You shouldn't be trying to make out the rings when aiming, not even making out the bull; that's why it's often referred to as a distraction bull!
With a pistol you should concentrate only on keeping your sights aligned and your aim is an "area aim", usually somewhere below the black, as when too close, the sights and the black of the target blend together.
Many pistol shooters use a "sub six" aim, ie. just under the 6 line. Personally I prefer sub 3 or even 2 as then the distraction bull is out of my line of sight/concentration. It's all personal preference; there is no "right way".
If all this sounds strange, try setting up a blank sheet of paper, aim generally sub 6 and you will be surprised at the groups you can make. Imagine a target superimposed on your group and think what the score could have been. Pistol targets are for scoring, not aiming!!
Regards glasses, if you can see the sights clearly and the target is blurred, you don't need them, only maybe a blinder for the other eye. If you need to try glasses, a good starting point is 1/2 diopter less than your normal "reading" strength. I use off the hook readers with a +1/2 lens in the right side and the left side whited out. Total cost +/- £15.
Walther CP-2 Match, FAS 604 & Tau 7 target pistols, Smith & Wesson 6" & 4" co2 pistol, Crosman 1377,
Baikal IZH 53 pistol, Gamo CFX Royal,177, Umarex SA-10 CO2 pistol.