I for ISO. ISO is one of the elements of exposure triangle. ISO stands of International Standards Organization, which sets the sensitivity of sensors on digital camera. Sensitivity of the image sensor depends on the ISO number and smaller the number, less sensitive the sensor is. The lowest number is 100 and it could go up as high as infinity. The highest ISO on a camera varies from camera to camera.
The rule of thumb is to keep the ISO low but in low light conditions, in indoor sporting events and in low light conditions when the photo is taken with out a tripod, ISO comes to recuse to well expose the photo. Also, ISO is used when freezing time/motion because the shutter is fast, resulting in less light going into camera. In such cases wider aperture and/or ISO are adjusted to well expose the photo. The drawback of high ISO is that the photo gets noisy as the ISO increases. One must have noticed, often times night pictures usually tend to have a grainy finish. That is the noise due to high ISO.
Today's photo was in a subway station where lighting was low. I didn't have a tripod and had to use a fast shutter to avoid a shake. ISO came to my rescue. Settings of the photo - ISO 12800 (very high ISO number), Shutter Speed - 1/100sec, Aperture - f/2.8. Here are high ISO pictures from the archives.
Picture of the day in 2013: Flowers Picture of the day in 2012: Hallway
Picture of the day in 2011: Cherry blossoms