Understanding your camera’s ISO control

Dave Johnson, TechHive
4 April, 2012
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Photography is often called “painting with light.” But what do you do when there isn’t any light? Well, unless you’re shooting inside a closet or at the bottom of a mineshaft, there’s always some light around. A photographer needs to make the most of whatever light you have access to. There’s a way to maximise the natural light in your scene: Using your camera’s ISO control.

ISO in a nutshell

An enlarged detail of a photo taken at ISO 1000. Notice the rough, sandpaper-like quality of everything in the scene, including the wall and the girl's complexion.

I get a lot of questions about ISO – many photographers don’t seem to understand exactly what it does. Your camera’s ISO control determines how sensitive the camera’s sensor is to light. On most cameras, ISO starts at 100 and goes up from there; the higher the number, the more sensitive the sensor will be.

Of course, that begs the question: Why wouldn’t you always just leave the ISO as high as it can go all the time?

That’s because ISO is a bit of a mixed bag. Higher ISO values give your camera a better light response, so you can take sharper photos with shorter shutter speeds in low light, but this comes at the expense of more digital noise in your photo. On the same camera, a picture captured at ISO 800 will tend to look noisier – random pixels that resemble grain on an old film camera or static on a television screen – than a photo shot at ISO 100.

Use ISO strategically

It’s a good idea to always shoot with the lowest possible ISO you can get away with. On many cameras, that means dialing in ISO 100 and leaving it there unless you have a good reason to increase it.

What reasons? Imagine you’re outdoors late in the day trying to take some photos and your flash won’t illuminate the scene because it’s too large or far away. In that case, crank up the ISO until the camera stops giving you a slow shutter warning. I’d suggest going with the lowest ISO that’ll give you a satisfactory photo in order to avoid introducing too much noise in the image. But don’t fret too much about this: It’s a lot better to capture a sharp photo with some noise in it than a shaky photo that was shot too slow for the available light.

You might also be able to rely on your camera’s Auto ISO setting. Check your camera’s user guide for details. On many cameras, you can set the ISO to Auto and it’ll dial the ISO up and down on its own when you shoot in certain modes (like Automatic exposure mode). I’m not a huge fan of Auto ISO because I don’t know exactly what the camera is doing, but it’s a convenient way to ensure you get the sharpest results without sweating over the settings.

ISO math

What, for example, is the difference between ISO 100 and ISO 200? Thankfully, cameras use a fairly consistent set of conventions, so that doubling the ISO doubles the light sensitivity. In this sense, ISO is like shutter speed or aperture. If you go from ISO 100 to ISO 400, that’s two stops of exposure change (doubled and then doubled again), so that’s equivalent to changing the shutter speed from 1/60 second to 1/15 second.

Put another way, suppose your camera is currently trying to take a photo at a shutter speed of 1/15 second at an ISO of 100. Change the ISO to 400 and the camera will now be able to take the same photo at 1/60 second, which is probably good enough to take a sharp photo. Change the ISO to 800 and the shutter speed will be 1/125 second.

ISOs go really high now

I’m old enough to remember when an ISO of 800 was very aggressive and 1600 was all but unheard of. These days, the party has barely started at ISO 1600. Camera manufacturers have made dramatic improvements in sensor technology in just the past few years and these days many cameras come with ISOs as high as 12,800. That’s a range of seven stops and it gives you incredible freedom to freeze the action with a fast shutter speed in extremely dim settings.

There are cameras that offer even higher ISO – the Nikon D3s, for example, goes as high as 102,400, which is ten stops of exposure control. Of course, you will see a significant amount of noise at those stratospheric ISOs, so if you use them, it pays to use some noise reduction software.

Cut down on noise

The noisy photo from earlier in this article, after it met a noise reduction filter.

Many photo editing programs, such as Adobe Photoshop Elements, have a noise reduction filter that cam significantly reduce the stray pixels of noise you get when shooting at a high ISO.

That said, you’ll get better results with a program that’s designed specifically to eliminate noise. My favourites – and ones I’ve recommended many times before – are Noiseware (available as a plug-in for Photoshop) and Noise Ninja.


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