What Bob said.

One quick thing you can try is using the split prism to focus on something at infinity. If the lens and the split prism agree, you're probably good. If you notice a discrepancy, be aware that there are many possible causes, including the lens, mirror, focusing screen, diopter, and your own eyes!

Personally, when I get a picture that’s out of focus or otherwise unsharp, it’s almost always because of an error on my part.