Mark Crabtree: you are very correct to state that focusing them is a problem with SLRs but some are better than others. - David Lyga
That's true. I've used some pretty good finders, but they are still dependent on the lens speed. I'm happy with my 35 f2.8 Distagon, but not with focusing it indoors. With a Leica, or similar, that isn't a factor.