ISO can be used as a tool to adjust exposure, and you can even use an automatic ISO setting. I’ll explain how you can use auto ISO in the best possible way.
Today, we take auto ISO for granted and see it as a tool to achieve a correct exposure. It is the digital counterpart to the ASA/°DIN that indicated the sensitivity of negative film and slide film. Although the ISO setting on our digital camera has nothing to do with the sensitivity of a sensor, it does work in a similar way. With a high ISO setting, you need less light compared to a low ISO setting.
Compared to film photography, we now have three settings to think of when acquiring a correct exposure. The aperture controls the amount of light that goes through the lens. The shutter controls the time that the sensor is exposed to light, and the ISO determines (the digital version of) sensitivity. Each value can be set differently for each shot.
Auto Aperture and Auto Shutter
I remember my first camera that had the ability to set an auto shutter. This setting had the letter "A" on the shutter dial. It meant the camera could set the correct shutter speed for me, according to the light value it measured with the built-in light meter. Back then, the letter "A" could be considered the abbreviation of "auto shutter," but now, it's commonly known as "aperture priority."
This auto shutter setting gave the ability to anticipate changing light situations more quickly. It was soon followed by the auto aperture option, which worked in the opposite way. With that, I could choose the shutter time, and the camera would set the appropriate aperture.
These automatic ways of setting exposure are now common, although many amateur photographers have returned to manual because it’s widely believed to be the only professional way of operating a camera. The funny thing is, those who believe so, do use manual with an auto ISO setting.
Limitations of Auto Aperture (Shutter Priority)
I want to return for a moment to the auto aperture setting, which now is known as shutter priority, indicated with the letter S on the PASM dial. It allows the photographer to choose a shutter speed, and the camera will find the aperture that allows for a correct exposure.
There are some limitations to this setting that you need to be aware of. The range of settings that the camera can choose from depend on the lens and its maximum aperture. The lens opening of most common lenses is limited to f/2.8, which is considered a sensitive lens. Most photographers use lenses that are even f/4 or f/5.6.
This has consequences for those who are using the shutter priority setting. It means the range of exposure values the camera can choose from is limited. Let’s do the math. If you have a f/2.8 lens, the camera can use approximately 7 stops (f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22) to set the exposure, with f/2.8 being the largest aperture.
If you choose a slow shutter speed, this won’t be much of a problem. But if you need a fast shutter speed like 1/1,000 sec or faster, the widest aperture isn’t often wide enough to allow the required amount of light to pass through the lens for a proper exposure. Fortunately, there is the possibility to raise the ISO setting.
Using the Auto ISO Setting
If you’re using the shutter priority and a fast shutter speed, it may be necessary to play around with the ISO setting to compensate for the limitations of the widest aperture. You often have to choose a high ISO setting if you’re shooting fast action.
This way, you need to keep an eye on the light situation and raise the ISO only if needed. After all, we want the ISO to be as low as possible to keep the noise levels within limits. That’s where auto ISO comes in. It allows the camera to raise the ISO setting when the appropriate exposure can’t be reached with the largest aperture available. This way, you can concentrate on the action without worrying about the ISO setting. It will always be at the lowest setting possible.
Another huge benefit from auto ISO is guarding the shutter speed when shooting with aperture priority. Although aperture priority has a much larger range of stops available (at least 18 stops if you take the complete range of shutter speeds), there is a point when you need a tripod to prevent camera movement. Auto ISO can be used to prevent the camera from using a shutter speed that is too long for handheld photography.
How it works is easy. You set the lowest shutter speed that allows steady handheld photography. If the correct exposure needs a shutter speed below this limit, the camera will raise the ISO until the exposure with that setting is reached. It is also possible to let the camera decide which minimum shutter speed is set. It will take the focal length into account for that.
Auto ISO is a solution to the limited amount of stops for the shutter priority or the risk of motion blur with a longer exposure time with the aperture priority. But does it have any use for manual exposure as well?
Using the Auto ISO With Manual
As mentioned, there are photographers who use the auto ISO setting for the manual exposure setting. This way, the photographer will choose a shutter speed and aperture to their liking and relies on the auto ISO to achieve the correct exposure.
In theory, this way of setting exposure would give complete freedom of choosing any shutter speed and aperture. The auto ISO will make sure the right exposure will be set. In reality, there are too many limitations to make it work properly.
For example, you can’t choose 1/2,000 sec and f/11 and expect the ISO to fill in the gaps. You will probably end up with the highest possible ISO setting to get a proper exposure. Or, you can’t even get the ISO high enough to reach that proper exposure. Also, you can’t choose 1/60 sec and f/2.8 in full sunlight and expect the ISO to create a correct exposure. Even the lowest ISO setting isn’t enough to prevent overexposure in that situation.
In other words, the combination of shutter speed and aperture has to be set near the correct exposure for the auto ISO to work properly. The ISO range of most cameras allows only about seven stops before noise becomes a problem, although higher-end cameras will have a larger range of ISO settings available. That is not enough to make it work for every manual setting you choose.
The Best Use of Auto ISO
Although auto ISO can be used with every setting on the PASM dial, I think it’s most effective when used with shutter priority and aperture priority. Both these settings allow the most flexibility and take away the limits of those settings.
In my opinion, using the auto ISO for manual exposure setting is possible, but not always the best choice. The reason for manual exposure is the ability to keep the exposure settings fixed, no matter how the exposure meter is influenced by light or shadow. If you would add auto ISO to the mix, you lose this control, and it will be similar to aperture priority or shutter priority.
If you’re confident that manual exposure settings are the only way to get full control over exposure, auto ISO undermines this belief. After all, it will change the ISO according to the exposure measurement. Even the use of auto ISO to get a proper exposure in manual doesn’t make a lot of sense. Why use the limited amount of stops, when aperture priority, with or without auto ISO, allows so much more flexibility?
You have to choose your preferable way of using auto ISO, of course. Either way, auto ISO is a great feature on every modern digital camera. But it would be wise to use it to its full ability instead of limiting the auto ISO with something like manual exposure. But perhaps there is a reason for auto ISO in manual that I never thought of.
Do you use the auto ISO possibilities of your camera? I would love to know how you use auto ISO, with what exposure setting, and why in that particular way. Please share it in the comments below.