A reader sent me a lens screening raws from a Sony 16-35mm f/2.8 GM on a Sony a7RII. He tested with a smallish target at 16, 19, 24, 28, and 35 mm. He used f/4 for all the shots but the 35mm ones because his target would be too small in the frame if he’d backed far enough away to use f/2.8. I’ll give you his comments and mine after each composite image.
16mm: Looks pretty great to me, with the right top corner being a little bit softer.
Me: I agree.
19mm: Performance is not as good at f/4, especially the on the left and bottom. Stopping it down to f/8 improves it quite a bit.
Me: I also agree. The asymmetry is consistent, and indicates a minor assembly defect.
24mm: Pretty solid performance at f/4, bottom corners lacking a bit. Stopping down to f/8 gives a bit of improved sharpness and contrast in the corners.
Me: I don’t think the bottom corners are all that bad.
28mm: Okay performance at f/4, corners are not super sharp but look pretty uniform. Stopping it down to f/8 doesn’t improve performance by much.
Me: f/5.6 might be the sweet spot.
35mm: I shot a series a f/2.8, which looks pretty average to me.
I agree. The corners are pretty soft. It’s reasonably symmetrical, though, which indicates that the problem is more with the design then the assembly.