As per below post, the situation being observed may not be at all related to the use of auto ISO, but, rather, the result of a reduced shutter speed that is being avoided when the operator manually sets ISO. For example, assuming aperture priority mode and manual ISO set to 400, the exposure might have the SS set to 1/100 sec. If auto-ISO were set up with a minimum SS of 1/50 sec and an ISO range starting at 200, the exposure for the same shot would result in a SS set to 1/50 sec at ISO 200. This slower SS could be the cause of the problem. In fact, auto ISO is often used to prevent the selection of a shutter speed that too slow for a hand held shot. This is done by setting the auto ISO minimum shutter speed to a value that is fast enough to avoid negative effects from camera shake. I don't often use auto ISO, but have used it to assure selection of an adequately fast SS in Aperture Priority mode.
SharpShooter wrote:
I don't think you have enough facts.
There should be NO correlation between A-iso and focus.
Perhaps to keep the ISO low, the camera is dropping the SS lower and she gets a bit of blur. When she sets her own ISO, at what is it set? Maybe, she has it set manually a bit higher it allows a higher SS, thus eliminating blur???
All purely guesses!!
SS