# 04. What is the basic formula for the conditions under which an image is in focus?

## Description

This article is from the
Photographic Lenses FAQ, by David Jacobson with numerous
contributions by others.

# 04. What is the basic formula for the conditions under which an image is in focus?

There are several forms.

1/Si + 1/So = 1/f (Gaussian form)
(Si-f)*(So-f) = f^2 (Newtonian form)

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