ISO is a measure of the sensitivity of the camera’s sensor to light. The lowest ISO setting of most digital cameras is 100 or 200. At this setting, the camera’s sensor is least sensitive to light. This is the sequence of ISO numbers: 100, 200, 400, 800, 1600, 3200, 6400 On some cameras the ISO doesn’t go as high as ISO 6400, while on others it goes higher. Can you see a pattern? Each number is double the value of its predecessor. As you double the ISO, you also double the sensitivity of the camera’s sensor to light. At ISO 200, the sensor is twice as sensitive to light as ISO 100. This means that only half the amount of light is required to obtain the correct exposure. At ISO 400, the sensor is twice as sensitive to light as ISO 200. Again, you only need half the amount of light to obtain the correct exposure. This sensitivity of the sensor doubles every time you double the ISO, until you reach the top ISO setting of your camera. Learn more at and
Explaining stops Each doubling (of halving) of the ISO is referred to as a stop. If you increase the ISO to 200 from 100, you have increased the ISO by a stop. If you change the ISO from 800 to 400, you have decreased the ISO by a stop. You may remember that aperture and shutter speed work the same way. Each change in value between the main settings either doubles or halves the amount of light that reaches the camera’s sensor. The term stop applies to aperture and shutter speed too. Why do we talk in terms of stops? A stop is a convenient measurement that applies equally to aperture, shutter speed or ISO. For example, if you need to increase the exposure by a stop to obtain the correct exposure, you can do so by changing either the aperture, the shutter speed or the ISO. Using stops makes it easy. All you have to do is choose which setting to change. Learn about photography at 
What ISO does Like aperture and shutter speed, increasing ISO does more than increase the sensitivity of the sensor to light. That’s the primary effect. But the secondary effect is that it increases the noise levels in the photo. Noise is a form of electronic interference. It’s a bit like the interference you get when the television has poor reception. While it’s not as drastic as the effect that poor reception has on a television image, it does mean that the image quality gradually deteriorates as you increase ISO. Noise reduction For this reason, the general recommendation is that you keep the ISO as low as possible when you take a photo. Only raise it when you have to. This advice stems from the early days of digital photography, and from film. Back then, you really could see the difference between ISO 100 and ISO 200. Jumping up to ISO 400 made an even bigger difference. But as digital cameras have improved, the difference is not so noticeable. If you own any camera made within the last five years I doubt you will be able to see much difference between ISO 100 and ISO 400. You may not even notice the difference at ISO 800 or 1600. The newer the camera, the more leeway you have when it comes to increasing ISO. That’s good news for all photographers.