The ISO is one of the standards that measured light sensitivity of film, which has also been adopted for the digital camera sensor. Different ISO’s were used to suit specific situations and lighting conditions. But the good thing about digital cameras is that, unlike with film, you can change your ISO setting with each shot.
A lower ISO can provide you with greater image detail. The downside is that you need more light coming in through the lens to provide that detail. That means an appropriate shutter speed and aperture opening. As you raise the ISO, you would require less and less light.
To illustrate, below is a series of pictures of the same subject. All pictures are of the same frame with different exposure setting, except for the aperture, which was locked at f/8.
At ISO 100, you can see a fairly clear image. However, the camera’s on board computer set the shutter speed at a fairly slow 1/15 seconds. I would not have gotten a sharp enough exposure without a tripod.
At ISO 200, the camera shot at a slightly higher speed at 1/25 sec. It’s hardly visible even if you’re zoomed in the image 100%, but digital artifacts do start to appear, particularly along the darker parts of the background.
The trend continues at ISO 400. And the noise is now more noticeable.
By ISO 800, the noise level has already significantly affected the whole image.
The Nikon D80 reaches the end of its normal ISO range at 1600. As you can see, the imagehas become covered with noise all throughout.
Conventional wisdom dictates that photographers use the lowest ISO they can get away with. True enough, generally speaking, cameras are at their best at the lowest ISO setting, which is usually 100. This is typically raised when shooting under darker conditions. A higher light sensitivity can allow the camera to shoot at faster shutter speeds. However, that comes at the cost of losing detail, essentially replaced by noise.
It is also important to note that despite what the acronym ISO implies, performance does vary among cameras of different makes and models. Older digital cameras like the Nikon D80 used for the pictures here have difficulty in maintaining image quality at ISOs beyond 200.
Rowse, D. (undated). ISO settings in digital photography. Retrieved from http://digital-photography-school.com/iso-settings.
Rowse, D. (undated). Learning about exposure – the exposure triangle. Retrieved from http://digital-photography-school.com/learning-exposure-in-digital-photography.
Youn, J. (undated). 99c Photography Guide (Kindle version). Retrieved from http://www.amazon.com