Having trouble with blurry indoor pictures? Your flash might help, but flashes can wash out your colors, leave your friends with demonic-looking red-eye, or wake that beautiful sleeping baby you're trying to photograph. Fear not, friends. Digital cameras come with a tool to help you capture that low-light picture: your camera's ISO setting. This article discusses what causes blurry images, examines how your camera's ISO setting works, and then offers some practical tips for using ISO to improve your photography.

What causes blurry pictures?

Before we tackle ISO, let's begin by discussing what causes blurry pictures. More often than not, blurry picture problems have nothing to do with focus. The problem is motion. As your camera was capturing the picture, either a) your camera moved, b) your subject moved, or c) both. One possible solution to this problem would be to put the camera on a solid base like a tripod. Tripods eliminate the camera movement problem but don't do anything to eliminate blur caused by subject movement. To eliminate blur caused by subject and camera movement, you really only have one possibility: capture the image quicker. If you can force your camera to decrease the amount of time it takes to capture the image, then there will be less time for anything to move. The result will be sharper pictures.

So, to eliminate blur, we need to use a faster capture speed. To understand how to increase capture speed, we need to understand a little bit about how a camera exposes images.

Exposure 101

All digital cameras include a built-in light meter to help the camera produce a properly exposed picture (i.e. not too dark, not too light). Before you take a shot, your camera's light meter examines the scene you're attempting to capture and measures how much light is available. Once it understands how much light is needed to give you a properly exposed image, your camera can adjust three settings to allow just the right amount of light in. Those three settings are shutter speed, lens aperture, and ISO setting. Let's take a look at each.

Shutter Speed

Some cameras use a physical piece of metal between the lens and the sensor - a "mechanical shutter" - to block light. When you press the shutter release button to take the picture, the shutter flips open and allows light to come through the lens and hit the sensor. After a period of time has passed, the shutter flips closed again (making a pleasant "click" sound) and no more light can hit the sensor. That duration of time while the shutter is open is called the "shutter speed". Long shutter speeds allow lots of light in. Short shutter speeds allow less light in.

Some digital cameras today don't use a physical piece of metal for a shutter. Those cameras use an "electronic shutter". All this means is that the camera turns on the sensor chip for a period of time to capture light and then turns it off again. This period of time is still called "shutter speed" and works the same way as mechanical shutter speed. If the sensor stays on a long time (i.e. a slow shutter speed), it will capture more light. If the sensor stays on only briefly (i.e. a fast shutter speed), it will capture less light.

Lens Aperture

The second setting the camera can change to adjust exposure is lens aperture. Inside your camera's lens, there is an adjustable hole that can be made bigger or smaller. The hole or "aperture" is pictured below.

If the aperture is larger, like the picture on the left, more light can come through the lens and hit the sensor. If the aperture is smaller, like the picture on the right, less light comes in.

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