[petrochemist]

With my old film SLRs manual focus was the only option, and was easy to achieve. Unfortunately more modern cameras assume you are using autofocus, so do away with the focus aids (split diagonal/microprisms) , many also give you a cheaper-darker-smaller view finder which makes manual focusing more difficult.
(My recent upgrade improves on matters here a penta-prism is significantly brighter than the penta-mirror in my older DSLR and I'm tempted to fit a third party focus screen giving me the other focus aids).

I have had a fair bit of practice manual focusing with my older DSLR, and whilst manual wasn't quite a good as the autofocus in good light, it could prove better in challenging conditions (where the camera tended to hunt excessively).

The autofocus circuitry can generally be used as focus confirmation on manual lenses, even working as catch in focus where the camera shoots when the lens gets into focus - a benefit with telescopes. I have found that some of my macro teleconverters don't give sharp enough images at infinity for the focus confirm to ever see them as focused...

My current autofocus lens collection maxes out at 300mm, whilst workable MF options have certainly taken me to at least 1000mm though the contrast has dropped off due to the cheap glass.

With the super wide angle lenses focusing is not too much of an issue. My 10-17 can almost focus from 10mm to infinity when stopped down at the wide end (it needs care not to scratch the lens on anything only 10mm away)