Patents were found for a Sigma 13mm f/2 APS-C and Sigma 18mm f/2 Full Frame lens. Sigma has a lot of patents for lenses that they haven’t brought to market, but these sound like they could be useful designs.
Example 2 (for 13mm F2 APS-C)
Focal length 13.45
F number 2.06
Total angle of view 2ω 99.09
Image height Y 14.20
Lens length 75.01
Example 6 (for 18mm F2 full size)
Focal length 18.62
F number 2.06
Total angle of view 2ω 104.45
Image height Y 21.63
Total lens length 90.81
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