I love to use the Holga and Diana cameras. Focus is pretty imprecise with these cameras in that you guess focus, and they have more depth of focus than regular lenses. I use all formats, but have made my best images with the Diana, partly because I use it more and partly because of the feel of the images.
Keith Carter's situation might be informative. In this video, the pertinent part starts at about 4:00.
Personally, I am growing cataracs, as many do. So far, only the right eye is really affected, but, of course, that is (was) the eye that I used to focus. So, I've had to make adjustments. I learned to focus with my left eye, and that will work until ... At some point, there will be the surgery, and then (hopefully) I'll see better than I have in most of my adult life.
The only adjustment I've had to make in the darkroom is to sometime use a binocular visor (Optivisor) http://www.doneganoptical.com/products/optivisor to look at 35mm negs on the light box. As bad as the right eye is, my overall vision is still better with both eyes than just the left eye alone.