Some caveats from an eye doctor... for single vision (that is, distance or reading, but not both) with little or no astigmatism, it is a simple process to get the Rx correct. If the lens power is from -4.00 to +4.00 in the “sphere” you don’t need to sweat the PD. Use 62 for distance or 58 for near. (See Prentice’s Rule if you can to calculate the induced prism, but take my word for it, it’s nominal and not relevant if your PD is off by 3-4mm). As you get over 4 “diopter” of power two factors start coming into play. First, PD becomes increasingly important. More important is “vertex distance” which is the distance from your eye to your lens. Most prescriptions are written with an assumed vertex of about 13mm. But the “effective power” of a lens changes with distance. Think of a magnifier lens — as you move it away it magnifies more. Same effect. So your Rx will not be correct if your higher powered lenses are too close or too far from your eye. That’s a function of how the frame and the nose bridge fit.
As for bifocals, two issues. On traditional flat top (FT25 or FT28) the issue is the height of the bifocal. Too high puts it in your line of sight and will annoy you a lot. Too low and it’ll be hard to get your eyes into the reading area.
More commonly today people get progressives. These are challenging to fit correctly in the office, much less mail order. And their are a dozen or so different designs that vary, with some being much better for some people than others. They vary by width, rate of change, and the nature and amount of peripheral distortion. High end progressives have far less distortion. Critical in the fit is ensuring that your pupil is directly over the center of the progressive zone. this requires a measure of your -monocular- PD, the distance from the center of your nose to the center of each pupil. Few of us are symmetrical. It may be 30mm in one eye and 33mm in the other. If not precisely measured and made the progressive glasses will be anywhere from suboptimal to unwearable. However, even if measured and fit correctly, if the TYPE of progressive is not right, it will also be from suboptimal to unwearable. Computer use requires a very different type of progressive than reading a book, for instance. So, buyer beware on progressives!
