martinfisherphoto Loc: Lake Placid Florida

First and foremost using manual means all of the perimeters are fixed, Shutter speed, ISO and aperture, IMHO.... But to answer your question, Yes your ISO will float up and down if ISO is left in the auto mode. Try this simple test. Assuming your camera is in BBF mode, set your camera to aperture priority with a Fixed ISO. Now take the camera and look thru the view finder while Depressing the BBF button. Move camera left to right thru light and dark areas. Thru the view finder you should be able to watch as your shutter speed changes. You can try this in Shutter speed priority as well only difference your aperture will change, that is if your ISO is fixed.. Now if you try what you suggested, fixed aperture and shutter speed with auto ISO, you will see the ISO will change as you sweep your camera thru light and dark areas. Remember your camera will always Adjust for Proper exposure/change aperture, shutter speed, or ISO for correct exposure/ if given the oppurtunity weather in BBF mode, or shutter release mode, that's assuming your not in full manual mode...
Now just to clarify, if in Full manual, fixed ISO, Aperture and Shutter speed, your camera will not change any of these perimeters, in any type of shooting mode.

GrandmaG Loc: Flat Rock, MI

I understand and agree to what you're saying and just for kicks I did the experiment you suggested. I did it both ways, with BBF set and the focus reset to the shutter, with the same results, although the readings in the viewfinder were better with BBF.

I think perhaps I worded my response wrong. What I meant to say was that using BBF is not the REASON your exposure changes (one part of the 3 parts required, depending on the mode used, changes).

All in all, IMHO, back button focus is the way to go and you must set ISO MODE to AF-C