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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ISO Setting
The final setting the camera can use to adjust exposure is the ISO setting. ISO is simply a measure of how sensitive the sensor chip is to light. At a low ISO setting (like 100), the sensor is relatively insensitive to light. At higher ISO's (like 400 or 800), the sensor is more sensitive. The more sensitive the chip is, the less light it needs to properly expose the picture.
Film shooters should remember the concept of ISO from the ISO rating on their film. Film comes rated at different ISO numbers (100, 200, 400, etc) based on how sensitive the film is to light. Digital ISO is the same concept, but with digital we're talking about the sensitivity of a chip instead of film. With digital, you can change that sensitivity anytime with the press of a button. With film, the photographer has to put a whole new roll of film in the camera to change light sensitivity.
Getting The Shot
The camera knows that it needs a certain amount of light to capture the image and, as we discussed, it has several ways to capture that amount of light. It could open the lens aperture to its largest setting and use a fast shutter speed. It could close the aperture to a smaller hole and use a slower shutter speed. Or it could turn up the sensor chip's sensitivity (i.e. the ISO setting) and get away with a smaller aperture and a faster shutter speed. Any combination of those three settings could produce a properly exposed picture, so what's the "right" answer?
In this case, we're trying to eliminate blur by getting the camera to capture the image faster. That means that we'll need a fast shutter speed. If the shutter is open for a shorter period of time, less light comes in, so we'll need to adjust the other two settings - aperture and ISO - to make up for that loss of light.
If you're shooting in a brightly lit environment - outdoors in daylight, for example - your camera can probably compensate for a fast shutter speed by simply opening up a larger lens aperture. Unfortunately, there is a limit to how wide the aperture can grow. If your scene has lower light - indoors or in strong shade, for example - opening a large aperture may not be enough. In this case, you'll need to increase your camera's ISO setting to get a fast enough shutter speed to ensure sharp pictures .
Let's take a look at two pictures that illustrate how ISO can affect the sharpness of a picture. The pictures were taken indoors with no flash. In both pictures, the aperture of the lens is set as wide open as it will go, so the only variables here are ISO and shutter speed.
Click for full size
In the picture on the left, ISO was set to 100 (the lowest/least-sensitive my camera will allow). With this ISO setting and a wide open aperture, the camera set the shutter speed to 1/25th of a second. Unfortunately this was too slow and the shooter couldn't hold the camera perfectly still. The result is a blurry picture.
For the picture on the right, ISO was turned up to 800. With ISO 800 and the same wide open aperture, the camera set the shutter speed to 1/200th of a second. Note that ISO was turned up to make the sensor 8 times more sensitive (ISO 100 to ISO 800) and the shutter speed became 8 times faster (1/25th of a second jumped up to 1/200th of a second). At this higher shutter speed, the shot was taken fast enough that movement wasn't a problem. The result is a much sharper looking picture.
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The Tradeoff
So if a higher ISO setting lets you use a faster shutter speed, why not always use the highest ISO setting your camera can set? There's got to be a catch, right? Yes, unfortunately there is. The cost for high ISO is NOISE.
As I discussed in "The Megapixel Myth" article, noise shows up in your pictures as little off-color spots throughout the picture. For example, in this beaver picture, noise appears as red, green, and blue dots all over the image.
Click for full size
Because I shot this picture in near darkness, I had to crank up my ISO setting to its maximum value just to get the shot. The high ISO "dots" are quite distracting and really make this a low quality image. Because of the noise, this picture will never hang on my wall. On the other hand, if I hadn't cranked up ISO and captured the picture, I wouldn't have gotten the shot and no one would believe that a beaver had just moved in behind my house.
In most digital cameras, low ISO settings (50, 100) are pretty noise-free. The higher the ISO goes, the more noise shows up. Cameras differ greatly in how much noise they exhibit at higher ISO levels. Take some sample shots with your camera at each ISO setting to get a feel for how much noise to expect at each level. This way you'll know how high you can set your ISO and still get satisfactory image quality.
Recommendations:
Now that we've covered the basics, let's conclude with a few practical ISO recommendations to help you get the best shots:
1. Many consumer-level cameras have an "Auto" ISO setting. This lets the camera pick an ISO setting for you depending on the light level. Give this setting a try. Much of the time, the auto setting will pick a pretty good ISO setting for you. It will generally try to keep ISO low when there's a lot of light (to reduce noise) and will raise ISO when there's less light. Unfortunately, the auto setting may avoid the highest ISO settings to avoid noise. This could result in blurry pictures.
If "Auto" ISO isn't producing satisfactory results or if you have a high-end camera that doesn't have an "Auto" ISO setting, these next few guidelines are for you:
2. In most outdoor daylight situations, you will have more than enough light to get a quick shutter speed using your lowest ISO setting. Thus, when shooting in daylight, start with ISO at the lowest setting to reduce noise. Only if blur becomes a problem should you turn ISO up.
3. If you're shooting with flash, the camera should have plenty of light without increasing the ISO setting. For flash photography, use your camera's lowest ISO setting to keep noise low.
4. If you're shooting indoors without flash, start cranking that ISO up. Yes, you may start to see noise if your ISO gets high enough, but noise is vastly preferable to blur. Much better to get the picture with a little noise than to destory the picture altogether. Experiment to find the lowest ISO you can use and still get a sharp shot at a particular light level.
5. You may notice that some of your zoomed pictures look blurry, even in strong daylight. Zoom actually magnifies the camera shake problem, so you may need an even faster shutter speed (or a tripod) to take those extreme zoom shots. If your zoom shots are coming out blurry, even in daylight, try punching up the ISO a couple levels to force a faster shutter speed.
6. If you're shopping for a new digital camera, be sure to compare the high ISO capability of each model that you're considering. Take a number of sample shots in the high ISO range (400, 800 and higher) and look for noise. Be sure your sample shots include dark areas (like shadows) as noise is most noticeable in darker tones. If you can find a camera that is relatively noise-free at high ISO settings, you'll have found a camera that should perform well in low light.
