Image Quality

In the following sections we will use DxO data and real life samples to determine how the updated Canon 16-35mm F2.8L III compares to its predecessor.

Sharpness

Sharpness At 16mm the lens performs very well wide open, so much so that we start to see the effects of diffraction by F5.6. As you move through the focal lengths there's a drop in overall lens sharpness, especially wide open; the worst of which can be seen at 28mm where you have to stop the lens down to F5.6 to achieve the best result. Corner sharpness suffers as you move toward the longer end of the lens. By 35mm the best overall result is at F5.6, with a tiny cost to central sharpness.
Chromatic Aberration The updated Canon lens does a very good job with respect to the handling of CA across nearly the entire focal range. At 16mm you do see a bit of CA in the corners wide open, which persists as you stop the lens down, but overall the lens is very well behaved.
Vignetting In terms of vignetting the lens sees a two and one-third stop decrease in the corners at 16mm when shot wide open. This improves by F5.6, but is never entirely eliminated. As you move through the focal range you do see a decrease in overall vignetting and it all be disappears one you stop the lens down to F5.6 at focal lengths beyond 24mm.
Distortion At 16mm the lens suffers from a fair amount of barrel distortion; especially in the corners. By 24mm this barrel distortion disappears and the lens has a slight amount of pincushion distortion at focal lengths at and beyond 24mm.
Transmission (T-Stop) The lens' F-number is a theoretical value, and the actual light transmission value, known as the T-stop, is always fractionally lower due to light losses within the lens. Lenses with more elements, like a complex zoom, tend to be slightly more effected. The measured T-stop for this lens is F3.1 which means the lens is letting through a bit less light than the F2.8 rating suggests.

How does the Canon 16-35mm F2.8L Mk III compare to the Mk II?

It's fairly well known that the previous iteration of this lens had its fair share of issues. Corner sharpness, the handling of CA, coma and overall central sharpness throughout the focal range proved to be major drawbacks for the lens though the issues never got in the way of the lens' popularity. With that said Canon had a fair amount of work to do with respect to producing a lens that could improve in many of the areas where the previous iteration faltered.

In terms of sharpness the updated version of the lens is a huge improvement over its predecessor; especially wide open at 16mm. The central sharpness seen in the updated lens is downright impressive when shot wide open, so much so that you will begin to notice the effects of diffraction as early as F5.6 at 16mm. Diffraction will always occur as you stop the lens down, but in this case the lens is so sharp wide open that this starts to be a limiting step rather than the underlying lens performance.

Corner sharpness at 16mm is also markedly improved over the previous iteration of the lens. At focal lengths beyond 16mm you see the Mk III's sharpness start to suffer. In order to achieve the best results you will have to stop the updated lens down to between F5.6 and F8 at a slight cost to corner sharpness. Overall the lens performs better than the previous iteration wide open in terms of sharpness at every focal length. With that said the updated version of the lens falters at the longer end and only shows marginal improvements in maximum sharpness over the previous version beyond 24mm.

Lateral Chromatic Aberration (CA) is an area where the updated version of the lens performs exceptionally well, especially at 16mm when compared to its predecessor. It only gets better over the remainder of the focal range. In terms of vignetting; at 16mm the lenses are fairly evenly matched when shot wide open. As you stop the lenses down at 16mm you see a slight improvement in vignetting in the new lens over the previous iteration. Beyond 16mm the lenses are very evenly matched; with all or most of the vignetting eliminated by F5.6.

Distortion is another area where we don't see much difference between the updated lens and its predecessor at 16mm. Beyond 16mm the updated version of the lens sees a slight improvement over the Mk II. The distortion is fairly simple, so should be largely correctable through your favorite post processing software.