Canon 16-35mm @ f/2.8	Canon 17-40mm @ f/4.0

Canon 16-35mm @ f/2.8	Canon 17-40mm @ f/4.0

Canon 16-35mm @ f/2.8	Canon 17-40mm @ f/4.0

What we’re seeing here is very interesting indeed. If you look closely at the graphs, the 16-35mm f/2.8 holds a flatter profile from the center out to almost ¾ of the frame towards the edge. Up until that point, the lens is covering much more evenly. After that point, however, the drop is fairly steep, and drops deeper than the 17-40mm f/4. The 17-40mm f/4 is a more gradual, and consistent decline from the center, but does not drop off so severely at the far edges.

Which is better? It’s hard to say. Overall, the edge-to-edge brightness at full wide aperture on the 17-40mm f/4 is going to appear to be more even, since it’s a more gradual transition and doesn’t go as deep. The 16-35mm f/2.8 will be, in fact, a more accurate and even coverage overall, but because it drops so precipitously, and deeper, the vignette effect will be more noticeable.