Steve,
you simply have to (at least when Canon DSLR's are in question)..
The image in finder is almost totally useless for focussing Unless you use RA microscope for that).
From time to time I am using modified 50mm F1.4 FD on my EOS (it is manual lens), and I needed to apply focus confirm circuit (
http://cgi.ebay.com.au/Focus-Adjusta...item35a797c77f), otherwise it is no go.