« on: April 19, 2013, 07:19:38 PM »
Canon cameras contain information about all the EF lenses and the commands to send to them. Third party lenses tell the camera that they are a Canon lens and then translate the command they receive to their lens. This can cause even another step that adds to inaccuracy, but it can be adjusted by AFMA as well.
Its a lot more complex that a person might think.
Again, all of that would be solved by closing the loop. Then all you'd need is a correction for sensor/vs AF array, which would be body specific and programmed by Canon at the factory.
It is sounding like the whole phase detect AF system is fully open loop, which really surprised me. Is Nikon like this too? Do they also have an AFMA type feature on their bodies?
Wish there were a Canon engineer I could speak to this about, would be a fascinating discussion!
Even with a closed loop, you may not be able to guarantee that it will always be correct since there's long term wear and tear, if the body is sent back to be cleaned and things shift slightly...plus it'll be different if you're using it in cold weather vs hot weather since parts expand/contract and at different amounts.