Lenses have published MTF specs which you can fudge into optical resolution specs. It's a bit subjective, but it works. (MTF curves carry a lot of information, and squashing it into a single lp/mm resolution means drawing a cutoff line on a graph which is inherently curved.) Sometimes you publish a couple different resolution values, for different contrasts. The actual MTF curves are more objective and directly reflect system measurements, which is why those are the ones published, instead of resolution specs which are easier to interpret. You can do the tests yourself at home fairly easily.