I play regularly with a 67 year old veteran USTA certified instructor who has serious vision problems but is still a solid 4.0. He can see the lines on his side of the court pretty well, but he will honestly (I believe him anyway) ask, "was that out?" when he, or more commonly his partner, flies one three or four feet out. When I play him I just call everything.
Pro Kennex 7G/Ki5X
Babolat VS Gut/TF Multifeel 16-Kirschbaum Proline II 17