MAubrey wrote
Canon's EF 85mm f/1.2L is 1/3 stop. It shows up as f/1.3 when you adapt it to other cameras with an electronic adapter, but on a native EOS body, it's f/1.2.
AFAIK, the EF protocol works by units of 1/8 stops. Each lens model tells the camera its max aperture as an integer number when it is initialized.
This information must be accurate for proper exposure, therefore the number sent must be following technical specification.
Canon cameras can instead display max aperture as per "marketing specification" (by the way, the lens product id is also sent to the body).
I have not found the patent for the 85L, but we known it is 1/3 stops faster than f/1.4.
I am therefore assuming the lens is sending the number 6 as its max aperture code, and the math performed by a third-party adapter is straightforward:
