A:AnswerYes it will fit. Yes it will increase clarity for a few reasons. It is a telephoto lens, it has stabilizer features , it has L series glass and is a fast Lens which means it can be shot in lower light.
A:AnswerCompared to the old Canon 70-200mm is that it's lower focal of 2.8L (great for dark scenarios). The image stabilization is better than the old model.
A:AnswerYes, though you should know that this lens is designed for a full frame camera, so with the 6ti's 1.6x cropped sensor you'll have the equivalent of a 112-320mm lens. Depending on your uses for this lens that could be a real plus.
A:AnswerThis is a fantastic professional grade lens made for professional grade full frame cameras. That said, my understanding from the Canon Community Forum is it should work with a crop factor(T2i not full frame camera). I would suggest going to the Canon forum to see what T2i users are saying about the use of this lens. https://community.usa.canon.com
