Optics Equation: What Assumptions Lead to n1f1=-n2f2?

[Prologue]

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?

 

[Bob S]

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

 

[Prologue]

Thank you.