What is ISO? | Settings in Digital Photography

admin November 3, 2020

ISO is one of three important settings on your camera that are used to take well-exposed photos.

The other two fundamental settings are aperture and shutter speed – and if you’d like to learn about these settings, check out our introduction to aperture and our beginner’s guide to shutter speed.

An introduction to ISO settings in photography

We regularly get questions about ISO from readers of Digital Photography School like these:

What is ISO and why is it important?
What is the best ISO setting?
Should I always choose the lowest ISO?

In this short tutorial, I want to answer each question in turn.

Let’s start with a definition of ISO:

What is ISO?

ISO actually has two definitions, depending on whether you’re discussing film or digital photography:

ISO in traditional/film photography

In traditional (film) photography, ISO (or ASA) is an indication of how sensitive a film is to light.

In film photography, ISO is measured in numbers. You’ve probably seen them on film canisters: 100, 200, 400, 800, etc.

The lower the number, the lower the sensitivity of the film, and the finer the grain that will appear in your photos.

ISO in digital photography

In digital photography, ISO measures the sensitivity of the image sensor.

The same principles apply as in film photography:

The lower the number, the less sensitive your camera is to light and the finer the resulting grain.

Higher numbers mean your sensor is more sensitive to light, and this allows you to use your camera in darker situations. The cost of doing so is more grain – although cameras are improving all the time. Today, many cameras can use high ISO settings and still get very useable images.

Here’s a time when you might want to choose a higher ISO:

If you’re photographing an indoor sporting event where the light is low and your subject is moving fast.

In such a situation, by choosing a higher ISO, you can use a faster shutter speed to freeze the athletes’ movement.

ISO settings and grain

Choosing higher ISO settings comes with a serious cost:

The higher your ISO, the more grain or noise that appears in your photos.

I’ll illustrate this below with two enlargements. The image on the left was taken at ISO 100, and the image on the right was taken at ISO 3200.

Can you see the difference? The high-ISO photo (right) is full of unpleasant noise, whereas the low-ISO photo (left) is completely clean.

ISO 100 is a “normal” or “standard” ISO on most cameras. It’ll give you lovely, crisp shots (with little to no noise or grain).

Most people tend to keep their digital cameras set to Auto mode, where the camera selects the appropriate ISO setting depending on the conditions you’re shooting in. However, most cameras also give you the opportunity to select your own ISO.

When you do override your camera to choose a specific ISO, you’ll notice that this impacts the aperture and shutter speed needed for a well-exposed shot.

For example:

If you bumped your ISO up from 100 to 400, you’ll notice that you can shoot at faster shutter speeds and/or smaller apertures.

Questions to ask when choosing ISO

When choosing my ISO setting, I generally ask myself the following four questions:

Light – Is the subject well lit?
Grain – Do I want a grainy shot or one without noise?
Tripod – Am I using a tripod?
Moving subject – Is my subject moving or stationary?

If there is plenty of light, I want little grain, I’m using a tripod, and/or my subject is stationary, then I will generally use a relatively low ISO rating.

If it’s dark, I want grain, I don’t have a tripod, and/or my subject is moving, I might consider increasing my ISO. This will enable me to shoot with a faster shutter speed and still expose the shot well.

Of course, the trade-off will be noisier shots.

Situations where you might need to push ISO higher include:

Indoor sports events, where your subject is moving fast, yet you have limited available light
Concerts, which are low in light and often “no-flash” zones
Art galleries, churches, and more, where there are rules against using a flash and the interiors aren’t especially well lit
Birthday parties, where blowing out the candles in a dark room can give you a nice, moody shot that would be ruined by a bright flash. Increasing the ISO can help capture the scene.

ISO is an important aspect of digital photography to have an understanding of if you want to gain more control over your digital camera. So experiment with different settings and how they impact your images. And in particular, learn more about aperture and shutter speed, which – along with ISO – are a part of the exposure triangle.

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