I'm reading the French documentation...
Regarding Aperture step-by-step moving commands [0x12,Value] on older Sigma lenses and [0x13,Value] on others, you can use signed values (positive to close the aperture and negative values to open the aperture) to go back and forth (BTW, there is an error in documentation, on the second element of the bullet identation).
In reality, as reflex automated lenses are working (EOS included), we will never need to use negative values to open aperture in steps from a closed one. From the start, the aperture is already open at her maximum, allowing a good light into the optical viewfinder. The incrementation/decrementation of the aperture is happening virtually on the firmware, inside the RAM memory and displayed on LCD according to the "double array" which contains the values and codes of the aperture (of course, the minimum and maximum values are depending on the lens characteristics - and I understand that on zoom lenses, the maximum and minimum changes and the lens must be interrogated for these every time an aperture change is involved ). When a picture is shot, the aperture always goes mechanically from the her maximum opening to the value set in memory by the user, and this is happening also when the aperture preview button is pressed. And reverts back to the starting value with a single command.
I understood correctly? What I am trying, is to learn how to use an EF lens in a
custom (read DIY) film camera, where the focusing is always on manual and because of that, I don't need to mess with focus related commands. The metering will be made with the sensor and firmware from here as a starting point
http://kadookacameraworks.com/light.html UPDATE: It seems that the account is suspended for now... last checked April 9, 2017. Meanwhile see it here
https://sites.google.com/view/vasihome/micro/kevin-kadooka-light-meterI will opt later for a full manual Leica II shutter, or an electromagnetic shutter recovered from a broke film camera (I have one in a defective Yashica AF-230). The mirror should be actuated with a servo...
Why not an (old and not in production anymore) EOS film camera? Well, I won't mind, but I hate film winders based on motors, that is always a source of troubles (one of the problems of my Yashica). A DIY camera is a camera that can be repaired/remade by you when it breaks but Cooke triplet lenses are a bit harder to build, and EOS lenses have the best electronic interface from all modern fullframe reflex cameras. Of course, full manual lenses (Rokinon, Samyang) can be also a solution, but it won't help with the automatic light metering , requires additional actions from the user.
