# Depth of Field needed for Fraunhofer Pattern

1. Apr 14, 2010

### rem2722

Say we define Depth of Field, DOF, as max de-focus allowed for decent Fraunhofer Pattern. Then what's the DOF of a (positive) lens with focal length f?

2. Apr 14, 2010

### Andy Resnick

The DOF is not a well-defined property of a lens, since perfect focus is achieved only at a single plane, and all other distances contain defocus.

However, some 'rules of thumb' exist, similar to the Rayleigh criterion being satisfied if the aberrations are less than 1/4 wavelength. The DOF is set by the f/# or equivalently the numerical aperture of the lens, and in the paraxial limit is given by DOF = 2* wavelength * n/NA^2. For larger numerical apertures (i.e. microscopy), the formula is slightly modified.

http://www.microscopyu.com/articles/formulas/formulasfielddepth.html