Photography Basics – Using Auto ISO

by Mickey Rountree

The exposure controls on any camera are shutter speed, aperture, and ISO. In manual mode we set all three to obtain our proper exposure. We have total control, and the only disadvantage is the time it takes the photographer to evaluate and make adjustments.

In aperture priority we set the aperture and ISO, and the camera selects the shutter speed to set exposure. As light decreases, the camera selects slower shutter speeds. The problem here is that the camera may select shutter speeds that are too slow for handholding or to freeze subject movement.

In shutter priority, we set the shutter speed and ISO and the camera selects the aperture. As light decreases the camera selects a wider aperture, and as light increases it selects a smaller aperture. The problems here are that as the camera changes aperture, the look of the image mage change considerably, and once the camera has reached the smallest or largest aperture it has no way to make further exposure adjustments and images may be over or under exposed.

Sometimes there are situations where the light is changing so quickly and drastically and photographic opportunities happen and then disappear so quickly that we may exceed the useful ranges of shutter speed or aperture, or we just don’t have time to change settings. This is when Auto ISO can come to the rescue. With Auto ISO set, the camera will change ISO as needed to maintain proper exposure.

Once we set Auto exposure, there are some additional settings we can make to assure our shutter speed doesn’t become too slow, or our ISO doesn’t go too high. The issue of high ISO is less important than it once was. My earliest digital cameras were pretty terrible above ISO 800. Also in my early days of digital, we didn’t have the software tools to reduce the noise of high ISO. Most cameras made in the last few years are pretty good up to ISO 3200 or even 6400. Images shot at even higher ISO can be more than acceptable with any of the good noise reduction software available. The important thing to remember is that it is better to have a sharp image with noise than to have a low noise image blurred by camera or subject movement.

I’ll be showing examples of how to use Auto ISO on Canon cameras, since they are what I’m familiar with. If you use another brand Auto ISO probably works similarly, but the menus to set it will be different. Check your camera manual, or go to YouTube and search your camera model and Auto ISO and most likely you’ll find a tutorial.

Set Auto ISO either with the top camera controls or through the menu.

In the menu go to Auto ISO Settings and set the highest and lowest ISO you want the camera to choose from. I’m not afraid to go as high as 51,200 in dim light with moving subjects.

In Aperture priority the camera will lower shutter speed down to a shutter speed you select or automatically based on the focal length you are using. I prefer to set it manually and usually set the minimum at 1/250 sec because I can usually hand hold most of my lenses at that speed. I might set higher if I have fast moving subjects. Once it reaches this shutter speed it will increase exposure by increasing ISO. In Aperture priority you can still dial in exposure compensation as you normally would.

If you choose to let the camera select the minimum shutter speed automatically it will select the reciprocal of the focal length. So with a 100mm lens, it would select a minimum shutter speed of 1/100 sec. At least on my Canon, you can fine tune this to select faster or slower shutter speeds.

In Shutter priority, as light decreases the camera will select wider apertures until it reaches the lens’ widest aperture and then begin increasing ISO. There is no adjustment you need to make other than setting high and low ISO. In Shutter priority you can still dial in exposure compensation as you normally would.

When I use Auto ISO, I almost always use manual mode, so that I have the most control. I set shutter speed, aperture and the camera adjusts ISO using the minimum and maximum I have set. The biggest draw back on most of my cameras is using exposure compensation. Normally in manual we add over or under exposure by watching our meter in the view finder. But in Auto ISO the camera always adjusts for a “normal exposure”. If you want to increase or decrease exposure you have to go to the camera’s menu and select exposure comp/AEB and dial it in there. That can be pretty slow when things are happening quickly. On my R5 there is a third control wheel and I can adjust compensation quickly while looking through the viewfinder.

Here are a couple of shoots where I feel Auto ISO really helped me get consistently good shots. Back in 2018 I went to the Photographer’s weekend at the Durango Silverton Railroad. For two days the train would go to scenic locations, let the photographers off and back the train up and make a pass or sometimes two. I knew the light would be constantly changing, but that I wanted 1/250 to freeze the motion of the train and f/8 for adequate depth of field. For two days and over 1000 images I had very consistent images that required minimal processing. ISO ranged from 200 to 6400.

I shot my second rodeo in 2022 in Dunlap, and knew I’d be dealing with very fast action and pretty low light. I set my shutter at 1/1000 for motion and my lens’ widest aperture of f/2.8 and Auto ISO. ISO varied from 100 when there was still daylight to 16,000 when there was only LED lighting. I was shooting an older 7D Mark II and there was very obvious noise, but with Topaz DeNoise I was able to get very nice images.

This spring (2025) I went to the Okefenokee Swamp and took two sunset boat excursions. I was in a moving boat and often had to shoot between other passengers, so I had less than a second to compose and shoot. I was using my 28-300mm Tamron lens, so I set 1/320 sec and f/8 with Auto ISO. For the boat excursions ISO was between 100 and 4000. However, while I was waiting for the boat I shot a rack of life preservers in very heavy shade and wound up at an ISO of 51,200. The original was very noisy and flat, but after two minutes of editing and noise removal it was a very nice image.

In short, I don’t need to use Auto ISO all of the time, but there are those times when it is invaluable. Give it a test run so that you understand how to set it up, how it works and its limitations. Then when you have one of those difficult situations you’ll be prepared.

Photography Basics – Using Auto ISO

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