Why on earth a DT ? An inexpensive 50 is a fine thing to have in the
lineup for both FF and APS-C users; not an ideal FL for APS-C where
it's neither "normal" nor "portrait" ... so why limit it to APS-C ?
I get this. The Sony 50 1.4 isn't exactly expensive anymore - the
price has come way down. If you want an a900, which is a lot of
money, you simply spend a bit more on your 50. Considering the fact
that you've already spent a lot more on making sure that your other
glass and your PC are up to the job, cutting costs for the benifit of
APS-C users is fine by me.
My final thoughts there ... the FF user probably doesn't mind
spending a bit more on the 50/1.4 to get the brighter VF image &
bayonet shade ... but a new 50/1.8 could be an easy match for the
50/1.4 and the 50/1.4 is awfully soft wide open, so the slightly
brighter VF image and the bayonet shade (and maybe use of 55mm
filters that you might already have) are the only reasons to go for
the more expensive lens.
My bigger objection, though, is if you're going to make it DT, why
not make it a 60 or 70mm lens ? (Or gives us that 35/1.8