Short answer: Digital ISO is an exposure index that approximates the end result you would achieve if you were using film of the same ISO. The lower the ISO the sharper and clearer the image will appear. The higher the ISO the more noise and grain you will notice in the image.
ISO for film is a measure of how sensitive that film is to light. A film with an ISO of 800 is more sensitive to light than a film with an ISO of 100.
For digital cameras the ISO is a measure of signal gain. A digital image sensor, the equivalent of the film in a film camera, cannot change it’s ISO value like you can swap out one film type for another. The sensor has one value which will generally be the lowest value possible, for instance this is ISO 100 on my Sony Alpha 6000.
The ISO value on a digital camera indicates how much the signal from the image sensor is being amplified. E.g. setting the ISO to 1600 means the image is being brightened.
Why is ISO Important?
A high ISO will make noise and grain in your images much more apparent. For instance check out the below images:
A high ISO is not creating noise but making it more noticeable by amplifying the signal. More sophisticated, expensive, camera bodies will produce better results at higher ISO values and give you more flexibility. This is something to look out for when analysing sample images and choosing a camera body to suit your needs.
Tips for the Best Results
In general to get sharp images you want to use the lowest ISO you can. The auto setting on your camera will generally take care of this, especially if you are in bright sunny conditions. However, I’ve noticed my camera will bump up the ISO unnecessarily in some lighting, like overcast weather, and prioritise a quicker shutter speed. With this in mind, ISO is a very important metric to be aware of when you’re shooting and something you will want to override some of the time.
Times when you want to use a higher ISO value are when you want to freeze motion with a fast shutter speed and your aperture is wide open but the image would be dark unless you bump up the ISO.
- Here’s a great 2 minute video from Apalapse on YouTube, I recommend checking out: https://www.youtube.com/watch?v=iagWndFwcz8&ab_channel=Apalapse
- Digital Photography School: https://digital-photography-school.com/iso-settings/
- Wikipedia: https://en.wikipedia.org/wiki/Film_speed#Digital_camera_ISO_speed_and_exposure_index