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Your refraction is one number, so 20/20, 20/40, 20/30. What that number indicates is the smallest black and white letter that you can correctly identify. The problem with that number is it only measures one size of cells in the visual system that are processing what we see and what we know, especially since the 1960s from vision science, is that our visual system is made up of cells tuned to different ranges of sizes or frequencies just like the hearing system is tuned to different ranges of sound frequencies and why the audiologist had to go to pure tone testing of different sound frequencies because these cells are independent in the hearing system. We know the same thing today in vision. Only the difference between the hearing system and vision is in vision we have two-dimensional cells, but these are functionally separate. So even though I have a measure of how well a person can see a tiny black and white letter, it tells me that size cells are doing something, but it is telling me nothing about the other size cells and what we know, from actually research that I did and others since, is that it is the part of the contrast sensitivity curve that really never gets measured by Snellen that is very important for how we actually see in the everyday world.