When you use Auto ISO and Manual Mode, you can choose any aperture with any given exposure time and the A7r will adjust ISO in the predefined range (standard is 100-6400) and keep the selection you made. This is a very cool function. Lets say you do street, use the 55/f1.8 and like to shoot with f5.6 and an exposure of 1/250. You dial that in and depending on the lighting, the camera will adjust ISO accordingly to keep those parameters.
If you want to use bracketing, I recommend you select an ISA value manual, or even better, you choose Aperture priority.
Cheers
Jan