Vignetting, or the darkening of the frame at wider apertures involves both corners as well as overall brightness. Wide open at f/2.8, the overall brightness is about 2/3 stop darker than one might assume, with some moderate darkening of the corners over and above the overall effect.
Stopping down to f/4 eliminates the issue as a practical matter, though there remains some modest corner darkening. Stopping down to f/5.6 evens things out to near-perfect (and f/5.6 is also the aperture at which peak optical performance is reached, assuming an optically correct sample).
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