The principal is called "Dawes' Limit". This determines the "resolving power" of the lens... but in arc-seconds (an angular measurement.... e.g. degrees, arc-minutes, and arc-seconds.
Assuming perfect optics ... no flaws ... it's the diameter of the lens that determines it's resolving power.
But since focal ratio for a lens is the focal length divided by the diameter, the lower the focal ratio, the larger the physical diameter.
So for example... Canon makes an EF 200mm f/2L lens. "f/2" at 200mm means the physical diameter is about 100mm (that's 4").
Compare that to say... a 55-250mm f/4-5.6 lens which, at 200mm is f/5.6 (200 ÷ 5.6 = 36mm).
The "f/2" version of the lens has nearly 3x times the aperture as compared to the f/5.6 lens ... and that means that assuming the optics in both lenses are good, the f/2 lens will be able to create a much sharper result.
This is related to diffraction-limited optics and Releigh Criterion: https://en.wikipedia.org/wiki/Angular_resolution#Explanation
One interesting bit about this is that it's possible to have a sensor whose pixel sizes are smaller than the resolving power of the lens ... which basically means it is wasted sensor resolution.
Keep in mind that this is all based on laws of physics, the wave nature of light, etc. and all based on the assumption of perfect optics.
Thank you, yes I see what you are saying and I have heard a couple that do reviews keep mentioning "the useable megapixel" of the lens. where it might only be able to I guess resolve 75% of the sensor.
Here is a link to guy that seems to have a large enough brain to understand all this, I just want sharp bird photos 100+ feet without breaking the bank and well, my brain is mostly mush.
I get what they are saying (sort of) but I cant relate it to which camera and lens to purchase. If I spend $2000, is it better to get a high pixel count sensor and a 200mm lens then crop (like birds, not printing anything, just show my photos on the tv) or get a better 300mm lens with a camera that has fewer pixels (but maybe a larger sensor)?