Now that I have both a 40D and 5D, I'm trying to decide the best wide angle lens choice-

I have a Tokina 11-16 f2.8 for the 40D right now. Would the 17-40 f4 L mounted on the 5D be at least as good or better than my current combo. I loose 1 stop of aperture, but the 5D is at least 1 stop better in low light.

I could sell the Tokina and get the f4 L without too much cost difference.

So what is the better combo-

40D / 11-16 f2.8

or

5D / 17-40 f4

5d and 17-40 is a more versatile combo. It goes from ultrawide to moderate wide, while the 11-16 is pretty much an ultrawide. F4 on FF is probably also a DoF equivalent to 2.8 on 1.6x sensors? Somebody with more tech knowledge than me can inform better about that.

BTW, my other lenses include 24-105 f4 IS and the 17-55 f2.8 IS; however, I haven't made up my mind if I will keep the excellent 17-55 f2.8 IS glass since it can only be used on my crop body. If I get the 17-40 f4 L, it might be a bit easier to let the cropper glass go.

Take a look at the Tamron 17-35 f/2.8-3.5 It's surprisingly good value. The corners are crappy on FF wide open but are decent once you've stopped the lens down some.

The aperture will matter for shutter speed, but not for DOF, as you'll have even shallower DOF on the 17-40L at f/4 on full frame for the same shot as you will at f/2.8 on the 40D.

I had the Tokina 12-24 on my 30D, and when I got my 1Ds II I swapped it for the 17-40L, and I've been extremely happy with the switch. The 17-40L is sharper in the center than the 12-24, but should be about the same as the 11-16. You might lose a small bit of corner sharpness going to the 17-40L, but you'll more than make up for it with the beautiful color rendition and then there's the benefits of the better camera sensor. Overall, I think you'll be pleased with the switch. Don't expect night and day differences with results, and you'll have to correct the 17-40's barrel distortion in post, but that's a one click fix for me (I use PTLens). You'll actually lose a bit of weight with the 17-40...it's a surprisingly light lens.

17-40mm offers great image quality when slightly stopped down, and more flexiblity on your gear. Both 40D and 5D will take 17-40, while 5D is incompatible with 11-16 whatsoever.

Thanks all. My excellent copy of the Tokina 11-16 will be for sale on the B/S forum soon.