G Ashton... I am also an available light freak. When it comes to low light digital ... Canon rules the night like a v&ire drinking light.
ISO 1600 is a breeze for both the Canon 1DMKII and 1DsMKII with f/1.4 and f/1.2 lenses mounted ... and ISO 3200 is actually usable. Using L lenses like the 35/1.4, both cameras can AF on a black cat in a coal mine at midnight. These are the two cameras I carry to shoot weddings ... where I have no alternative but to get the shots all the time, every time.
But our newly discovered use of Zeiss glass on the Canon 1DsMKII is a revelation of clarity and beauty. It is harder to focus these manual lenses on the 1DsMKII compared to the lightening fast AF of the Canon lenses, but the sacrifice is worth it for some work. It can be seen as extending the investment in such an expensive digital body by providing even more options to achieve desired results. BTW, I recently acquired a Novoflex Hasselblad to Canon EOS adapter to use those MF Zeiss lenses on the 1DsMKII also.
While in NYC for just two days, I carried a Zeiss 25/2.8, 60/2.8 Macro, & an 85.1.4, (thought of mugging DJ for his Zeiss 35/1.4 ; -)... and never resorted to flash while shooting 5.56 Gig of RAW images day and night !!!
I am processing the NYC shots now, and while I did miss some shots due to sloppy user errors, the over-all impression is... well ... WOW!!! Zeiss Rocks on the 1DsMKII ... a digital camera able to really show what these lenses are capable of.
Here's a shot taken on a dark side street in NYC. The light is what you see and nothing else. I set the 1DsMKII to ISO 3200 AND set the compensation to plus 2! (I was using the Contax/Zeiss 60/2.8, and would have killed to have DJ's 35/1.4). I ran a simple noise reduction plug-in to clean it up a bit, but not so much as to compromise sharpness. on the full resolution file I can clearly read the "ring bell" sign on the wall behind the girl smoking a cigarette...