How is the success of a cataract operation measured - how well the implanted IOL has settled into the capsular bag, or the difference between the pre-calculated and actual visual acuity or something else?
In general, a patient measures success by how much better he or she sees afterwards as compared to before.
People judge their new, post-cataract surgery vision by comparing it, consciously or not, to how they were accustomed to seeing most of their life. For example, a patient may have been quite nearsighted all his life. A side benefit of reducing or eliminating that nearsightedness with the new lens implant used in cataract surgery will often be appreciated as much (or even more) than the actual elimination of the cloudy natural lens!
For somebody with a mature, severe cataract, almost any lens implant used will make him please with the results.
Clinical trials and refractive cataract surgery require even higher standards of success, as based on the elimination of all refractive problems as well as visual acuity.