Hello ,
I have this situtaion, I hope someone can share some insigths.
I ve been using the 18-55 mm and set up the ISO and F to change with the back button dial.
THen I changed to the 27 mm fixed lens, no problem, but after I changed again to the 18-55 mm, when using the back dial it wont change to aperture, only the ISO.
Cheers
Pablo