In the first comparison both lenses are in their widest aperture which is F2.8 for Tokina and F5.0 for Sigma. At this comparison Sigma seems to be doing better. Tokina has some vignetting and also softer in the corners while their center performance is close.
When they are both stopped down to F6.3, Tokina's vignetting improves and now Sigma's vignetting looks heavier especially in the corners. Yet its sharpness also looks a little better than Tokina, especially in the corners while centers looks very same.
Stopped down further, their performance gets more closer to eachother. Its not easy to decide which photo is better in the third comparison so I'll leave the decision to you.
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DXOMARK Score
Best at f=11mm & f/4 [?]
Lens Metric Scores
Sharpness
10 P-Mpix
Transmission
3.8 TStop
Distortion
0.6 %
Vignetting
-1.1 EV
Chr. aberration
22 µm
DXOMARK Score
Best at f=10mm & f/4 [?]
Lens Metric Scores
Sharpness
5 P-Mpix
Transmission
5.3 TStop
Distortion
0.7 %
Vignetting
-1.5 EV
Chr. aberration
18 µm
DXOMARK Score
Best at f=12mm & f/4 [?]
Lens Metric Scores
Sharpness
10 P-Mpix
Transmission
4.4 TStop
Distortion
0.4 %
Vignetting
-1.2 EV
Chr. aberration
12 µm