What is ISO in Photography? Taking the Mystery Out of ISO Camera Settings

It's the camera setting that most beginner photographers deliberately avoid, but ISO can open creative doors

Apr 4, 2017   |   Justin Katz
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This article is part 4 of the Complete Guides to Learning Photography for Beginners.

Three basic camera settings determine how light or dark an image is — and while two of them (aperture and shutter speed) can be explained by physical parts inside the camera, the third is impossible to touch, yet plays a big role in the final image. To make things worse, that third setting is a confusing acronym: ISO. So what is ISO? What does ISO stand for? And how do ISO camera settings affect the photograph?

While ISO has all the marks of a headache-inducing concept, that third exposure element isn’t rocket science. Broken down, ISO is a simple way to increase or decrease your camera’s sensitivity to light — here is what you need to know about ISO.

What is ISO? ISO and exposure

The concept of ISO, just like shutter speed and aperture, stems from film. ISO is how sensitive a piece of film — or a digital sensor — is to light. In film photography, to change the ISO, you would have to swap out the entire roll of film for another. In digital photography, however, ISO is an adjustable camera setting that determines how sensitive the sensor is to light.

Think of a camera sensor as a solar panel. When you use a low ISO number, like ISO 100, that solar panel is gathering little light. If you adjust your camera settings to ISO 3200, on the other hand, that same panel will gather much more light, even in the same weather.

Just like that theoretical solar panel would use a high ISO to gather more light on a cloudy day, high ISOs are great for shooting in limited lighting. A low ISO, on the other hand, means that the camera’s sensor is not very sensitive to light — it’s a good setting for using outdoors on a sunny day.

High ISOs will dramatically brighten a photograph, often enough to allow the camera to shoot in low light without a flash. ISO is just one setting that affects the photograph’s exposure, or how light or dark it is. ISO is often used to balance the image’s exposure with the aperture and shutter speed. By making the sensor more responsive to light, high ISOs allow for shooting in limited lighting with a shutter speed fast enough to prevent blur. Low ISOs, on the other hand, allow for using wide apertures for out-of focus backgrounds without overexposing the image.

What else does ISO affect?

While ISO gives a single camera sensor the flexibility to shoot in a variety of lighting situations, there is a trade off. The more sensitive the camera is to light, the lower the image quality becomes. That’s because high ISOs introduce what’s called noise or grain in an image, or tiny dots that look like this:

[Cropped photo showing noise at a high ISO]

Noise is sometimes used as an artistic effect (often in black and white or to age a photograph) but for the most part, those tiny dots degrade the image’s quality. At high ISO levels, the noise will be so abundant that the camera won’t capture the smaller details like textures and fine patterns.

ISO actually stands for International Standards Organization — that means that ISO on one camera will have the same effect on the exposure as the same ISO on a completely different camera.

While one ISO setting has the same effect on exposure, the way that ISO affects image quality is not universal. A DSLR from five years ago will have a higher level of noise than a DSLR released this year, since technology improves the way that the camera sensor handles different ISOs. While the difference will be less dramatic, you may also notice a difference between one DSLR and a different model from the same year and even manufacturer.

Advanced cameras with larger sensors will also have less noise than a camera with a smaller sensor. For example, a DSLR has a much lower noise level at ISO 1600 than a smartphone camera at that same setting.

That’s why knowing your gear is essential. Take a photo at each ISO on your camera, then open them up and compare them. Find out where you really start to notice the drop in image quality, and what ISO settings should only be used in tough lighting and which ones shouldn’t be used at all.

As a general rule, whenever the lighting is sufficient, use the lowest ISO possible (on many cameras, that’s ISO 100 but the ISO range differs on each model). If the lighting is challenging (and you can’t use a slower shutter speed because you are shooting action or don’t have a tripod), use a higher ISO, but keep it as low as you can for the light that’s available.

How to change your ISO

On digital cameras, changing the ISO is as simple as adjusting a setting, with no need to load a different type of film. Every camera is a bit different, but on most, ISO isn’t hard to find. On advanced DSLRs, the ISO will be displayed in the viewfinder and on the LCD (when lit) or on a secondary screen on the top of the camera. Pressing the ISO button and turning the dial near your right thumb often flips through the different ISO settings.

Don’t see an ISO button? Look for an ISO option in the menu (or the quick menu, if your camera has one). Many cameras without a physical ISO button will let you assign a control to ISO by going into the custom settings menu.

Still stuck? Every camera is a bit different, so look for ISO in your camera’s manual to find the exact steps. (If you don’t still have the physical paper manual on hand, you can often find a copy online).

ISO Camera Setting Examples

Often, choosing an ISO setting is about prioritizing one setting over another. In limited light, you may have to choose between the noise of a high ISO and the blur from a slow shutter speed. A sports photographer, then, may often choose a higher ISO to freeze the action, sacrificing some noise for a sharp image.

If, on the other hand, you are using a tripod to take a photo of a still scene — such as a landscape or a product photo — there’s no reason not to use the lowest ISO possible for the highest image quality. The tripod will prevent any blur, which means you’ll get a better shot with a slow shutter speed and a low ISO.

Of course, there’s more than just a low and high ISO — an indoor photo with decent lighting (either the artificial type or via windows) is often good at a happy medium, like ISO 800.

What if the lighting is low, but you don’t want to introduce lots of noise into the image? Adjusting the shutter speed and aperture also affects the exposure — learning how to manage all three will give you full creative control over your images.

A flash can also help keep the ISO down. A flash will help by both adding more light into the scene and by freezing motion to prevent blur.

ISO isn’t a physical camera part like the aperture or shutter, but understanding ISO is just as essential to learning photography basics. In well-lit scenes, or when shooting a still object with a tripod, keep the ISO as low as possible. When lighting is limited, however, don’t be afraid to push the ISO to avoid blur.

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