I have noticed that those who have used auto exposure modes exclusively sometimes misuderstand the ISO setting. They think it alters exposure. In fact it does not, unless you are using 'manual mode' It simply alters the shutter speed/Aperture combination. The ISO number is a measure of the sensors sensitivity. Raising the ISO permits faster shutter speeds at a given aperture size (f stop eg f2.8) whilst maintaining correct exposure. Or smaller apertures at a given shutter speed. The converses are true when the ISO setting is reduced. High ISO numbers tend to increase noise, therefore, it is good practice to use the smallest ISO setting that will give a faster enough shutter speed. With the Canon 20D noise control is so good that I don't even think about it up to IS0 400 and I would not hesitate to use ISO 800 if I needed too.
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