Why Auto ISO Is So Amazingly Awesome

As someone who regularly pontificates about the benefits of manual exposure control, it’s not often that I sing the praises of an automatic camera setting. And sure, I use autofocus and even automatic exposure modes such as Aperture Priority and Shutter Priority regularly. But there’s one auto-exposure control I practically can’t live without: Auto ISO. Here’s why.

Auto ISO can be used with manual shutter speed and aperture settings or in the aforementioned shutter priority and aperture priority auto modes. It does what you might expect: automatically adjusts the ISO to achieve what it believes to be the correct exposure.

Auto ISO is a function not only of the digital revolution (after all, how is a film camera going to change the sensitivity of the emulsion mid-roll?) but also a beneficiary of the amazingly low noise that can be achieved even at high ISOs.

You see, Auto ISO wouldn’t be nearly as useful if ISO 400, 800 or 1600 delivered images that were noisy and not nearly as pretty as those shot at ISO 50, 100 and 200. But the reality is, I have a hard time distinguishing between the noise profiles of an ISO 100 image file and an ISO 1600 image file unless they’re greatly enlarged. So, in practice, for lots of things I shoot, whether it’s ISO 100 or ISO 800 is immaterial.  

Auto ISO

But because I relish total control, it’s typical for me to want to dial in a specific aperture (ƒ/2, for instance, if I want to minimize depth of field without risking missing focus as much as I might at ƒ/1.4) and specific shutter speed (1/250th at a minimum when I’m handholding, and often as much as 1/500th or more, even for simple portraits in diffuse natural light). You see, I hate missing focus and I hate blurry photos (when they’re not part of my creative vision, of course). So, I take control over my exposure and ensure I’ve got the aperture and shutter speed dialed in exactly where I want them.

But as anyone who has ever traveled with a camera slung over their shoulder and shot indoors and then outdoors and, soon enough, indoors again…or who has photographed with clouds moving and changing the lighting by the minute…or who simply wants to nail the exposure every time…as all these folks know, having some automatic exposure assistance is immensely helpful. That’s where Auto ISO comes in. It’s the perfect automatic camera setting for us control freaks who don’t want to take a chance at the wrong aperture or shutter speed in a pinch.

To use Auto ISO, most cameras simply have an AUTO setting atop their available ISOs list. Scroll to it as you would when choosing any other ISO. But wait, there’s more.

You can set the parameters of the Auto ISO settings so the camera won’t fall below a certain ISO or stray above a predetermined limit. This is a great way to ensure you won’t wind up in one of those whopping ISO settings like 51,200 that most definitely produces noticeable noise.

Auto ISO parameters


That said, I almost never set a limit on the maximum ISO—or I set it at the camera’s maximum. The whole reason I’m using the feature is that I know for certain exactly what aperture and shutter speed I need to ensure the image is sharp in all the ways I want it to be. Noise is the least of my concerns. I’d much rather get the shot and ensure the motion is sharp and focus is correct and then deal with noise-reduction in post rather than having a nice, clean, noise-free image of a blurry subject that ruins the shot. If minimizing noise is especially important to you, you may want to dial in a limit to the Auto ISO range. If that’s the case, you may also prefer to set a very tight range—say ISO 50 to 800, perhaps—and instead of using a fully manual aperture and shutter speed, use aperture or shutter priority exposure modes. This way, the camera will use the manual exposure setting you dial in (say, a shutter speed of 1/1000th in shutter priority mode) and default to the widest aperture that would keep the ISO within those narrow parameters.

When I use Auto ISO, I like to make fine adjustments with the exposure compensation dial. On my Canon EOS 5D Mark III, that’s done with the +/- button and the thumbwheel. You can even preset it when you’re setting the Auto ISO. On my Sony A7R3, it’s a dedicated dial atop the camera and easily reachable with the thumb. That camera also allows me to choose my ISO parameters right from the same menu setting where I select Auto ISO mode.

Whatever camera you use, familiarize yourself with the controls that Auto ISO provides, as well as the exposure compensation dial, to quickly fine-tune Auto ISO shots to make them slightly darker or lighter as needed. And, of course, if you’re shooting RAW you’ve got a lot of leeway. So as long as you’re close in-camera, you can make those final fine adjustments in post.

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