>>In the AF fine tune menu, there is both a saved value and >a >>default value. Saved value is for individual lens, default >is >>the global value for all lens. >> >>If your saved value for individual lens is maxed out at >-20, >>then set the global default value also to -20. Then you >have a >>total of -40. > >I have seen that claim made elsewhere, but I believe it to be >incorrect. As far as I know, the "default" value is >only applied when using a lens that does not have its own fine >tune value saved.
I am 99% sure you can get to -40. Was just playing around with it last night. Changed one, it affected focus, then changed the other, and it affected focus again. Didn't take any pictures and compared side by side, but just be seat of the pants testing, that was how it worked.