Simon- I have decided I will go for the new CZ16-35mm. I have tried the old Sigma 18-35mm today and the 10-20mm which is not ff- but nevertheless gave it a try on the A900 in cropped mode (11mp). Unfortunately I tried it against the CZ 24-70mm @ 35mm and there were worlds between them- even closed down to f8/ f9.
I might not buy the CZ16-35mm straight away as I really want to see how often I use the wider end of my lenses. 24mm of my current CZ is already pretty wide for landscape and I am not sure how much wider I really need (not want
I assume very much that Sony will have to come out with some really good lenses in the future particulalry if they come out with more ff cameras. In all honesty- I think when you deal with this kind of resolution you should really go for the very best there is.
Anyhow- let's wait and see what happens at the PMA soon!