It's all relative to how much noise you're willing to tolerate. The upper ('expandable') endof the available ISO range is unavailable by default and needs to be made available via C.Fn for a reason - basically, you shouldn't use them unless you're desperate for light. But useable ISO is also relative. The 'normal' range for the 1DIV is100 - 12800, whereas the 'normal' range for your T2i is 100-6400. Also, as an example, ISO 3200 on 1.3x crop like the 1DIV is going to be less noisy than the same ISO on your T2i, and the same ISO 3200 on a full frame camera like the 5DII will be even cleaner. The high ISO noise performance of your T2i is essentially the same as on my 7D. Personally, I find 1600 to be useable, 3200 to be barely tolerable. (Side note: I'm jealous of the T2i's capability to set a ceiling for the Auto ISO!!). However, ISO 3200 on a 5DII is very useable.