A patent application showing the optical formula for an EF 10mm f/2.8 prime lens has been filed in Japan.
Japan Patent Application 2018-233223:
- Focal length: 10.30mm
- F number: 2.88
- The half angle of view: 64.54°
- Image height: 21.64mm
- Lens length: 124.58mm
- Back focus: 38.00mm
It looks like this patent may have been a part of the EF 11-24mm f/4L USM development.