want to work out their combined focal length when placed together, just convert to dioptres, add them together then convert back again:
e.g. a 300mm lens and a 400mm lens together. We already know that 300mm = 3.3D. 400mm = 1/0.4 = 2.5D so the combined power is 5.8D.
To convert back to focal length, 1/5.8 = 0.172 metres or 172mm.
Another rule of thumb is that a 1 dioptre lens will focus parallel beams of light at a point 1 metre away. Double the power to 2 dioptres and the focus point halves to 0.5 metre, etc.