# Image distance for a thin lens

For a thin lens we have:

$$\frac{1}{p}+\frac{1}{q}=\frac{1}{f}$$

Where $p$ is the object distance, $q$ is the image distance and $f$ is the focal point of the lens. Having worked with lenses a little bit, I was wondering how the image distance is defined. I know that for a lens, if you stick a screen anywhere after the lens, you can get some sort of image to appear on it. Is the image distance the distance where the resolution of the image is a maximum, or is there a different way of defining that distance?