ISO (pronounced “eye-so”) allows you to control the camera’s light sensitivity based on a numerical system. The lower the ISO number (e.g. 200 or 400), the less sensitive the camera will be to light, so you need good, strong lighting to capture your subject well. The higher you set the ISO numbers (e.g. 1,600 and beyond) the more sensitive the camera’s sensor becomes, so you can use the camera in low light without a flash.
However, as you increase the ISO, you will have a slightly reduced quality of the image as it will get grainer. The grainy look is called ‘electronic noise’. A customer mentioned that decades ago when she was shooting film, she used to buy color negative film and slide film that was rated at speeds of 25, 64, and 50… yet her digital camera did not have an option below 200 ISO. Why?
The majority of DSLRs tend to start around 200 ISO and stop at 6,400 ISO. Each camera has a base ISO which provides the cleanest image for that camera. The ISO range available on your digital camera is called its ‘native setting’ and the engineers designed it to work at its most optimum level within those limitations. In some cameras you can select ‘Extended ISO’ mode, which opens up additional settings.
This will let you drop below 200 ISO, or extend the range to a ridiculously high ISO number such as 51,200! But extended ISO is really just a marketing gimmick. Ultra high ISO is quite impressive, but all it means is that your camera can do some software manipulation to take it beyond the normal range. And when you go beyond the ‘native settings’ you will end up with reduced quality.