Exposure – ISO explained

This is part of a four-part series on exposure.

Click here to read an overview of exposure, and for shutter speed, and aperture.


Once your aperture has allowed your chosen amount of light into the lens, and the shutter speed has controlled the length of time the light has passed through the lens, the ISO controls how much of that light is absorbed.

The ISO is the camera’s measurement of light sensitivity. In analog photography, a higher ISO film absorbs the light faster, and film with a lower ISO number absorbs light more slowly. In digital photography it works in the same way, just with an electronic sensor in place of the film. A low number equals slower absorption.

Like both shutter speed and aperture, the ISO increases in stops that double or halve the light absorbed.

standard digital ISO range

Standard digital ISO range

Some newer dSLRs have a higher range than this, but the principle is still the same. These stops correlate with the shutter speed and aperture stops. One ISO stop up or down equals one f/stop up or down, or one shutter speed stop.

Okay, that’s interesting, right? But how is it useful?

Well. Lets say that I develop a sudden need to take a photo of the wooden elephants on my desk. It’s 6pm and the sun is sinking fast, but I need that photo now, or else. No idea or else what, but, or else.

Wooden elephants from Nepal - with motion blur from a low ISO and slow shutter-speed

(Photo taken at 0.5", f/2.0, ISO 100)

Even with my aperture open to f/2.0, the shutter speed needs to be at half a second to capture enough light for a correct exposure. There’s no way I can take a handheld photo at a shutter speed this slow and not have it blur dreadfully. But wait, the ISO is down to only 100. If I bump that up two stops, I can use a shutter speed that’s two stops faster.

As so:

Wooden elephants from Nepal - ISO 400, no motion blur.

(Photo taken at 1/8, f/2.0, ISO 400)

Motion blur begone. 1/8 is still pretty slow, but it’s enough for me to brace my arm against the desk and keep the camera still. In this way, if you’re struggling with low light, increasing your ISO will often help you achieve the exposure you’re looking for.

Of course, changing the ISO affects not only the amount of light absorbed, but also the amount of grain in the image. A low ISO provides a low-grain, smooth effect. As the ISO setting gets higher, grain begins to appear in the photo. Depending on your camera, the grain shouldn’t have too much impact on your image until you start to go higher than about ISO 400. From ISO 800 onwards, you’ll begin to notice pronounced grain.

Here’s an 100% view of one of the helicopters I saw the other week:

100% crop of helicopter - high ISO 800 with lots of grain

(Photo taken at 1/4000, f/4.5, ISO 800)

As you can see, at ISO 800, the colours are far from smooth.

Sometimes a high ISO can be exactly what you need for getting those shots you would have otherwise missed due to low light. This rather creepy photo is taken deep underground in the Paris catacombs (I laughed at the warning sign as we queued that said the tour, “could make a strong impression on people of a nervous disposition”, from now on I will heed signs like that, no matter how quaint the wording), with virtually no lighting around me. I could never have taken it if it wasn’t for a very high ISO setting of 3200.

Skull underground in Paris catacombes - ISO 3200

(Photo taken at 0.3", f/3.3, ISO 3200)

In most circumstances however, it’s advisable to keep your ISO as low as possible, unless you are purposely aiming for a grainy effect. It’s much easier to add grain to a photo in post-production than it is to deal with unwanted grain that’s there from the time you take the photo.

So where does this leave us?

While writing this four-part introduction to achieving a correct exposure, I have personally gained a greater level of insight into the balance between shutter speed, aperture and ISO, and I have found it fascinating to go through my old photos with an eye on the camera settings used. There are many photos in my folders that could have been improved hugely had I put a little more thought into the settings I used when I took them.

With this in mind, lets all make an effort to be more conscious of our camera settings and think about the creative effect we want in our images as we’re taking them, instead of as an after thought.

Play with the settings on your camera, try the same shot with a high DoF (small aperture), and a low DoF (large aperture), or experiment with creative motion blur. In digital photography we don’t have to pay for film or processing – take as many photos as you can on different settings, and watch the quality of your shots improve as you gain more control over the final result.

That’s what I plan to do, and I’d love to have company along the way!


2 thoughts on “Exposure – ISO explained

  1. Another really simple and easy to read explanation, merci! I would say I’ll play with the settings on my camera too, but since still only have my out of charge camera with no charger. Will definitely be less lazy about it when it is working.

    And oh, Les Catacombes de Paris! Quite frightening.

  2. Pingback: Shooting in Manual – the easy way! |

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