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As one of the three components of the exposure triangle, ISO is important to understand for any photographer.

But while ISO can be incredibly useful, if not utilised correctly it can have a detrimental effect on your photos. So, it is vital that you understand what ISO is and how it can impact your images.

This guide will give you everything you need to know.

What is ISO?

Before the invention of digital sensors, ISO was used to describe the sensitivity of camera film to light. In other words, the higher the ISO the more sensitive the film was and so you were able to take photos in low light conditions at faster shutter speeds or narrower apertures. This method was then replicated for digital sensors, but the principle remained the same and ensured similar brightness levels on a digital sensor as you would get on film.

It wasn’t until 1974 when ISO (an acronym for “International Organization for Standardization”) for film was created. Before this, there were two film standards. There were the US ASA and the German DIN systems.

What the global body that created ISO did was to combine these systems into one universal standard for film by using numbers.

How does ISO work?

Every photographer has probably seen ISO numbers on the back of their camera. But many don’t know what those numbers mean and what happens when you increase them.

Simply put, every time you double the ISO you require half the amount of light to achieve the same exposure level.

So, if you raise your ISO from 100 to 200, you are effectively doubling the brightness of the image.
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Why is ISO useful?

You might be wondering why this is useful for photographers? For example, why would you need to double the level of exposure in your shot?

Well, imagine you are taking a photo of a fast-moving subject like at a sporting event. To get a good exposure level you have to set your ISO to 100 with a shutter speed of 1/125 sec. But while this gives you good exposure, because of the fast-moving action, your subject becomes blurred. The only option is to increase your shutter speed. Increasing your shutter speed will mean less light entering the camera and so your photo becomes underexposed (too dark).

This is where ISO comes in handy. By doubling your ISO to 200 you can now also double your shutter speed to 1/250 sec and still get the same exposure. If that isn’t fast enough you can double your ISO again to 400 and increase your shutter speed to 1/500 sec. The key here is that your exposure level remains the same while your shutter is faster.

By understanding the basics of ISO, you can use it to allow you to capture the shots that you want to take. That is why it is so important to understand ISO as well as shutter speed and aperture (known collectively as the exposure triangle).

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A word of caution…

Even though ISO is so useful for photographers, it also one of the biggest potential culprits for poor quality photos. Back in the days of film when you used a higher ISO film, it resulted in more “grain” in your image. This is similar to digital sensors these days.

The higher your ISO setting the more “noise” you will see in your photo. This can affect the sharpness and clarity of the photo. Even with the noise reduction capabilities of editing software these days, it is impossible to recover the same quality in a photo taken at high ISO as you would get with a low ISO.
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It’s also important to know that the type of camera you have (i.e. the sensor size) also has an impact on the noise level that high ISOs will produce. The bigger the pixel sizes (or bigger sensors) will produce less visible noise at high ISOs than say small compact cameras.

Digital cameras have come a long way since they were first launched. It wasn’t unusual to see noise in some early digital cameras even at ISO 800. Nowadays, most high-end cameras can produce pretty good results even at ISO 1600+.

But while it might be tempting to increase your ISO all the time, you should treat it with caution. Think of the ISO as a balancing act to give you what you need. But always aim to keep it as low as you possibly can. It’s the best way to ensure the quality of your photos is not affected.

When to increase or decrease ISO?

There is a wide range of ISO levels. In some cameras, this can be as low as 50 and as high as hundreds of thousands.

So when should low ISO be used and when should the ISO be increased?
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Low ISO:

You should use a low ISO wherever possible. This would include any time you are using a tripod where your shutter speed isn’t of any importance. For example, if you are photographing a landscape scene you can have a slow shutter speed.

Or if you are photographing outside on a sunny day then shouldn't need to raise your ISO much (keep in mind that raising your ISO slightly isn't going to show any noise).

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Raising ISO:
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On the other hand, for example if you are photographing a city scene at night and want to freeze the action (i.e. cars and people) you will have no choice but to raise your ISO. Another scenario that will require you to raise your ISO is when you are photographing anything handheld. Most people will struggle to hold a camera steady enough to avoid camera shake at anything slower than 1/60 sec.

So, if you find that is the case, then increase your ISO to the point where it will allow you to take the photo handheld.

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What about auto ISO?

Most cameras will also have an auto ISO setting where your camera will set the ISO based on your shutter speed and aperture. This setting can be useful when you are photographing fast-moving scenarios that won’t allow time to change your settings. Auto ISO can ensure that your photos are captured at a good exposure level. But you should also be aware that there is a danger that the camera will suddenly bump up your ISO really high.

To combat this issue, some cameras have a setting that allows you to set a limit on the maximum ISO the camera can select. But the obvious downside is that sometimes the photo might be underexposed. So, you should make a decision based on the scenario that you are photographing and set the maximum ISO accordingly.
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Test your camera

As mentioned earlier, different cameras produce different levels of noise. A good way to know for sure is to test your camera and see the results. Set up your camera on a tripod and focus on a stationary object manually. Keeping your aperture the same, take photos at the different ISO levels that your camera is capable of (adjusting your shutter speed accordingly).

Then zoom into your photos in editing software and see at which level too much noise becomes visible. This will give you an acceptable limit to work with when you are out and about shooting so you can always aim where possible to be below that limit. This is a test that I do on any new camera that I buy.
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Next steps

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Hopefully, you now have a better understanding of ISO and can see why it is so important to your photos. Like anything in photography, with experience you will grasp when to raise your ISO and when to not.

But as rule - always err on the side of caution and only raise your ISO as much as you have to.

All images by Kav Dadfar.