The resolution of lenses has been deliberated over the years. John William Stutt, 3rd Barron Rayleigh (English scientist Astronomer Royal 1812 – 1842 Nobel Prize 1904). He published the Rayleigh Criterion, the theoretical resolving power of lenses.
This study, and method is still valid today. The lens is caused to image ruled lines. The width of the lines and the spaces between are the same. The lens is rated based how the lines are resolved.
The resolving power (R. P.) of a lens in lines per millimeter (lpm) = 1392 ÷ f/number
This is the value for green light chosen for photographic lens evaluation.
Table of R.P for wavelength 589 millimicrons as to resolved lines per millimeter.
f/1 = 1392 lpm
f/2 = 696 lpm
f/2.8 = 497 lpm
f/4 = 348 lpm
f/5.6 = 249 lpm
f/8 = 174 lpm
f/16 = 87 lpm
f/22 = 63 lpm
The resolving power of any lens decreases as the lens is stopped down due to diffraction. This is due to the light rays passing in close proximity to the blades of the Iris diaphragm (aperture). These close passing rays bleed into the shadow regions and into the path of light that clears the aperture. The result is the boundaries become less clearly defined.
Note: A lens operating at apertures f/8 and larger exceeds the resolving power of pictorial film.