Many of us use Canon 16-35, 17-35, or 17-40 wide angle zoom lenses, and none of us are really happy with it. The cropped sensor folks can't get wide enough, and the full-frame guys (myself included) are unhappy with edge performance.
"Canon's engineers have completely redesigned the optics on the EF 16-35mm f/2.8L II USM to deliver higher contrast levels and improved resolving power." [Canon's Press Release] Of course, they are referring to a redesigning of the Canon EF 16-35mm f/2.8 L USM Lens - a very good and very popular lens itself. The biggest physical appearance change is in the II's wider lens barrel at the objective end - going from 77mm to 82mm in filter size. The 16-35 L II is .2" (5mm) wider (for the just-noted reason), .3" (8.6mm) longer and weighs the same as the original 16-35 L. They feel identical out to and including the focus ring. The II's zoom ring is more firm (I like this better). Minimum focus distance remains the same - Maximum magnification remains the same as well.
Barrel distortion is noticeably reduced at the wide end while pincushion distortion is slightly increased at 35mm. Flare is reduced over the entire focal length range - improving contrast in some comparisons. Overall, I consider the 16-35 L II sharper than the 16-35 L I, but this amount of difference varies throughout the focal length range and distance/direction from the center of the image. The II is at least as sharp or sharper in the center of the image at all focal lengths. The II is especially improved in f/2.8 non-center sharpness on the wide end. For the most part, I would consider the II an improvement in non-center sharpness overall though the 16-35 L I holds its own or even surpasses the II at certain focal lengths/apertures/points within the frame.
The II has slightly less vignetting than the I. CA is very slightly reduced - but looks different as it is primarily present in the corners which are now sharper at many focal lengths and points within the frame. Both lenses deliver exposures about 1/3+ stop brighter than usual.
What I'm looking for in a review is a comparison of the 16-35 L II vs the 16-35 L. Maybe I'll try to get ahold of one for a review...