It may surprise anyone who reads certain posts to discover that whilst I may well own and get to do all sorts of things, I live on an income that according to Government tables is below the poverty line, the tehnical term they use being, "Destitution".
Obviously, I either do crime, claim benefits, OR am unusually resourceful. A means test I undertook at the C.A.B. Confirmed that it is of course, the latter as I do not do crime for my money, nor do I qualify for benefits. After filling in a big questionaiire designed to show where I could manage my money better the only thing they could recommend was that I have my six monthly haircut done cheaper by students. (So I bought a set of clippers from ebay using my O/H's account that were as cheap as a single haircut, and apparently they do an acceptable job.)
One thing I do not do is go to spec savers at 75 quid for ONE set of specs, oh no, I of course do my own eye test at the reading glasses stand in B&M!
I work though the pairs using the little numbers printed on them until I find a pair that gives me the best view right across the store, then setting those aside, I work through the pairs looking at the small print they give you to look at and find the best pair of those, IF there are choices of frame in those sizes, then using the mirror I pick the best looking frames for the reading glasses, and the best frames for the driving glasses, and I get two( or if I am flush and they have them, three) pairs of each. I just bought a year or twos worth of specs for under a tenner...
This only works for age related steady degeneration happening equally across both eyes, and opticians can measure down to a quarter dioptre wheres cheap specs come in half dioptres but I've had 30 cheap years of cheap specs wearing, and when my G/F's curiosity got the better of her and she MADE me have a proper eye test, about ten years ago, my figures were within a quarter dioptre of their prescription.
The effort of trying different specs and deciding which ones work best, I feel gives one a reassuring sense of knowing what works and what does not, as well as saving me a shedload of money. on a 3x2 inch cutout in a panel (or similar) with my regular glasses on I can apparently clearly see a discrepancy of 5 thousandths of an inch which the guy who made it needed to measure before he'd quit arguing.
If you have more complex problems then you will need a more specialist approach, but for most older people this is an opportunity to never be without specs again, but not being bi- or vari-focal, you always need to have at least the two sizes.
You really often do NOT need the services of a professional even with vitally important stuff, like your eyesight. I've been at this thirty years and in that time I've lost a half dioptre per decade I.E. my prescription would have changed three times, but I've enjoyed much better "spectacle availability" and done way less hunting for my specs than most people who do it "properly".
Also since shopping for aesthetics now costs 2.99 a pair maximum instead of 79.00 minimum I can and do buy the occasional pair on a whim, which makes the availability aspect even better and occasionally you get a really cracking nice durable frame with a really nice lens that last 7 years or like my best "long focus" ones currently have.
Just because I'm destitute and off grid, does not mean I need to suffer a reduced visual acuity at all! You don't need a degree to be able to tell whether your eyes are working correctly, nor do you need a degree to evaluate whether B&M glasses will work for you.
Of course wearing incorrect glasses will not be good for (or even damage) your eyes, so you make damn sure that you don't wear the wrong glasses, by testing which ones work for you...
Ker-ching!