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I am trying some exercises in optics and in my textbook (in the end of the book where are some unexplained formulae) i saw these:

$$ f = \frac {- f_1f_2}{\Delta}, \quad f' = \frac {f_1'f_2'}{\Delta}$$

  • It's something about the optical system (for many thin lenses), $f$ is of course a focal length, but what is $\Delta$, a length between $f_1$ and $f_2$ (and I am not sure what this means)? I have a language barrier and I haven't found anything in my language and I want to understand it.
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$\Delta$ is the distance between the two focuses.

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