The What, When, Why, and How of Auto ISO
So, you understand how to interactively use Aperture, Shutter Speed, and ISO settings to achieve proper exposure. You know how to control things like depth-of-field and the freezing or blurring of motion. Perhaps you also understand the camera modes: Aperture Priority, Shutter Priority, or Manual and know when using them which settings are fixed and which can fluctuate. But how often do you adjust ISO adjustment? The idea that you might let the ISO speed “float” with each shot is alien to many photographers. So what is Auto ISO? When and why might you want to use it, and how can you set it up to make better shots?
Back to basics – the Exposure Triangle
From the dawn of photography and the simplest pinhole camera to the most sophisticated modern DSLR, there have been three constants – Aperture, Shutter Speed and what we now measure with ISO – the Light Sensitivity of the media onto which the image gets recorded. All cameras are essentially boxes with a hole in them. The size of the hole (aperture), the length of time the hole is opened (shutter speed), and the sensitivity of the recording medium (ISO). When we allow light into the box to create an image on the sensitive media, we are making an “exposure.” It makes up the “Holy Trinity” of photography – The “Exposure Triangle.” Perhaps you knew all this? If so, feel free to skip ahead in the article, otherwise, keep reading.
A “correct” exposure
There are two basic things to consider when making an exposure:
- What is the correct amount of light to let into the “box” rendering all tones in the subject and capturing everything from the blackest shadows to the brightest highlights, and
- How can we use the three components of the triangle most creatively?
The first consideration is technical, the second creative.
A histogram shows us the 256 shades of gray for a given image. At the far right are the highlights, on the far left, the shadows. In theory, an image which stays “between the goalposts” such that none of the tones go off either edge is a “correct exposure.” In editing, we can redistribute the tones so long as they have not gone to “0” which is total black, or 255 which is total white. At those extremes there is no detail to recover; it is either totally black and “blocked up,” or totally white and “blown out.”
Creatively using the controls
How to use the elements of the exposure triangle creatively brings in some secondary considerations of how aperture, shutter speed, and ISO affect our image. Aperture is the hole in our “box” while the f/stop is the term we use to define the size of the hole. A good way to remember which is the “bigger hole” is is to think about any f-number as a fraction. If you like pie, would you rather have a 1/2 pie or a 1/16 slice?
Therefore, the bigger f/numbers like f/22 represent smaller apertures (holes), while the small f/numbers like f/2.8 or f/4 represent the larger apertures.
Creatively, we can use smaller f/stops to increase depth-of-field and larger ones to limit it. In a portrait, we might want an unfocused, simplified background with a limited depth-of-field, so a large aperture would be a good choice. In a landscape photo where we want front-to-back sharpness, a small aperture may be better.
The shutter speed you choose also offers creative possibilities. Remember, shutter speed is represented in whole or fractions of as second. A shutter speed of 1/2 second is a longer time the shutter remains open than 1/250th of a second. You might think of the shutter speed as the “slice of time” we expose the light-sensitive medium to light. Short (faster) shutter speeds will help us freeze motion by capturing a “thinner slice of time.” Longer (slower) shutter speeds can allow us to “stretch time” and cause moving objects to blur.
Adjustable ISO? What a concept !
Of the three components of the triangle, ISO choice has implications, but probably less so than the others. Like an audio amplifier, lower settings keep the background “noise” less while higher settings which amplify the signal also introduce more noise and distortion. ISO measures how sensitive we make the sensor in a digital camera. In the film days, film sensitivity was fixed. Put in a roll of ASA 64 film and that was what you lived with for the whole roll. It had less grain than did an ASA 400 roll, but it was also less light sensitive.
In the digital world, ISO can be changed whenever we like, even from shot to shot. Now making an exposure truly becomes a “three-ball juggling act.” We can change Aperture, Shutter Speed, or ISO with each shot if we like. We still must use those to make a “correct” exposure, but we can also better consider the creative implications of our choices. We can also choose which controls we want full control over and which we might relinquish to the camera. Auto ISO coupled with newer, better, and “less noisy” sensors has changed the ballgame. Let’s go back to our three-ball juggling analogy.
