key concept ISO (sensitivity) settings
Along with shutter speed and aperture, it controls exposure
Your main tools for adjusting exposure are the length of the exposure (shutter speed) and the amount of light (lens aperture). But digital cameras have introduced a third option – ISO.
Increasing the ISO setting on your Nikon is like turning up the volume. If the light is poor, increasing the ISO will amplify it and make it stronger (though this does also increase digital ‘noise).
Some photographers now include ISO as an exposure adjustment alongside shutter speed and lens aperture to produce an ‘exposure triangle’, because it works on the same basis – you can increase the ISO to compensate for a smaller lens aperture or faster shutter speed. This quickly gets complicated, though. It’s simpler to stick to shutter speed and aperture for routine exposure adjustments and only increase the ISO if the conditions are so poor than you can’t get usable settings any other way.
You don’t always need to increase the ISO at night. If you put the camera on a tripod, you can leave it set to its lowest ISO and use as long an exposure as you need.
Shutter speed, lens aperture and ISO settings have been cleverly arranged so that each setting delivers twice the exposure of the one before. You can use this to balance these settings up so that you keep the same overall exposure even though you’ve changed one of the values. For example, 1/1000 sec at f/4 gives the same exposure as 1/125 sec at f/11.