Sigma A 18-35 mm f/1.8 DC HSM
At the shortest focal length and the maximum relative aperture the brightness loss in frame corners amounts to 34% (-1.22 EV). On stopping down to f/2.0 the vignetting decreases to 28% (-0.95 EV). Stopping down by another 1 EV stop lowers it to 14% (-0.43 EV). By f/4.0 that aberration becomes invisible (8%).
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Very similar situation can be observed at 24 mm. Here at the maximum relative aperture you must take into account the loss of light of 36% in the corners (-1.29 EV). On slight stopping down to f/2.0 the vignetting decreases to 29% (-0.98 EV). That aberration becomes marginal by f/2.8, amounting there to only 11% (-0.32 EV). It disappears practically completely by f/4.0 (6%).
Also at the maximum focal length we deal with similar results. With the aperture fully open you have to accept the light fall-off of 37% in the corners (-1.33 EV). By f/2.0 that value amounts to 31% (-1.06 EV) and by f/2.8 – only 16% (-0.50 EV). The vignetting disappears by f/4.0 where it reaches just 10% (-0.29 EV).
The results of 34-37% the Sigma showed on the maximum relative aperture cannot be called low. On the other hand, though, compared to the potential rivals, they still look impressive. Let’s remind here that the 1.8/35 lenses produced by Nikon and Sony for smaller sensors, had results of 37-44% so they performed similarly or a bit worse than the Sigma.
The Sigma presents itself even better if compared to full frame 24-70 or 28-70 mm f/2.8 lenses. They, at their much slower maximum relative apertures, can lose in the corners from over 40 to even over 60% of light!
The Sigma A 18-35 mm f/1.8 DC HSM also shouldn’t be inhibited when compared to 17-50 mm f/2.8 lenses. Those constructions, being over 1 EV slower and designed for the same APS-C/DX sensor, can have the vignetting on a level of 30-40% so exactly the same as the tested, much faster Sigma.