Digital Photography and ISO
ISO (International Organization for Standardization) is a setting on your digital camera that controls the sensitivity of your camera sensor. If you photograph on a bright sunny day, you have lots of light. Therefore you can use the combination of a relatively fast shutter speed with a relatively small aperture to properly expose the image.
Add some clouds to the equation, and you have less light (a lower Exposure Value). In that case, you’ll either need to use a larger aperture (known as opening the aperture in photo speak) or a slower shutter speed to properly expose the image.
If the shutter speed is too slow for the subject you’re photographing, or the aperture is too large for the landscape you’re photographing, you have two choices: Come back when there’s more light, or increase the sensitivity of the camera sensor.
If you come from a film background, you may remember something known as film speed. When you were photographing in bright sunny conditions, you used a slow speed film. When you were photographing in low light situations, you used a fast film.
The problem with film was you were stuck with the film speed until you shot all the frames. This posed a problem if conditions changed rapidly. This is one of the beauties of shooting digitally. You can change the sensitivity of your camera sensor on the fly using a menu command or camera dial.
To increase the sensitivity of your camera sensor, you increase the ISO setting. Use a low ISO setting in bright conditions. Increase the ISO setting when shooting in dim lighting conditions, or when you need a smaller aperture or a faster shutter speed. For example, if you need to use the shutter speed to freeze the motion of a bird in flight on a cloudy day, increase the ISO setting.
Increasing the ISO setting is a dual edged sword. When you increase the ISO setting, you can capture photographs in lower light conditions. Increasing camera sensitivity, however, also increases digital noise. Some people equate digital noise to film grain, but they are not the same. Because film grain has a pattern, it adds interest and character to a photograph.
Digital noise, however, is random mishmosh created by electric circuitry that degrades an image. The amount of noise a digital camera generates varies depending on the age of the camera and the size of the sensor. Smaller sensors and cameras built with older technology are noisier than newer cameras.
The amount of megapixels a camera captures is also a determining factor. When a camera manufacturer crams lots of pixels onto a small sensor, the size of each pixel is smaller. This results in a noisier image at any ISO setting. Digital noise is readily apparent in the shadow areas of an image when the image is enlarged.
To determine the highest ISO at which your camera delivers acceptable images, place your camera on a tripod and take a series of images at night. Make sure that part of the scene you capture has dark shadows. Take the series of pictures starting with your lowest ISO rating up to your highest.
Download the images to your computer, increase the magnification to 200 percent, and then scroll to the shadow areas of each image. When you see an image that has unacceptable noise, note the ISO setting and never use this setting or higher unless you absolutely have to.