In the old days you loaded a film and that was the ISO you were stuck with. Then came digital and ISO became more flexible. This is how I have worked for about the last 5 years. Somehow, I am not sure how, I sort of discovered auto ISO. It can under certain conditions be a very useful tool. Set an aperture and shutter speed that you want, set auto ISO and carry on. With modern cameras and up to date software it is amazing what you can get away with in the ISO department if you have to.