The full frame Sigma on small APS-C/DX sensors doesn’t have any serious problems with vignetting. At the maximum relative aperture the light fall-off in the frame corners amounts to 20% (-0.66 EV). The problem disappears completely on stopping down to f/4.0 because the vignetting is only 8% there.
The situation is even better after attaching the converter. At the maximum aperture, which is then f/4.0, the vignetting reaches 11% and it decreases to only 2% on stopping down to f/5.6.
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