^Combination of lenses

^Combination of lenses

Two or more lenses are combined to

  1. Increase magnification.
  2. Make final image erect.
  3. Reduce certain aberrations.

If two thin lenses of focal lengths f1 and f2 are separated by a distance d then the effective focal length and power of the combination is given by:


2. P = P1 + P2 – d P1 P2

3. m = m1 x  mx  mx – – – – – – – – –

Let us prove the above relations.

* A Zoom lens is based on this result such that f can be varied 0 to ∞ by changing d.

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