Optivisors and similar magnifiers are a two-edged sword; the magnification can be a bit OTT sometimes, difficult to judge and the black plastic above can cut down the available light. Try just getting more light on the subject - or a hands-free magnifier with or without a built-in light (e.g. Lifemax or Draper). T'other option is to get some cheapo pound-shop +4 reading glasses, but they don't suit everyone.
I got one of these big magnifiers on an arm that could be fixed to the table. For very very small parts they are usefull. No sweat or pain at the forehead. It has lights that give perfect illumination on the spot. But I rarely use it, it's most time stowed away. Fortunately my eyes are still good enough to work without glasses or contact lenses. With 48 363/365 years I fear that will change in the near future.
I have enough old brushes that are not good enough for "real" painting but good enough to get some paint into the difficult places.