Put another way, suppose your camera is currently trying to take a photo at a shutter speed of 1/15 second at an ISO of 100. Change the ISO to 400, and the camera will now be able to take the same photo at 1/60 second, which is probably good enough to take a sharp photo. Change the ISO to 800, and the shutter speed will be 1/125 second.

I'm old enough to remember when an ISO of 800 was very aggressive and 1600 was all but unheard of. These days, the party has barely started at ISO 1600. Camera manufacturers have made dramatic improvements in sensor technology in just the past few years, and these days many cameras come with ISOs as high as 12,800. That's a range of 7 stops, and it gives you incredible freedom to freeze the action with a fast shutter speed in extremely dim settings. There are cameras that offer even higher ISO--the Nikon D3s, for example, goes as high as 102,400, which is ten stops of exposure control. Of course, you will see a significant amount of noise at those stratospheric ISOs, so if you use them, it pays to use some noise reduction software.

Many photo editing programs, such as Adobe Photoshop Elements and Corel Paint Shop Pro, have a nose reduction filter that cam significantly reduce the stray pixels of noise you get when shooting at a high ISO. Remember the noisy photo we saw earlier? Take a look at the photo , after it met a noise reduction filter.