**Scott_online** wrote in post #13850218I worked it out as the aperture at which the diameter of the Airy disk exceeds the pixel dimensions. For green light that's around f/2.8. How did you get f/8?

The diameter of the first ring of the Airy disc is given by the equation:

sin θ = 1.22λ/d, where θ is the angle of the divergent rays after diffraction, λ is the wavelength of light (520nm in the case of green light) and d is the diameter of the aperture.

The D800's photosites are 4.9 microns, or 4900nm across. Therefore, trigonometry gives us, approximately (since θ is small and there won't be much difference between the focal length and the hypotenuse of the focal length and radius of diffraction):

tan θ = 4900/F, where F is the focal length (in microns).

Therefore:

sin θ = 4900/(√(F^2)+(4900^2))

Since θ is small, for ease of calculation, we can approximate this as:

sin θ = 4900/F = (1.22 x 520)/d

F/d = 4900/(1.22 x 520) = 7.72, or an f-stop of f/7.72 as the diffraction limit for a single photosite.

Of course, this is just for one photosite - a pixel takes information from four different photosites, which effectively doubles our acceptable Airy disc diameter to 9.8 microns, giving us an effective diffraction limit of f/15.4.