Quote Originally Posted by Sean Setters
On a side note, the whole reason I purchased a 7D instead of the 5D Mark II is because the 24-70mm f/2.8 L IS didn't exist. I fell in love with the general purposefunctionality of my 17-55mm f/2.8 IS, and I couldn't bear to leave any of its useful traits (the focal range coupled with IS) on the table when upgrading to a full-frame camera.

You know, Sean, I said this very thing myself. Then I bought a 5DII + 24-105mm f/4L IS, and I couldn't be happier - it's a substantial improvement on the 7D + 17-55mm f/2.8 IS (although I still use the latter combination sometimes).


It makes sense if you look at the FF equivalent specs, given that the crop factor affects both focal length and aperture (in terms of DoF). That means that the 17-55mm on a crop body behaves like a hypothetical 27-88mm f/4.5 lens on FF, so the 24-105mm lens on a FF body has a focal range that's both wider at the wide end and longer at the long end, and a DoF that's 1/3-stop shallower, and it still has IS. Also, since ISO noise is inversely proportional to sensor size, you can compensate for the loss of 1 stop of shutter speed by bumping up the ISO without a noise penalty (in fact, you even get an extra 1/3-stop of ISO-driven shutter speed, if you need it).


So, overall when comparing the 17-55mm on a 7D with a 24-105mm on a 5DII, the latter combination is the clear winner. IMO, the only downside to the 5DII+24-105mm combo is the autofocus - the f/4 lens does not activate the high-precision, f/2.8-sensitive AF point (but I find the 5DII's center AF point to be very accurate, and it's actually noticeably better than the 7D in low light).