The S/N logic only applies if you want to get the largest capture of light by shooting as wide as possible. Its not terribly applicable to landscape shooters, for example. At
comparable apertures, in terms of the apparent depth of field, the 645Z is well ahead.
Most of the 645Z's new lenses have shake reduction in the lens (28-45 and 90mm). It would seem likely that forthcoming lenses will have in lens SR too. Sure, for handheld work the smaller FF retains significant advantages, but lens quality remains an issue with the smaller format at wider angles. I hope the Batis lenses will resolve this with the Sony line, but I have yet to use a wide on anything aside from a Leica M that offers the same quality as the 28-45mm DA.
Originally posted by Simen1 I know. It was a rhetorical question to stevebrot that said the reason to buy 645 was "Better S/N ratio and DOF (you know the usual stuff)." Actually you don't get better DOF and S/N ratio on 645 because it doesn't have as large apertures available as FF will get. 645 also lacks shake reduction. So in my opinion the FF offering will have key advantages over 645. On the other side 645 will offer a different package with better resolution as the main selling point. Even f/1,4 lenses stopped down to f/2 cant resolve as much as the 645 on f/2,8. Or at resolution optimal apertures for that matter.
---------- Post added 06-27-2015 at 07:19 PM ----------
I think #1 and #2 is good guesses, but #3 might hold some genuine Tamron knowledge.