The exposure is controlled by 3 (three) adjustments we make in the camera: shutter speed, aperture and sensitivity. Shutter speed and aperture ultimately control how much light comes into the camera (we can compare this to a water tap – how much you open the valve and for how long will dictate the amount of water that will flow)… and how much light is needed for a certain exposure is determined by the sensitivity of the medium used (today it has been expressed as ISO numbers – and as ASA not so long ago).
In the digital cameras world today, we can find ISO ranges from as low as 50 up to 204,800, being the normal range placed somewhere between 200 and 1600. These numbers have some qualities associated with them: it sets the amount of light needed for a good exposure, and the lower the number, the more light is required, and as a consequence for a fixed aperture, a slow shutter speed will have to be used; and will influence the amount of noise in the image.
So, if you have lots of light (or have the camera mounted on a tripod), the lower the numbers you can set, and on the other hand, when you do not have lots of it or you need a faster shutter speed (for action and sports shots, for example), you will need to raise the ISO (and this is what the AUTOISO settings in your camera do: adjusts the ISO settings so you end up with the correct camera measured exposure for a given situation. You normally set the minimum and maximum range you want the camera to automatically adjust the ISO based on your acceptable quality and speed requirements).
It is worth knowing that each time you double the ISO (for example, from 100 to 200 or from 400 to 800), half of the light is needed for the same exposure, and vice-versa.
As we mentioned above, noise levels will also be influenced by the ISO settings, and the higher the number is, the more noise and visible grain an image will have. We normallywant the images to have the least amount of noise as possible.
Today, most digital cameras can make good quality images at ISOs up to 800 or 1600 and above, but several aspects affect this, from the sensor type a camera uses (for example, the size of the pixels used on the camera’s sensor, which are larger in SLRs compared to the compact ones. Larger pixels result in less noise and SLRs have larger sensors with larger pixels) to the amount and type of noise reduction algorithms and systems used in the cameras.