Dawes' limit is a formula to express the maximum resolving power of a microscope or telescope. It is so named after its discoverer, William Rutter Dawes,Dawes, W.R., Catalogue of Micrometrical Measurements of Double Stars. In: Memoirs of the Royal Astronomical Society, Vol. 35, p.137 1867, 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 arcsecondsR = 116/D D in millimeters, R in arcsecondswhere D is the diameter of the main lens (aperture) R is the resolving power of the instrument
This formula agrees with the usual at a wavelength of about 460nm, somewhat bluer than the peak sensitivity of rod cells at c. 498nm.
See also Rayleigh criterion
References