Quite simply, ISO, when used in reference to digital photography, represents the light sensitivity of the image sensor. A lower value equals lower sensitivity, while a higher value equals greater sensitivity. The typical range of ISO values on a camera can vary, however just about all Digital SLR cameras have a range of about 100-1600. As newer cameras have been introduced in the past year or so, the upper end has expanded to 3200, 6400, and even 12800 in some higher end camera bodies. As the ISO value is turned up, the potential for noise in the photos also increases. Digital noise appears as grain and specs of color throughout darker areas of an image. Choosing the proper ISO is often a compromise because of the potential for noise. (See my post about reducing noise using Lightroom 3)
Along with the aperture and shutter speed, ISO is the third factor in the “exposure triangle”. These three settings work together to produce the exposure in an image. By exposure, I’m referring to the proper level of lighting in an image. Of course, exposure can be somewhat subjective based on the photographer and the scene in the photo itself. That being said, underexposed photos would typically appear too dark to the average person. An overexposed photo would appear too bright in most areas of the image and most likely have areas that were completely blown out to pure white.
Since the ISO value represents how sensitive the camera sensor is to light, it makes sense to use different ISO values based on the lighting situation you are in. I say this with the assumption that a flash is not being used and that you are holding the camera in your hands rather than using a tripod. A good flash can help to overcome challenging lighting situations and a tripod can be used to steady the camera and allow you to use a slow shutter speed. I’m sure I will eventually discuss using flash in a future post. 🙂 A good rule of thumb is to start at ISO 400 in normal daylight. If it’s a very bright day, you can change to ISO 100 or 200 and reduce the potential for noise to almost nothing. On the other hand, as lighting drops off, you will want to start increasing the ISO to a higher value. Indoor settings typically have lower lighting, so it’s not a bad idea to move up to ISO 800 or more.
Assuming you are using a fairly constant aperture setting, the main reason for changing the ISO setting is to allow for a decent shutter speed that will help to prevent blurry pictures due to camera shake. Let’s say for example that you are shooting indoors with no flash and your aperture is set to f3.5 with the ISO at 400. In order to make a proper exposure, according to the meter in the camera, you need to use a shutter speed of say, 1/15 of a second. Well, that’s a pretty slow shutter speed. There is a pretty decent chance that even if the lens has image stabilization built-in, you will end up with a blurry photo due to hand holding the camera. In order to get a faster shutter speed, you will need to crank up the ISO. By increasing the ISO to 800, you can get the shutter speed to 1/30 of a second. Better, but still kind of slow. Increasing ISO further to 1600 will get you 1/60 of a second. This is definitely a more useable shutter speed than 1/15. The trade-off, of course, is more noise.
I think that the easiest thing to do is to start at ISO 400, like I mentioned, and then experiment with your aperture and shutter speed. If you find that your shutter speeds are slowing down, i.e. slower than 1/60 of a second, try turning up the ISO. Go from 400 to 500 and see what that does for the shutter speed. Continue to increase to 640 and then 800. (note: some lower end cameras may not have the middle values so they may jump right from 400 to 800).
The more you experiment and learn how to use the settings, the more you will be able produce properly exposed photos in different lighting situations. When it comes to photography this is really the core of it; learning to get the right amount of light into the camera.
By the way, the photo above was taken at Plan B Restaurant in Glastonbury, CT, which has pretty low lighting. You’ll notice that I cranked up the ISO to 1600, which is the highest normal ISO setting that the Nikon D60 can do. Even at an aperture of f4 and ISO of 1600, I could only get a shutter speed of 1/5 of a second. The trick here was to rest the camera on the table so that I wouldn’t get any camera shake from hand holding it. I was also careful to press the shutter button firmly and not let it go until after the shutter had opened and closed. This also helps to reduce camera shake resulting from pressing the button and just letting go of it immediately. I liked how the lower angle, natural lighting and somewhat shallow depth of field produced a more interesting look rather than a straight on shot with flash.