# Optical resolution

By analogy with optical observations, e.g. in astronomy, the term optical resolution in structure determination,$$d_{opt}$$, is used to describe the expected minimum distance between two resolved peaks in an electron-density map.

$d_{opt}=[2(\sigma^2_{Patt}+\sigma^2_{sph})]^{\frac{1}{2}}$

where σPatt is the standard deviation of the Gaussian function fitted to the Patterson origin peak, σsph is the standard deviation of the Gaussian function fitted to the origin peak of the spherical interference function, representing the Fourier transform of a sphere with radius 1 / dmin, and dmin is the nominal resolution.

### History

The  term  was introduced into the protein structure determination literature by Vaguine, A. A., Richelle, J. & Wodak, S. J. [(1999). Acta Cryst. D55, 191-205SFCHECK: a unified set of procedures for evaluating the quality of macromolecular structure-factor data and their agreement with the atomic model].