Combination of Lenses
When we view an object through several lenses which are near each other, then it is possible to treat the combination of lenses as one lens.
Combination of Lenses
When several lenses with focal lengths f_{1}, f_{2},â€¦. arc near each other, then the combination has a focal length f_{total} given by

The quantity 1/f for a lens is called the power of the lens, measure in [m^{â€“1} = diopters = D]. This word power has nothing to do with the other definition of power, that is, energy per time. The point here is that the power of a combination of lenses is the sum of the power of the lenses.
Example
Dieter has an eye which, when the eye is at rest, focuses light to a point 0.024 m behind the lens, which is 0.001 m in front of the retina. What is the power of the corrective lens he must wear?
Solution
The power of Dieter's eye is P_{eye} = 1/0.024m = 41.67 D. The combination of lenses should have a focal length 0.024 m + 0.001 m = 0.025 m, so the power of the combination of the two lenses needs to be P_{combo} = 1/0.025 m = 40 D. Since P_{combo} = P_{eye} + P_{correct}, we have P_{correct} = â€“1.67 D.