# Newton's formula

## Newton's formula

An expression relating the focal lengths of an optical system (If the optical system is a lens in air −

*f*and*f*′) and the object*x*and image*x*′ distances measured from the respective focal points. Thus,*ff*′ =

*xx*′

*f*=*f*′ and the formula becomes−

*f*^{2}=*xx*′*Syn.*Newton's equation; Newton's relation.*See*fundamental paraxial equation; sign convention;**gaussian theory**.Want to thank TFD for its existence? Tell a friend about us, add a link to this page, or visit the webmaster's page for free fun content.

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