I ended up buying a 18-35 two months ago. I already have made a couple of hundreds of shots, but I've never taken out both the 18-35 and 16-35 at the same time. My impression is that the 18-35 does almost the same job, but it's much lighter and smaller.
Ok, today I went for a walk and made some comparison shots between the 16-35/4 and the 18-35/3.5-4.5.
All shots are on the D600 at base ISO and on a tripod. VR was off for the 16-35. I developed in ACR, CA correction was on, otherwise no lens corrections. Moderate sharpening (amount 40, radius 0.
applied equally on all the shots.
Scene 1: Close focus, bokeh, backlight
Because the physical length of the lenses is different, I ended up shooting a different focal length, but the shots are fairly similar and have the same aperture.
18-35, 35mm, f/5.6, 1/350s
16-35, 28mm, f/5.6, 1/350s
Commentary: Very similar rendering, bokeh reasonably smooth, good contrast, no flare, decent sharpness.
Scene 2: Mid- to far-field, extended DOF, fine detail, corner sharpness
18-35, 20mm, f/11, 1/10s
16-35, 20mm, f/11, 1/10s
Commentary: Again, very similar rendering with minor geometric differences. Both lenses render nice colors.
100% crops from the upper right corners reveal slight differences:
18-35:
16-35:
Commentary: The 18-35 renders the corners of the image with more sharpness and less geometric distortion.
Scene 3: Infinity (or close), wide aperture corner performance.
18-35, 24mm, f/4, 1/500s
16-35, 24mm, f/4, 1/500s
Crops from upper right corner:
18-35
16-35
Commentary: Again, the 18-35 delivers more sharpness in the corners of the frame.
Scene 4: Distortion
18-35, 30mm, f/11, 1/8s
16-35, 28mm, f/11, 1/8s
Commentary: Neither of the lenses is distortion-free around 30mm. The 18-35 shows barrel-type and the 16-35 shows pincushion-type distortion. In general, both lenses have distortions which are fairly easy to correct. The 18-35 is always barrel, the 16-35 changes from barrel to pincushion and is almost distortion-free somewhere close to 24mm, which is quite nice, but not tremendously important.
Interim conclusion: I'm not yet completely sure, but I'm leaning towards selling the 16-35 and keeping the 18-35. So far, I haven't encountered a situation in which the 18-35 disappointed me and for me the 16-35 doesn't really show many advantages in my type of shooting. The 18-35 impresses me with the sharpness even close to wide-open and in the corners. The contrast, color etc. is almost indistinguishable from the 16-35. Moreover, it is smaller and lighter, which makes it easier to carry and use.
Any other observations? Thoughts?