The diffracation limit of a telescope
is a function of the wavelength of the light
and the diameter of the telescope.
This tells us the angular seperation below which two objects cannot be distinguished because the diffraction pattern caused by the circular aperture obscures the objects. The human eye can be considered as a primative telescope.
The formula is approximatly.
D = (1.22 x wavelength)/aperture
Taking the aperture to be 2mm and the wavelength to be 555 nanometers the calculation yields an angle of about 1.7 arcmin,
the best telescopes in the world aim for sub arcsecond resolution.