Every camera has an ISO button. It stands for "International Standards Organization", which means absolutely nothing and yet is one of the most useful functions a digital camera offers. Basically, it means how sensitive your camera is to light.
In the days of film, if you wanted to shoot sunsets and then beach shots at midday, you needed to change film rolls, because the film that recorded light fast enough at dusk was too fast for bright sunlight. It was - and still is - rated as the speed at which the medium recorded available light: we wanted faster film to record low light and slower film to record bright light. ISO muscled into the act and set a scale. The higher the ISO number, the faster the film and therefore the better it was at capturing low light scenes. The same applies in digital, but we don't have to change films.
So, the basic rule goes like this:
low light = higher ISO. It's such a great rule that cameras in full auto mode automatically increase the ISO setting in "full auto" mode. Unfortunately, there's a downside.
Without getting too technical, ISO is merely a scale and what it measures is how much your camera amplifies what light it receives. At the base ISO setting of 100 (*), cameras do nothing. Anything higher and the camera doubles the received input by amplification. When you amplify a signal, you also increase the noise.
The greater the amplification, the greater the noise.
For us, this means higher ISO settings add crap to our images in the form of coloured dots which show up. In film, this was known as "grain". Sometimes noise adds an "old" feel to pictures, but as a general rule we try to avoid it. As a general rule,
try to keep your ISO as low as possible.
If ISO is so terrible, why is it so useful? Because it doesn't really matter for small prints and web posts. Plus, each time you double the ISO setting you halve the exposure.
Forget
aperture for a moment. There's a beautiful picture to be taken of kids dancing on stage, but your camera's light meter says the correct exposure is 1/10th of a second, which means your subject will be blurry. Easy! Double the ISO from 100 to 200 and your exposure halves to 1/20th second. In fact, double that to 400 ISO, which shortens exposure to 1/40th sec. and you may get a clear picture! To make things easier, most cameras feature high ISO noise reduction functions - and if they don't, image editing programs do.
It sounds too good true. Unfortunately, it is. The image quality of most compacts at ISO settings above 800 ISO (I would say 400 ISO) is unacceptable. Do you really want to print/post a furry image? My dumb rule of thumb goes like this: the $ value of your camera is as far as you can push the ISO setting. If your camera cost $200, don't expect any miracles at 800 ISO.
To return to the image of kids dancing, you can play with a combination of aperture and ISO to reduce the exposure time. With hard-earned experience with your camera and its lens (this applies to DSLRs too), you’ll find the right balance between ISO, aperture and the right exposure to keep things sharp. My advice is worry about aperture and speed first, then increase the ISO. No one said it would be easy, but it is fun.
A word about flash photography
When you use flash, set your ISO to 100. That's nine words. Believe them.
Instead of showing you examples, here's a concrete exercise:
Go indoors, set your camera to 100 ISO and shoo something. Then do the same thing at 200, 400 and 800 ISO. Make sure the aperture remains the same. You'll see that the exposure time halves each time and the quality of the image (at 100% magnification) deteriorates. I guarantee it!
About "Biff" notes:
Brenda referred to the last post as "Cliff" notes, something my limited knowledge of American education equates to "crib sheet for getting through Shakespeare 101". I almost like the idea, but I think "Biff" is closer to the mark. Anyone who's seen Back to the Future
will know what I mean.
(*) I know, some compacts offer an ISO setting of 80 and most Nikons start at 200.
