Excerpt with written permission from Robert O'Toole. Please see more of his wonderful images at
Auto ISO - The Third Auto Mode
Imagine if there was an exposure mode that would enable you to set both the aperture and the shutter speed and the camera will automatically change a third variable to adjust the exposure. This would be like having simultaneous aperture and shutter priority (via manual mode) with auto exposure. This mode is called auto ISO and when used in manual mode I don’t see any reason to use aperture or shutter priority. The power of auto ISO in manual is that you can set the both the aperture and shutter speed and the camera will set the ISO value for a proper exposure.
To read the rest of the article go to Robert's blog at: http://www.robertotoole.com/2011/12/30/auto-iso-the-third-