« on: January 05, 2013, 06:48:43 AM »
If you need a wide zoom for the 7D and you dont want to spend the $300 extra for the canon 10-22, then the Tamron 10-24 is a pretty good deal. You wont be able to change the aperture mid exposure (not too sure why you would need to either), but there may be a workaround depending on what you are doing. I do a lot of long exposures underground and one thing that I find to be very useful is screen stacking in photoshop. it allows you to combine multiple exposures as if they were all shot on the same frame. so if you had two 30 sec exposures, you would stack them and end up with the equivalent of a 60 second exposure with the content from both frames. if you needed two different aperture settings for the same photo you could shoot them to different files and stack them later in photoshop.
Just about any lens out there is going to be sharp at f/5.6. The Tamron is not weather sealed, but I have been pretty lucky with it so far. Most of the places I bring it to have really fine sand. I just do my best to avoid getting sand on my camera and I haven't had any problems in the 14 months that I have had the lens.
In my opinion, the 17-40 would not be wide enough for a lot of light painting situations, assuming any of that is going to take place in a tight space, such as underground or in a building. It is also not dust /sand proof without a UV filter on, so could on spending another $60 on a nice UV filter if you go that route.