With regard to your point about framing........wouldn't I get the same framing with a given lens size because of the crop factor? For example, an 18mm digital lens (like the 18mm position of the Canon f3.5 18mm-55mm telephoto lens) would have about the same viewing angle as my 28mm film camera lens: 18mm x 1.6 crop factor (Canon's) = 28.8mm.
Crop factor applies to both focal length and aperture equally. So just do as you did with focal length and substitute aperture. i.e. f3.5 x 1.6 crop factor = f5.6
So an 18mm lens at f3.5 mounted on a T3i will give you the same Field of View and
Depth of Field as a 28.8mm lens at f5.6 on a full frame camera.
So basically, stop your current 28mm f2.8 lens down two stops and that's pretty much what you'll get - Mark
P.S. If you want the details: remember that f number is defined as focal length divided by pupil diameter. So increasing focal length results in a corresponding increase in f number. e.g. a 100mm lens at f2.0 has a pupil diameter of 50mm. (100 / 50 = 2) If you, say, double the (equivalent) focal length (like a camera with a 4/3 sensor would) to 200mm -- 200 / 50 = 4 (note: obviously the pupil diameter doesn't change.) So a 100mm lens at f2.0 mounted on a camera with a 4/3 sensor (with a 2X crop factor) will give the same FoV and DoF as a 200mm lens at f4.0 on a full frame camera.
Note: this also works "in reverse". So if you want to duplicate the framing and DoF that your current full frame camera has when using your 50mm lens at f2.0 on a T3i (with a 1.6X crop), you would need a 31mm (50 / 1.6 = 31.25) lens at f1.2 (2.0 / 1.6 = 1.25). Canon's EF 35mm f1.4L would get you close for ~$1,500.