Originally posted by newarts Ben I think the diffraction limit of resolution is a function only of F-Number, ie. (1/(angle subtended by aperture)) times wavelength.

Resolution is not defined by the f-number (relative aperture), but by the real or effective aperture or entrance pupil (which is all the same). In astrophotography, which is the subject here, you usually talk about a fully open aperture (as scopes don't have a diaphragm), but ofcourse the basic formula applies to any lens:

res(angular in arcsecs) = 1.22 (W/D) 206265"

W = Wavelength of the light

D = Diameter of the lens (or entrance pupil)

the absolute numbers are only there to normalize for arcsecs.

Another, even simpler rule would be: res = (115" * 1mm)/D

this is normalized to mm as the measure for the diameter and the wavelength ids placed around 560nm through the fixed factor

Anyway: both lenses describe the "diffraction limited" angular resolution of a lens. The smaller the diameter gets (in a photographioc lens this would be to close the aperture) the bigger the resulting number gets, which is nothing else, but a shrinking resolution.

As you see, we are talking not about the f-number, as the focal length does not come into play. We are only talking about effective diameters of the lens.

If you substitute "aperture" in your formula with "diameter" it is correct.

Ben