With digital censors, ISO refers to the amount of amplification of signal and affects exposure of your image by affecting shutter speed and aperture combination that you use. ISO value indicates camera’s image censor’s sensitivity to light. That means ISO gives idea of how sensitive image sensor is to the light. High ISO value means your sensor is more sensitive with light and similarly low ISO value means less sensitive with light.
You can take pictures in low-light when ISO value is set to high in your camera. But at the same time high ISO value also introduces digital noise because at higher ISO, artificial electric gain is added to the signal. It’s not coming from the natural light source, its the light being produced artificially and amplified by camera using signal booster.
When ISO is set auto to your camera, camera will automatically detect light condition and increase or decrease ISO value as per needed. You can manually set ISO value to adjust proper exposure under different lighting condition. Higher ISO value allows you to set faster shutter speed to freeze motion when your aperture can’t be more wide to allow more light. But you also have to consider noise factor when increasing ISO value high enough. Lower ISO value means that there must have more light available to take picture and you can take photos with proper exposure using combination of faster shutter and wide open aperture. Increasing or decreasing ISO value by double (from 200 to 400, 400 to 800 etc or reverse) is same as increasing or decreasing exposure by one f-stop.
Using ISO values higher and higher means your camera sensor will capture images at very low light condition but also produces noises (grains) in the picture. You won’t notice this grain in small size picture but if you enlarge photo, you can easily notice effects of high ISO vs low ISO. So try to shoot within 400 ISO value or less as far as possible. But some cameras like Nikon D90 produces acceptable quality of picture at higher ISO as well.
If you are looking for Nikon D90 specific ISO settings, please click here.
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