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Oh, yeah, a good focusing screen is the best. Have you used an 850 or 900 with the M screen?
I wish I could, but never have the chance. None of my friend owned A850 or A900. If the alpha mount has shorter flange focal distance as EF, I'd consider buying one. To get every lens Leitaxed costs a fortune.
Actually A900 has longer flange focal distance than EF, so all Leica R lens need to be leitaxed. Certain Sony Alpha lens can be converted to EF, but not reverse as far as I know. Sony is bit easier way than Nikon, but not that easy as Canon EF (which for most cases needs just good adapter).
I think that was phuang3's point. If Sony had the shorter register of Canon, he'd be more interested because he wouldn't have to Leitax everything.
And Canon certainly makes it easy to use the simple adapters, but the FF mirror clearance issues with some of the best in the R line is not a problem the a900 has.