Auto ISO is one of those features that I ignored for a long time, considering it not much more than a gimmick. It wasn’t until recently that I decided to try auto ISO, and I quickly realized that I was missing out on a valuable and practical feature.
Auto ISO Explained
As the name suggests, in auto ISO mode, the camera will pick the correct ISO value for the scene being metered. Initially, the idea of letting my camera pick the ISO value seemed not only silly to me, but also like a genuinely bad idea, since I was afraid of winding up with grainy images if the camera chose a very high ISO. I was also firmly entrenched in a film shooter’s mentality, since I grew up in an era when using ISO 800 film was pushing the boundaries of grain and typically only used as a last resort. Obviously, the world changed a long time ago, but as many of us know, old habits die hard, especially for us photographers!
The beauty of using auto ISO is in the customization possible. The camera doesn’t simply pick any appropriate ISO for your exposure, but gives you a number of other options to ensure you don’t wind up with extremely grainy or blurry photos. In this article and video, I explain how these features work using a Canon EOS camera, but the basics will work with any camera that has auto ISO, although the customization levels will vary by brand.
Once you’ve set your camera to auto ISO, you can tell the camera the lowest and highest ISO it is allowed to use using the “Auto Range” menu. At first, I thought of the auto range as a high-ISO cap, leaving the low ISO at 100 and setting the high cap at around 3200, which I felt was the most grain I would want to see in my images. I quickly realized that this was not the best way to use the feature and now fine-tune it a bit more based on the specific shooting conditions and not just on acceptable grain levels.
For example, suppose you are taking portraits of sports, kids, or any subject that moves around a lot, outside on a bright sunny day. In this situation, the minimum ISO level is just as important as the maximum, because in general, you don’t want the camera to use ISO 100 for any moving subjects, even outside on a bright day. It’s much more practical to have the minimum ISO set to say 400 in this situation, since you want the camera to pick as fast a shutter speed as possible if you are when shooting in auto or aperture priority, and the difference between 100 and 400 for most applications is negligible. If the camera picks a slower shutter speed because you let it decide on an auto ISO of 100, for instance, the photo may come out blurry. As another example, if you know that you will be in a dark room where 100, 400, or even 800 ISO isn’t going to cut it, make sure that your low ISO auto range reflects that. In general, my suggestion is to keep the low and high range of the ISO closer rather than far for best results.
All of this might seem obvious, but it took me a while for it to sink in, probably because I come from a time when we did our best to use the lowest possible ISO at all times, when possible. Nowadays, I don’t think that applies anywhere as strictly as it used to, and I will even shoot at ISO 400 or 800 in my studio if the situation calls for it.
Minimum Shutter Speed
With a Canon EOS body, you can also choose whether the camera automatically or manually picks the minimum shutter speed it will use when you are shooting in program or aperture priority. When set to manual, you can tell the camera the slowest shutter speed you want to use is 1/500 s. This is another customizable safety measure that helps to ensure sharp photos, and the camera will not use any speeds below the one you dial in.
The Minimum Shutter Speed Auto setting, however, is much more interesting. With this setting, the minimum shutter speed is set automatically, but it’s based on the focal length of the lens you are using. By adjusting the menu slider, you can tell the camera to use a shutter speed faster, or slower, than the focal length of the lens attached. This way, if you are using a 200mm lens, for example, the camera will not use a shutter speed lower than 1/200 s in the default mode. By sliding the auto setting to “faster,” you can force the camera to use a faster shutter speed in one-stop increments, up to three stops. By choosing slower, the opposite is done. This, in my opinion, makes auto ISO a very practical and flexible feature, even for someone like me who primarily shoots manually.
Auto ISO and Aperture Priority: An Excellent Combination
By far, the most common way I use auto ISO is when shooting in aperture priority. In this mode, the camera picks the shutter speed and ISO, and I just dial in the aperture that I want. As a portrait photographer, selecting a wide aperture is almost always my main concern, and with two small children, I find this to be one of my favorite ways to shoot. I love not having to worry about the camera picking a shutter speed that is too slow for fast-moving kids or an ISO setting that’s so high my images are too grainy. By dialing in my auto ISO settings, I am able to retain creative control in situations where I don’t have a ton of time to fiddle with settings, in other words, any situation where kids are involved! I have found myself using Aauto ISO with aperture priority more and more when I leave my studio and work in natural light, whether it's taking some snaps of the kids or a concert in a dimly lit venue.
The last benefit of using auto ISO is that you can still dial in exposure compensation and fine-tune your image. I often like to intentionally over- or under-expose images to create certain effects, which is why I generally shoot in manual mode. But, as with my other misconceptions regarding auto ISO, I initially thought it would cancel out exposure compensation, and this is not the case. This adds to the flexibility of auto ISO even more, since I am still retaining a good amount of creative control. And, I often find myself changing ISOs on the fly during a shoot, especially if the lighting situation changes fast, so auto ISO is an extremely practical tool that can save you a lot of time.
Auto ISO, like any other feature in a modern camera, is something that should be experimented with. It’s not for everyone, but I think that a lot of photographers may dismiss this valuable feature as not for them. I know that I did, and I’m glad that I finally tried it out, because I have added a valuable tool to my photographic toolbox.