i had a 17-40mm that i used for many things including star shots. it was okay. then i replaced it with a 16-35mmv2. it was better only because of now i was at f2.8, and it was noticeably(just) wider. i've since sold it and now have a pro-optic 14mm. I haven't gotten a chance to do much of anything with it yet, but i'm hoping to give it a go with the night sky over the christmas holiday.
@risc32: Thanks for sharing your experience with the 17-40. I am pondering on the Sigma 12-24 F/4.5-5.6. I know it is very slow but it seems attractive by its lenght (24mm, good for 500/600 rule photography 600:24 yields 25 seconds of exposure, and at the wide end you are at F/4.5). Can it compensate a 16-35 2.8 wide open? My current 28 F/2.8 does pretty well, although it is quite narrow. As I have never stopped it down to 4.5 I am asking a kinda silly question...Sorry. I'll try to test it out during christmas holidays to get to a better conclusion, but your comment would be welcome anyway. Thanks and cheers, Pedro.
Here's a sample of my Canon 28 F/2.8 with the 5D3 @ISO 10.000 F/3.2 25s exposureZ96A0762aALTxMasterKLEINDEF
by Peter Hauri
, on Flickr