Most digital cameras have ISO speed control built within them. In film cameras ISO speed is a gauge of a films sensitivity to light. The higher the ISO number the more sensitive the film is. This is why photographers will use lower ISO Speeds such as (100 or 200) when shooting outdoors in bright conditions but use an ISO of 800 when taking photographs at night.

However, higher film ISO speeds often lead to a ‘grainier’ look due to a difference in film construction. Often this look is desirable and adds to the overall feel of a higher ISO speed photograph if the photographer is using film.

With digital photography a higher ISO speed also allows you to take photographs at night making the CCD chip in your camera more sensitive to light. And like film cameras the digital camera which uses a higher ISO speed will also have a ‘graininess” look to it. The major difference with digital photography graininess is that it looks terrible. If you were to enlarge a photograph which was taken with a high ISO speed from a digital camera you would notice that the graininess isn’t so “grainy” at all. In fact, it looks like little bunches of random pixelated colors. It’s nothing more than picture distortion.

For the highest resolution possible using digital cameras (even cameras with a low megapixel count) try using the smallest ISO speed possible to ensure you get rid of this color distortion.

As a photographer you need to make quick decision. Lower ISO speeds are not always possible in low lighting situations. Sometimes you’ll just need to use higher ISO speeds in order to make your CCD chip more sensitive to light. The alternative is using a lower ISO speed and using a longer shutter exposure time. This obviously won’t work if you’re trying to capture a moving object but it will be helpful if you’re trying to capture a static object by allowing more light in to “expose” your CCD chip for a longer period of time.

I hope this helps.

(The first image is an ISO 50 image and the second was taken in the same lighting but used ISO 800. Both photographs are blown up 500% to help show the effects of ISO speed).