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| p.1 #5 · Strange sharpness behavior with 16-35mm L /2.8 mkii |
It's possible you're not focusing accurately enough to take advantage of the large-aperture performance the lens offers. Increased depth of field makes very accurate focus less necessary at smaller apertures.
Wide-angle lenses are hard to focus accurately with phase-detection autofocus. With a tripod, a distant subject, and the lens set to 16 mm, try manually bracketing the focus in tiny increments while shooting at several f-stops per focus distance. This will rule out missed focus and establish which f-stop performs best when the focus is accurate.
Note that the required focus increments are so small that stiction may preclude simply turning the focus ring in tiny movements in one direction, from close to far; instead, you may need to jiggle the ring back and forth to settle on a new focus distance very slightly farther than the previous photo, paying close attention to the distance markings to know where to aim at. I suggested a focal length of 16 mm to minimise the accuracy needed: on zoom lenses the focus ring is effectively "slower" at shorter focal lengths.
Alternatively, borrow a 5D Mark II or similar and test your lens with Live View, which will give you a clear indication of focus.