# Optics Equation

I keep seeing this equation pop up and I have no idea what it is or where it comes from.

$$n_{1}f_{1}= -n_{2}f_{2}$$

It looks like a take on snell's law, but what assumptions have to be made to get to this equation from snell's?

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## Answers and Replies

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The formula is related to an optical aberration called field curvature. See attachment.

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Thank you.