ISO in Digital Photography
In digital photography, the term ISO refers to the image sensor’s sensitivity to light. The lower the number, the less sensitive the image sensor is to light.
As you may have noticed, the higher the ISO the more grainy an image becomes. This is why it’s often not considered to be good practice to only adjust the ISO in the absence of light. Most photographers (depending on the situation and their camera) will have to lower their shutter speeds, aperture and raise their ISO to compensate for the absence of a significant light source without making the image appear extremely grainy.
An ISO of 100 is generally accepted as the standard ISO and will result in very crispy photographs with little to no grain. Most people tend to keep their digital cameras set to ‘Auto’ which allows the camera to select the most suitable ISO depending on the conditions in which they are shooting. The camera will try to keep the ISO as low as possible, as would a photographer when manually adjusting the settings (in most cases).
As a part of the exposure triangle, when you adjust the ISO it changes the exposure of your photograph which means you may be more able to adjust the shutter speed and aperture. For example, if you’re shooting in decent daylight and are aiming for a well-exposed shot, you could choose to raise your ISO slightly so that you can increase your camera’s shutter speed. The purpose of increasing the shutter speed is so that you can capture movement such as sports, however, if you were to raise the shutter speed without adjusting ISO, the result would be an image of lower brightness.
When choosing ISO, I generally ask the following questions:
Light: Is the subject already well lit?
Grain: Do I want to avoid having any grain in my image?
Tripod: Can I use a tripod?
Situation: What am I photographing? A portrait or action sports?
If there is an abundance of light, I would prefer to have as little grain as possible so will use the lowest possible ISO and adjust my aperture and shutter speed to ensure that my image is still bright enough.
On the other hand, if it is dark, I will need to increase my ISO to ensure the image is still well-lit as this will still allow me to use higher shutter speeds to avoid capturing a blurry image.
ISO in Traditional and Film Photography
In film photography, ISO (or ASA) is the indicating of how sensitive a film is to light. Just as for digital photography, the value was measured in numbers and the lower the number, the lower the sensitivity of the film to light (and hence the less grainy the image produced)
What is ISO in photography?