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I need to know if there is a way to put lenses in front of each other to make their focal length shorter. I tried putting them directly in front of on another, but I wasn't sure if there was a better way. I don't know anything about lens physics. Thank you in advance!

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  • $\begingroup$ one way is change the refractive index of medium they are in .Google for lens maker's formula $\endgroup$ – Who Jun 7 at 19:35
  • $\begingroup$ Look up telecompressors or focal reducers. They are fairly common on amateur telescopes. $\endgroup$ – D. Halsey Jun 7 at 19:57
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If you put the lenses very close together and if the thin lens approximation also holds, then their dioptric strengths (inverse of the focal lengths) are approximately additive. If they're not very close together, then in general the combination will not show behavior that's equivalent to that of any simple lens.

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