ISO is one of the three pillars of the photography along with the shutter speed and the aperture. When we are talking about the camera with digital censor, ISO refers to the amount of amplification of the signal. It affects the exposure of the image by affecting the shutter speed and the aperture combination used in camera. ISO value indicates the camera's image censor's sensitivity to the light and gives us an idea of how sensitive the image sensor is to the light. High ISO value means image sensor is more sensitive to the light and similarly, low ISO value means sensor is less sensitive to the light. You can take pictures in a low-light condition when the ISO value is set to high in your camera. But at the same time, setting up high ISO value also introduces the digital noise in the picture because when we increase ISO value, artificial electric gain is added to the signal to provide more lights into the image. It's not coming from the natural light source but the light is being produced artificially and amplified by the camera using the signal booster.
When the ISO is set to Auto, camera’s metering system will automatically detect the light condition and increase or decrease the ISO value as needed. If you are using your camera into semi-manual mode or Manual mode (M), you can also manually set the ISO value to adjust the proper exposure under different lighting condition. Higher ISO value allows you to set the faster shutter speed so that you can freeze the motion when your aperture couldn't be more wider to allow more lights into the sensor. But you also have to consider the noise factor when increasing the ISO after certain value. Some cameras specially consumer level DSLRs are highly susceptible to the noise with higher ISO value whereas full frame cameras can take much higher ISO value without producing noticeable noise in the picture. When you have enough lights available, you should use lower ISO value and adjust the proper exposure using the combination of faster shutter and wide open aperture (smaller f value). Increasing or decreasing ISO value by double (from 200 to 400, 400 to 800 etc or reverse) is same as increasing or decreasing the exposure by one f-stop.
Focal Length - 50mm Aperture - f/1.8 Exposure - 1/1.3 sec ISO - 800
All of these pictures are taken in an Aperture Priority (A) mode with f/1.8 aperture and only changing the ISO value. The first picture was taken with ISO 800 and the camera adjusted the shutter speed to 0.769 seconds automatically to balance the exposure. This picture is almost noise free and much more cleaner than the second shot where the ISO was bumped to 6400 and then the camera accordingly adjusted the shutter speed to 0.0769 seconds to adjust the exposure. By increasing ISO value, I was able to shoot with much faster shutter speed but it produced the noise in the picture as well.
Focal Length - 50mm Aperture - f/1.8 Exposure - 1/13 sec ISO - 6400
I cropped the corner area of the second image to demonstrate the digital noise in the picture. If you look at the picture below more closely, you can clearly see the grainy noise all over the picture introduced by high ISO.
Focal Length - 50mm Aperture - f/1.8 Exposure - 1/13 sec ISO - 6400
Using higher ISO value allows your camera sensor to capture images at very low light condition but also produces noises (grains) in the picture. You won't probably notice it in a small size picture but if you enlarge the image, you can easily notice the effects of high ISO vs low ISO. It is always good practice to shoot with lower ISO value possible and increase it if you cannot get more lights by adjusting the Shutter speed and the Aperture value. But with more and more innovation everyday, modern DSLR cameras produce acceptable pictures quality even at higher ISO.
If you are looking for Nikon D90 specific ISO settings, please click here.
