Auto ISO in photography exposure – pros, cons and how to use it

Auto ISO – controlling exposure, part 5

Using auto ISO can be helpful when there’s a lot going on and you don’t have enough time to respond to changing light conditions quickly enough, especially for new photographers.

ISO controls the sensitivity of your camera’s sensor to light. By changing the ISO we affect the brightness of the image.

Further reading: The exposure triangle – what role does ISO play?

Auto ISO can be used in these shooting modes:

• Aperture priority mode
• Shutter priority mode
• Manual mode
• Program mode

We’ll have a look at how to use auto ISO in the first three shooting modes.

Further reading: What are the best shooting modes to use and why?

Why use auto ISO?

Using auto ISO is an automated way of being able to control the brightness of an image while you can concentrate more on achieving:

• sharp focus
• freezing movement
• or blurring movement
• and getting the depth of field as you’d like it

I changed the shutter speed with each photo, the aperture remained the same and the camera was set to auto ISO. As you can see, the brightness of the images is the same throughout.

Using auto ISO in aperture priority

PRO: As you would normally do with aperture priority, set your aperture and your camera will set the shutter speed for the correct exposure. The difference with using auto ISO is that you don’t need to set your ISO as well.

As long as you’ve set your maximum ISO and your minimum shutter speed, your camera can adjust the ISO, as well as the shutter speed, to ensure the image is neither too bright or too dark. When you reach the minimum shutter speed that you’ve set, the camera will push the ISO higher.

Your camera’s order of priority when it comes to your settings: 

• Aperture
• ISO
• Shutter

CON: If you don’t have enough light at your minimum shutter speed and maximum ISO, your camera will use a slower shutter speed than the minimum you have set. It won’t go over the maximum ISO you set.

Further reading: The exposure triangle – what role does aperture play?

Using auto ISO in shutter priority

PRO: As with shutter priority mode, you set the shutter speed and the camera will use the appropriate aperture for the correct exposure. Again, you don’t need to set your ISO as the camera will also select this.

Your camera’s order of priority when it comes to your exposure settings:

• Shutter
• ISO
• Aperture

CON: As with aperture priority and auto ISO, your image might be too bright or too dark if you don’t keep an eye on what’s happening.

The aperture settings can’t go beyond the capability of your lens.

If your lens can only go to F5.6, for example, and you need more light than the maximum possible ISO you’ve set, your image will be underexposed.

The opposite is true if you want to use a very slow shutter speed that requires a smaller aperture than your lens is capable of, or an ISO lower than the minimum ISO of your camera, your photo will be too bright.

Further reading: The exposure triangle – what role does shutter speed play?

Using auto ISO in manual mode

PRO: In both manual mode and shutter speed priority mode, even after setting your minimum shutter speed under ISO sensitivity settings, you can set your shutter speed as low as you wish.

The minimum shutter speed setting is relevant only in aperture priority and program modes.

CON: If you push your exposure settings, either the shutter speed or the aperture, beyond the minimum ISO or the maximum ISO you set, your image could be too light or too dark respectively.

When to use auto ISO

It all depends on how you like to work.

Some photographers swear by it and use it all the time. I prefer to do everything manually, but there are circumstances where I would use auto ISO. If the light conditions are changing rapidly and I don’t have time to keep up with shutter speed, aperture and ISO, I would use auto ISO.

Examples of ideal times to use auto ISO:

• Sports
• Rock concert – or any concert or show with changing lights
• Lifestyle family photography in a dense wood with busy children

A lifestyle family photoshoot with young kids on an overcast day in thick woodland is a very busy shoot. Every now and then there was a clearing in the tree canopy, so the light quality changed constantly. Ideal time for using auto ISO.

Things to think about when using auto ISO

These days DSLR cameras are capable of so much more at high ISO settings, especially if you have a higher end camera. If you have an entry level camera, you need to pay attention to your ISO setting, as a high ISO setting will affect the quality of your photos.

When you use a high ISO:

• Noise increases
• Dynamic range decreases
• Color accuracy decreases

How you set up your auto ISO settings will help to control loss of quality by preventing your camera from using the extremely high ISO settings.

Setting up auto ISO

Maximum ISO

Set the maximum ISO that you are comfortable using, rather than leaving it for the camera to go to the absolute maximum it’s capable of using.

Minimum shutter speed

You can set the slowest shutter speed that you are comfortable using. This way you prevent the possibility of camera shake if your camera selects a shutter speed that is too slow.

ISO sensitivity

Set your ISO sensitivity to what you would ideally like.

Then, when you are in either aperture priority mode or shutter priority mode, your camera will take this into account and adjust the aperture and shutter speed as much as possible to accommodate your ISO setting.

Regardless of whether I use shutter priority mode, aperture priority mode or manual mode, when my aperture and shutter speed settings force the ISO to change from the ISO sensitivity that I set, “Auto ISO” flashes in my viewfinder.

This warns me that the ISO is no longer at the sensitivity that I set.

Problems with using auto ISO

Scenes that are difficult for the camera to meter

You could end up with images that are too light or too dark, as your camera’s exposure meter may be confused by the scene. For example:

• Your camera will want to underexpose a bright scene, such as a sunny beach
• By the same token it’ll want to overexpose a dark scene, such as a close up of a black cat

You can use exposure compensation to overcome these difficult situations. Set your exposure compensation for:

• Bright scenes to plus one or more stops
• Dark scenes to minus one or more stops

Further reading: How and when to use exposure compensation – controlling exposure part 3

Auto ISO may be limited

Your auto ISO setting may limit your camera’s performance in any of the shooting modes, so you could end up with images that are too light or too dark.

Exposure indicator not visible

When using auto ISO in aperture priority or shutter priority, your exposure indicator doesn’t show potential over or under exposure, even if you have reached the limit of the ISO setting.

My solution – custom ISO button

Create a custom button so that you can easily alter your ISO, without having to take your eye away from the viewfinder.

As the ISO setting will be displayed in the viewfinder, you will always know what ISO you’re shooting at.

Because you can always see your settings, and therefore if you’re over or under exposed, you can make quick decisions and adjustments.

Unless the light is changing too rapidly to control all three settings, you can still get a well focused image with the depth of field you want, no camera shake and your choice of frozen or blurred movement.

In photography there’s always a “but, a “however” and an “unless”. That’s what makes it so interesting!

Trying out different ways of photographing is how we figure out what works best for us, so give it a go. Auto ISO might be exactly right for you.

In case you missed the rest of the series on controlling exposure…

1 – How to read your camera’s exposure meter

2 – Metering modes and how exposure metering works

3 – How and when to use exposure compensation

4 – How to control exposure in all shooting modes

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