A Patent was found for a Sigma 13mm f/2 APS-C lens. Sigma has a lot of patents for lenses that they haven’t brought to market, but a 13mm could be competitive on X-mount.
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
via SonyAddict, digicame-info