Learning to juggle
Watch a video clip of a juggler throwing three balls, and you will see at any given time, one ball is in the air, and the other two are in each of his hands. He has “control” of two of them, and the third is in “float.”
Now, when you use Auto ISO, it becomes that “third ball,” the component you let be in “float.” Fortunately, ISO has the least creative potential, and with modern cameras, the least penalty of choice. So it often makes sense to let it be the “ball in float.”
Let’s bring this back to the practical. You’re shooting dancers on the stage in an auditorium. The stage lighting varies with each scene and even as the dancers move to different spots. They are not allowing flash here, so you must live with the lighting conditions.
You want a reasonably high shutter speed to freeze the motion and a moderately small aperture so you have sufficient depth of field. Which of the three “balls” makes the most sense to let “float?” Auto ISO to the rescue! Situations where lighting changes quickly and the action you’re capturing won’t wait while you manually adjust settings is perfect for using Auto ISO.
Setting things up
I shoot with a Canon 6D most of the time so I will use that as my point of reference and for the menu shots below. How, (or even if your camera supports Auto ISO at all) will vary between make and model so you will need to dig a bit deeper to learn that. You might even have to get out your camera manual! The method may differ with your camera, but if you can grasp the general concept, the rest is simply navigating your camera’s menus.
Usually, there will be a button or menu where you can set Auto ISO. If you go to the low end of the scale, past the lowest (smallest) numbers of ISO you will likely find “A” for Auto ISO. Set the camera there.
Now you will want to set some “boundaries” as to when and how Auto ISO will be implemented and how high you will allow it to go. You should know that the higher ISO settings may allow you to shoot in very low light but may also introduce more image noise. How much is too much noise and what settings are impractical will depend on your camera and you. Shoot some high ISO images and evaluate them, so you know how much is too much for your liking.
With this information, you will want to find the menu item where you can set the specifics for how Auto ISO behaves. On my Canon 6D, I tap the Menu button and then roll the small top dial to the third camera menu icon from the left. I then roll the larger Control dial down to the second item, ISO speed settings, and hit the Set button to get to the menu below.
Again, your camera may differ, but you will set several things here:
- Confirm the camera is in Auto ISO – ISO Speed
- Set the full ISO speed range the camera will use – ISO Speed Range
- Choose the lower and upper limits of ISO you will allow – Auto ISO range (you will usually enter the lowest ISO as the minimum and the highest as that ISO you think will not have excessive noise). For my 6D, I typically enter 100-3200 here.
- Choose the minimum shutter speed you will allow before Auto ISO changes the ISO setting – Min. shutter spd.
For this last setting, whatever you enter here is the slowest shutter the camera will allow before jumping to a higher ISO setting.
You will note “Auto” is an option here. If you pick this, your camera will detect the focal length of your lens when the image is about to be made and use the formula 1/focal length to set the minimum.
The idea here is you should not shoot slower than this (especially if handholding your camera) if you want to prevent camera shake blur. For example, let’s say if you are shooting a 24-105mm zoom lens and are zoomed all the way in. If Min. Shutter Speed is set to Auto, your camera will start to increase the ISO if the required shutter speed drops below 1/100th.
How it works in each mode
So you have this all set up. Now how will it operate? It depends on what camera mode you are shooting in. Let’s look at each.
Full Auto (Green) Mode
What you Can Adjust – Nothing, in Full Auto Mode the camera adjusts Aperture, Shutter Speed, and is in Auto ISO.
What the Camera Adjusts – Everything. This is a true “Point-and-Shoot” Mode with the camera making all adjustments.
Exposure Compensation Possible? – No
Pros/Cons – You are letting the camera make all your exposure and creative decisions. You are in Auto ISO and perhaps didn’t know it!
Program (P) Mode
What you Can Adjust – Everything, but as you adjust one item, the others will change too depending on lighting.
What the Camera Adjusts – Everything. The camera will seek to maintain proper exposure.
Exposure Compensation Possible? – Yes
Pros/Cons – This can be confusing when used with Auto ISO. I don’t recommend it.
Aperture Priority (Av or A) Mode
What you Can Adjust – Aperture. Lock in your Aperture setting and Shutter speed will adjust to maintain exposure. If the required shutter speed is lower than your minimum, ISO will increase up to the maximum you have set.
