Auto ISO: Friend or Foe
Auto-exposure mode tells your camera you want it to pick what it thinks is the optimal aperture and shutter speed for each image. Most modern cameras also support even greater exposure automation via auto-ISO. Once set, your camera controls all three variables that determine exposure.
There's a relationship between shutter speed, aperture and ISO known as the Law of Reciprocity that states that a change to one exposure variable has to be compensated for by an inverse change to one of the others to retain the same exposure. Open up the aperture by one stop to create a more shallow depth of field and you also allowed in more light during the exposure. Unless you really wanted to have the image come out one stop brighter, you could decrease the exposure time by one stop to compensate. Leaving a small hole for an aperture open for a long time versus selecting a wider hole but closing that hole sooner would yield the same amount of light in total either way. These variables have to retain a reciprocal relationship if you want to retain the same exposure.
Back in days of yore (back when cameras shot film), reciprocity was usually described in terms of just aperture and shutter speed since it wasn't practical to adjust ISO between shots. Once you loaded a roll of film of a particular ISO, you were stuck with it until you next changed rolls. For that roll of film, all we really had to work with back then were the two variables of aperture and shutter speed. Digital cameras have opened things to new possibilities since its possible now to also change the ISO setting — the third variable in the exposure reciprocity triangle. Instead of decreasing the exposure time by one stop to compensate for opening up the aperture by one stop, you could slow down the ISO by one stop, making the sensor that much less sensitive to the increased amount of light it will see during the exposure. A wide aperture hole plus a long exposure time isn't really the whole story these days since we can control the ISO too.
Three variables are better than two, right? Perhaps.
The first issue we need to discuss is precisely what decreasing the ISO means on a digital camera. With a film camera, it meant putting in a new roll of film with an emulsion that was less sensitive to light when compared to what we had been shooting with. But it obviously can't mean replacing a digital camera's imaging sensor in a similar fashion. That would be crazy. And it doesn't mean putting sunglasses or a filter over the sensor to cut down on light transmission either. No, decreasing the ISO on a digital camera is purely electronic in nature. Essentially, the camera sensor is still just as sensitive to light as it ever was, but we instruct the camera to ignore some of that light.
And this leads me to the second issue affecting the usefulness of automatic ISO. Your camera wants to make sure your images don't end up suffering from motion blur. Shoot under low light and your camera will likely attempt to avoid the necessary low exposure time even with a wide open aperture. Of course long exposures can also result in image noise, but auto ISO won't give you the choice of which you might prefer. It's just going to avoid long exposure times on principal by bumping up the ISO. Even if your camera is firmly mounting on a tripod to stop camera shake, the auto ISO will still be doing its thing. And of course if you're trying to blur motion through the use of long exposures, best of luck to you — you'll end up fighting against your camera's auto ISO logic.
There's no doubt about it: auto ISO can make creative control of your camera more difficult. It can mean you don't have to worry about achieving an adequate exposure, but at what cost?
Camera makers have started taking steps to improve the usefulness of automatic ISO. My Nikon lets me specify the maximum ISO I am willing to let it amplify the signal to as well as the minimum shutter speed I am willing to retain before it adjust the ISO. These sorts of settings can help, but they operate as hard and fast rules and still rob you of control for deciding what you think is best.
Of course a lot of this depends on your shooting style and needs. If you're a wildlife photographer or photojournalist whose subject matter tends to move faster than you can adjust your camera, you'll be grateful for the added help in achieving a useful exposure setting more quickly. But if you can afford the time to take matters into your own control I'd recommend doing so. Only you know what you want a given image to end up looking like. And every choice you or your camera makes to achieve exposure for an image also impacts the creative look of that image. If you want to be in full control of your images, you won't want your camera trying to wrest that control away from you.
Personally, I love the ability to change ISO manually. This gives me a third variable I can use to achieve what I am after. Within a certain degree of latitude, I can use aperture to control depth of field, shutter speed to determine how motion will be rendered, and ISO to balance these first two to get the exposure I need. Back in film days, I often had to accept the tradeoff that came from only having two variables to work with. If I wanted maximum depth of field I had to accept that some slight blurring might result from the necessary long exposure time. Having the ability to break the hard and fast reciprocity between aperture and shutter speed is a great thing. Newer cameras can yield great results from a fairly wide range of ISO settings and I love being able to leverage that potential. But I want that control for myself. But perhaps that's just me.