I have a 50mm lens, should I get a 35mm or 28mm?

Look at the Sigma 30mm f / 1.4. I have one, primarily as a result of facing the same dilemmas you are, and it splits the difference between 28 and 35; to my mind, 35mm is probably too close to 50 to be really useful.

Mine back-focused slightly, but I sent it and my t2i to Sigma, and the combo focused beautifully when they came back.

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