Originally posted by UncleVanya My reading on the subject leads me to think that they are hard to nail focus with because mirror lenses have thin depth of field as per their focal length - and lack autofocus, and often (newer ones perhaps more than older ones) have low precision focus rings that don't have enough throw and are poorly dampened. Also given the slow aperture and the low degree of focus screen contrast with standard dslr screens focusing can be hit or miss. But I'd listen to those with direct experience and ask them if these factors cause problems or not.
I wish a few compact mirrors were out and autofocus. The only af one I know of is a Minolta that works on Sony A mount I think.
None of the focus screens are optimized for f8 low contrast lenses so they are all bad in their own way. The stock screen is reasonably bright because it's designed for slower lenses. The low contrast makes seeing the focus point hard. I guess I'd describe it as no particular help in focus, but no big downside. If you want to use a TC, it's probably the best option because the other types have blackout/darkness trouble. The EE-S style screen gets really dark over the whole surface, and again, the focus point is not obvious. I only tried this one briefly. The split prism will show you a point of focus or sometimes black out. You can often adjust eye position slightly to fix the blackout, but it can be fussy.
Live view is not great either. I usually get mine dusted off for moon shots, and by the time I decide focus is sort of OK, the moon has slipped out of the frame.