what is auto ISO?

Addressing this issue is a little more complicated than I first realized. First off, I'll admit that I don't have much experience using Auto ISO. Secondly, the Auto ISO function varies from camera to camera, some allowing adjustments to be made in the Auto ISO function, and some not. Also, it is a challenge to keep abreast of the fast-changing camera technology. And finally, there is a variety of opinions on whether Auto ISO is useful or not. Some photographers make compelling arguments for its use while other recommend that you never use it. There are probably good reasons for both views.

 So, what is Auto ISO? When you set the camera to Auto ISO mode, the camera uses algorithms established by the manufacturer to automatically set the ISO to give the "correct" exposure given the aperture and/or shutter speed setting(s) you have selected (within limits).  It will start with the lowest ISO and increase as needed for the lighting condition and lens used. It appears that most, if not all, DSLR cameras recognize which lens is in use (or what zoom setting you are using) and makes appropriate changes in the algorithm for that lens or zoom setting.  For example, in Auto ISO my Canon 5D-4 selected a settings of 1/40 sec, F/5.6, at ISO 100 for a 24mm zoom setting on a 24-105mm lens. Zooming the lens to 105mm it gave settings of 1/125 sec, F/5.6, at ISO 320 for the same lighting condition.

The best way to determine how your camera sets the exposure parameters (ISO, aperture, and shutter speed) is to test your camera in various light with different lenses while using Auto ISO. This information is probably published by the camera companies somewhere but, in my experience, is not readily available. Some cameras will let you set the maximum and/or minimum Auto ISO limits; for example, not allowing settings above ISO 6400 (or higher). Some also let you set the minimum shutter speed to use with Auto ISO. You'll have to check your camera manual or menu system to see what adjustments are available in Auto ISO mode for your camera. 

What Mode Dial settings work with Auto ISO?  In my opinion, the three modes listed below are the most usable modes when using Auto ISO, although Auto ISO will function in all modes (except possibly flash, depending on the camera).

1. Aperture+Auto ISO: You manually select the aperture and let the camera set the shutter speed and ISO.

2. Shutter+Auto ISO:  You manually select the shutter speed and let the camera set the aperture and ISO.

3. Manual+Auto ISO:  You select both the aperture and the shutter speed and let the camera set the ISO. Some older cameras many not permit setting both aperture and shutter speed, but this combination offers you the best control of the camera. Most, if not all, the newer DSLR cameras allow this option.

Of course, you can also use Auto ISO in fully-automatic or program mode but why would you? In these modes you have no control over the camera since it will be selecting two or three of the exposure parameters based on its own criteria (algorithm) which may not be what you want.

When to use (or not use) Auto ISO? Here are some recommendations:

1. Use Auto ISO in any situation where there is fast action (moving subjects, wildlife, sports, etc.) where you don't have time to manually adjust the ISO for varying light conditions. I suppose you could say the basic premise is "noisy photos are better than blurry photos", unless of course you are trying to create a pleasing blur.

2. Use Auto ISO in rapidly changing light such as sun and clouds, or subjects that move in and out of shade and sun, or people moving from inside to outside.

3. Turn off Auto ISO while using a flash unless you have tested it and know it will function properly.

4. Turn off Auto ISO in strong back-light conditions and very bright conditions which could lead to overexposures.

5. Turn off Auto ISO while using multiple light sources, such as in studio lighting, or when trying to shoot a silhouette.

6. Be cautious using exposure compensation while in Auto ISO mode unless you have tested this situation.

Since the usable ISO settings keep getting higher and higher, and noise reduction software has gotten so good, I think use of Auto ISO might offer a good alternative for many conditions. Try it out!