Dawes' limit is a formula to express the maximum resolving power of a microscope or telescope. It is so named for its discoverer, W. R. Dawes, although it is also credited to Lord Rayleigh. The formula takes different forms depending on the units.
| R = 4.56/D | D in inches, R in arcseconds |
| R = 11.6/D | D in centimeters, R in arcseconds |
| where | D is the diameter of the main lens (aperture) |
| R is the resolving power of the instrument |
