I thought I would post up some observations of the 5DIII. I bought this camera with a trip to the arctic circle in mind, aiming to photograph the Aurora Borealis (northern lights). We were in northern Finland, well into the arctic region for a six day trip. We were lucky to see amazing light shows on numerous occasions.
For me this trip was about learning the camera in extreme lighting conditions: landscapes covered in deep snow with bright sunlight, and night skies with no light or other pollution.
We took a 5DIII (and Three Legged Thing carbon "Eddie" tripod) with L lenses of which the one that actually got used was the F4 24-105. Plus a 40D and a Fuji point and shoot. Used a Lowepro Photo Sport 200AW Backpack, which was fantastic: invaluable in the conditions and very comfortable in use (trekking through deep snow). Used for all cold to hot transitions (didn't bother with plastic bags against condensation) and had no issues at all. Highly recommended.
Daytime temperatures ranged from -10C to -20C. Night temperatures from about 10pm onwards were rarely above -30C and the coldest night was -37C outside our cabin, so probably around -40 or so in the open. Hardly any wind, so very little extra wind chill.
The Fuji froze at -25C: the snag with such cameras is the metal screens which protect the lens will freeze very easily, as will the buttons. Despite this the camera coped extremely well with daytime shooting in very bright conditions when you would expect the sun and snow combination to create exposure problems.
The 40D survived lower temperatures but the enthusiast grade USM zoom was not happy and we had some focus issues occasionally as a result.
The 5DIII was a revelation. We were out with a group on a frozen lake covered with about 2 feet of snow (skidoo tracks were found for the tripod sessions) and the 5DIII kept on going pretty much whatever happened. At -37C or below out in the open I was taking 15 to 30 second exposures of the northern lights. Battery life was excellent. The cheap Canon remote release worked perfectly. The 3LT tripod was excellent (though the legs froze eventually, so it could not be collapsed). Eventually I began to get a problem with the info button as the info screen would come up when required, but then could not be turned off again. This problem persisted even when I removed the battery and replaced it: so I expect the button was frozen. Camera still worked though.
Other photographers with us had problems. Canon 650D would not work at -30 or below. New Canon 6D with L lens froze at minus 30 as well (not helped by the owner breathing on it a lot!). Nikon D800 for some reason seemed to be working but just gave smudges of light rather than proper shots. This may well have been operator error. Live view on the Nikon seemed to be useless most of the time at very low temperatures, but again I suspect that this was a photographer with all the gear and no idea.
We got lots of AB shots but I have learned a lot and would do better next time. Lessons learned:
1 Buy a faster lens. I wished I had had a better and faster lens with me, especially as we had lots of portrait and animal opportunities where I would have liked to get much shallower DOF. I think I will buy a fast portrait lens: any suggestions?
2 Avoid groups. People tramp about and the slightest vibration affects your tripod when they come close! And they always come close becuase they want to see what shots you are getting, exs[ecially if their camera has frozen!
3 Practice in advance with high ISO shots. I ended up pushing ISO much further than I had expected, largely because I wanted less star trails and less vibration risk. This was my first ever experience with photography in almost total darkness and I learnt a lot. Made quite a few mistakes.
4 Get a low power head torch. Our bright LED torches were far too bright! You need to check your camera settings (especially focus on the lens) but you don't want to wreck your night vision or disturb others.
I invested in special gloves as it was essential to wear them all the time when using the camera at night.
In summary, the 5DIII is quite an amazing bit of kit. Very usable in extreme conditions. We used it extensively for video too (including whilst riding skiddoos and dog sledding) and it gives amazing results. My next challenge is learning how to use the autofocus capabilities better.
Adrian