A 4x5 focusing screen is big, and an 8x10 one is even bigger, but they are often a bit dim and so critical focusing can be a pain. Many photographers use a loupe to magnify the ground glass, but good ones can be very expensive. I've used cheap ones, which work fine, but it is yet another item to keep track of and worry about scratching in your pocket between shots.
So what about strong reading glasses? I have no idea why they aren't more popular for this purpose, but I've found them to be a great solution. You can find them on Amazon and eBay for very little cost, and at strengths of 4 or even 6 diopters. I guess they are often overlooked because they are only found in lower strengths in the local drug stores.
Here's why I like using reading glasses for focusing:
- They can be kept on your head when not in use. No need to fumble looking for a loupe in your pocket.
- Super cheap so you don't have to worry about them and you can carry spares.
- Compact and often come with a storage case.
There is a drawback, however. A traditional loupe is placed directly on the glass at a fixed distance, but glasses have the extra variable of eye-to-screen distance. I find that I have to focus on the etched lines on the ground glass first to know I am focusing on the correct plane.