ISO is the value that represents the speed in which a photographic negative is exposed. In other words, the higher the number, the less light is required to properly expose the image (and the grainier the picture is, which we'll talk about in a minute).
This concept has been translated to digital cameras as well. Back in the old days of film photography, each film canister would have a number on it such as 100, 200, or 400. This concept is now a setting in each digital camera and most cameras can select between a wide range of ISO settings. Typical ranges are from 200-1600; however, newer cameras have a wider range such as 100-6400. The beauty of digital cameras is that this setting can be changed for each image, where before, you were stuck with one speed of film per roll.
How ISO affects exposure
The ISO is one of three factors in determining the correct exposure. The other two are the shutter speed and the aperture settings. When the ISO is set to a low number such as 100 or 200, the camera requires more light to properly expose the shot. This is compensated by either a larger aperture setting or a slower shutter speed. However, the opposite applies when the ISO is set to a high setting such as 1600 or 3200. The camera doesn't need as much light, so the shutter speed can be set much faster or the aperture set to a smaller setting.
Image noise and its relationship to ISO
When somebody talks about noise in an image, they're referring to how grainy the image is. This is similar, but different to a picture being overly pixelated. Technically, noise is any aspect of the image that is created by something other than light.
The lower the ISO rating, the more light is required, but the less noise is introduced into the photo. Therefore, it's a good idea to keep the ISO as low as possible, while still being able to properly expose the photo.
How the image sensor's size affects ISO
The size of the image sensor affects the range of ISO settings the camera can have. The larger the sensor, the greater the ISO range is. Point and shoot consumer cameras often have a sensor that is 1/1.8 inch in size (roughly 7.2 x 5.3mm) while most DSLR cameras have a larger sensor that is 23x15mm. High end digital SLRs may have a full frame sized sensor that is 24x36mm.
Why this is important is because the larger the sensor, the larger the pixels can be. When the pixels are larger, they receive more light which reduces noise. It also makes it so the range of ISO settings are greater.
Scenarios for when to change the ISO setting
- When shooting indoors with low light, often you may need to increase the ISO in order to get a good exposure without using a flash
- When shooting outdoors in sunny conditions, drop the ISO to the lowest setting to reduce noise and end up with a better quality photo
- When using a telephoto lens, where its necessary to shoot with a fast shutter speed, increase the ISO to compensate for less light
- When using a wide angle lens, where its possible to shoot at a slow shutter speed, decrease the ISO for better results
- When shooting lightning, where the bolt itself can easily overexpose a photo, decrease the ISO for both better quality and to reduce the amount of light coming into the camera
ISO represents the speed of the film or digital sensor; i.e. how much light is required to expose the photo. The larger the sensor, the better. DSLRs have a much larger sensor than consumer point and shoot cameras, which is one of the reasons why they are able to produce better images.
Use a higher ISO to compensate for low light situations, but use the lowest ISO setting possible for each specific situation to reduce the grain in the image. Most cameras have an "Auto ISO" option that is good until you get the hang of the different settings.