We have been taught that Auto ISO is evil. But is it? If you know how to use it correctly, it can be used for the force of good! ;)
The Nikon D610 (and of course, many other cameras) has an Auto ISO feature which allows the user to set the maximum ISO it can go to and the minimum shutter speed to prevent camera shake. I set the maximum ISO to be 6400 as the quality at that ISO for Nikon D610 is still acceptable to me. As for minimum shutter speed, I leave it at the middle of Auto, which will ensure the shutter speed does not fall below 1/focal length of the lens I am using. I can vary it by up to 2 stops if for instance I am using Vibration Reduction lenses, or I can just manually select a shutter speed as slow as 1s.
The Auto ISO feature really shines when you are shooting wildlife esp. birds. A typical scenario will be using shutter priority to set your shutter speed to say, 1/2000s, and the camera will first ensure the lens is wide open before boosting the ISO. Exposure compensation will work as normal as well, so for example if you are shooting a bird in flight against a bright sky, you may +1 or 2 EV and the ISO will also increase accordingly (assuming aperture already wide open).
But what if you don't want to shoot at wide open aperture? Take my Tamron 150-600mm as an example. It is sharper at f/8 than f/6.3. So if there is sufficient light, I would very much prefer to shoot at the former rather than the latter. But you can't do that in shutter priority as the aperture is not controlled by you. Although you can do it in aperture priority, you have to be careful of your shutter speed, and to make things worse, any exposure compensation will affect the shutter speed. So this is not ideal for birding.
However, this is where shooting at manual mode comes to the rescue. Simply set your desired shutter speed e.g. 1.2000s and aperture e.g. f/8 and enable Auto ISO. When performing exposure compensation, the ISO will automatically increase or decrease accordingly. Voila! :)
The last thing to note is to always manually set your ISO to the lowest you desire e.g. 100 before enabling Auto ISO. This does not affect manual mode, but if you are on aperture or shutter priority, that ISO you set will automatically become the floor of your Auto ISO. In other words, if you set it to be 3200, then you will always shoot at ISO 3200 or higher no matter the lighting condition unless you are in manual mode.