When it comes to exposure in photography, it seems that most photographers have a reasonably good understanding of the impact of both the shutter speed and the lens aperture. But the role of the ISO setting can be a bit more mysterious.
Of course, the shutter speed determines the extent to which motion in the frame is frozen or blurred. And the aperture primarily affects depth of field or the range of the scene that will appear in sharp focus. But what exactly does the ISO setting do?
The need for speed
The ISO setting on a digital camera is often compared to the ISO (or ASA) speed rating for film. Back then, the higher the speed the less light was needed to make an exposure. Higher ISO (or ASA) ratings meant faster filems. After all, with a higher ISO for both film and digital captures you are able to achieve a faster shutter speed than you otherwise would. However, with digital photography, the ISO setting is actually more similar to push processing of film than it is to the ISO rating for a film’ sensitivity to light.
With film photography, the ISO rating relates to the sensitivity to light for that film. With digital photography changing the ISO setting doesn’t actually alter the sensitivity of the image sensor in the camera. Rather, when you raise the ISO setting on a digital camera you are boosting the exposure by applying amplification to the signal recorded by the camera.
When film is push-processed, the images captured on that film will have greater contrast, more visible grain structure, and reduced resolution. Similarly, when you raise the ISO setting for a digital capture, the photos will exhibit reduced dynamic range and increased noise.
(Editor’s note: Push processing means increasing the time the film is in the developer chemistry to effectively add more exposure.)
When you raise the ISO setting on a digital camera, what you’re really doing is capturing photos that are under-exposed. Each doubling of the ISO number represents one f-stop of light. So raising the ISO from a base level of 100 on a given camera to a value of 800 means that you will be under-exposing the photo by three stops. That’s one stop for raising the ISO from 100 to 200, one more f/stop for 200 to 400, and a third stop for 400 to 800.
To compensate for the light that was not captured due to the shorter exposure time or smaller lens aperture, the signal gathered by the image sensor in the camera must be amplified. Because there was less information — meaning less light — in the original capture, the image with amplification applied will have more noise than an image captured at a lower ISO setting.
Noise is caused by underexposure
It would actually be fair to say that raising the ISO setting isn’t really what is adding noise to the image. Rather, it is the underexposure (sometimes extreme underexposure) that creates the additional noise. Noise, after all, is the opposite of information, and with less light being captured there will be less information in the photo.
By keeping the ISO setting at the lowest setting, you will be capturing photographic exposures based on the true sensitivity of the image sensor in your camera. That, in turn, helps to minimize the amount of noise that appears in the image. Raising the ISO setting enables you to use a shorter exposure durations or a smaller lens aperture size (or both), resulting in an image the is under-exposed and therefore contains less information than an image captured at a lower ISO setting.
The three exposure settings
The key is to keep in mind how each of the three settings for overall exposure affects your photos.
- The shutter speed enables you to determine whether movement within the scene is frozen or represented with a degree of blurred motion.
- The aperture on the lens allows you to choose how much of the scene will appear in sharp focus.
- The ISO setting primarily enables you to achieve a faster shutter speed or greater depth of field (or both), but at the cost of increased noise and reduced dynamic range in your photos.