What the Camera Adjusts – Shutter Speed and then ISO
Exposure Compensation Possible? – Yes
Pros/Cons – If control over depth of field is your priority, this is the best option. Used in combination with the minimum shutter speed setting, it allows you to lock in the Aperture, set a base for the shutter speed, and have the camera adjust ISO increase when light goes below the shutter speed minimum you set.
Shutter Priority (Tv or S) Mode
What you Can Adjust – Shutter Speed. Lock in your Shutter Speed setting and Aperture will adjust to maintain exposure. If the required aperture is more than the maximum for the lens used, ISO will increase up to the maximum you have set.
What the Camera Adjusts – Aperture and then ISO
Exposure Compensation Possible? – Yes
Pros/Cons – If control over shutter speed is your priority, this may be the best option. Used in combination with the minimum shutter speed setting, it allows you to lock in the Shutter speed. The camera will adjust the aperture as needed and call on an ISO increase when you reach the maximum aperture of the lens used.
Full Manual (M) Mode
What you Can Adjust – Shutter Speed and Aperture. Lock in both your Shutter Speed and Aperture settings and ISO will adjust to maintain exposure. The exposure display will stay centered and ISO increase or decrease as needed to maintain proper exposure. If the required ISO exceeds the minimum or maximum set, the indicator will move off center showing an under or overexposure.
What the Camera Adjusts – ISO
Exposure Compensation Possible? – Camera Dependent
Pros/Cons – This gives maximum creative control to set both shutter speed and aperture and thus control both freezing/blurring of motion and depth of field simultaneously. ISO will “float” to adjust exposure up to the limits set. With some cameras, no exposure compensation is possible in this mode. However, with newer cameras, the “center point” may be adjusted thus supporting compensation.
When to use Auto ISO
When you have time to be a bit more leisurely with your image making, you can slow down and think through each of your settings. What are your objectives? Freezing action? Increasing or limiting the depth of field? Is the light changing?
When time permits, and you have a good understanding of each element in the exposure triangle, use full manual and set your ISO to the lighting conditions, staying as low as possible to limit noise. For landscape, portrait, still life, architecture, or other kinds of work where time permits and lighting is reasonably constant, Auto ISO isn’t much additional help. Ditto if you’re doing long exposures on a tripod where shutter speeds will be longer.
Where Auto ISO really shines is in conditions where the action is fast, the light is changing or particularly low, and you are blasting away without time to think through each setting.
In that case, Auto ISO may be the helping hand you need. If lighting permits and your camera supports exposure compensation in Full Manual, this could be the ideal method. Lock in both Shutter Speed and Aperture where you like and shoot, counting on Auto ISO to handle any fluctuating exposure conditions.
Sometimes Auto ISO in combination with Aperture Priority will be a good choice. I work part-time at an auto dealership photographing cars for the web. Set up like this, I can go from shooting the exterior of the car in bright sunlight to the much darker interior with no adjustments, letting Auto ISO kick up the speed for the darker interiors.
Sports and Action can be an excellent time to use Auto ISO, especially in changing lighting conditions. It was a mixed cloudy day, and the light on the river where these kayakers were running was changing. I wanted to be sure my shutter was fast to freeze the action. Shutter priority plus Auto ISO was the ticket.
What if Auto ISO goes wild?
Some photographers, especially those trained with the mantra “Auto Anything is Bad,” have a hard time invoking Auto ISO. Good photographers control everything, right? What if the camera goes up to a crazy high setting and all my images are too noisy?!!
It could happen. But, then again, remember you can limit the upper end of the ISO setting.
Also, newer cameras have such good sensors that your “upper limit” may be much higher than you think. Finally, what if you shoot at too slow a shutter speed and get blurry shots or don’t get the depth of field you wanted? There are many good noise reduction programs, but no apps I know of to fix a blurry, out-of-focus, shot with insufficient depth of field. I’ll take a noisy image over an out-of-focus image any day!
If you’re an old film guy like me and Auto ISO feels funny, or you’re worried about what it will do, or just haven’t been able to fully get your head around it, I suggest you relax and give it a try. Take your camera out on a non-essential shoot, turn on Auto ISO and just play. I’m going to bet you might just come away with a new trick.