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What the Hell is ISO?
What ISO Does.
Well, I guess the first thing we're going to discuss today is ISO. On a digital camera, ISO refers to the sensitivity of the sensor to light. This is adjustable from either 80 or 100, depending on the camera, and goes up to 6,400, or even 12,800 on some cameras. The lower the ISO number, the less sensitive the sensor is to the gathered light, so usually a longer shutter speed is needed to compensate for this. With a higher ISO, a faster shutter speed can be used to capture the same amount of light for a proper exposure, though one is usually sacrificing quality in doing so, as noise starts to appear in some cameras as low as an ISO of 400, or on mine (Canon 60D) around 800 there appears visible noise.
The image at right was shot with a Canon 60D, at an ISO of 1250, with minimal noise. It was shot with 30seconds of open exposure, no flash. Years ago, this would have been an impossible shot at an ISO that high, but with today's technology, ISO can go up quite high without there being too much discernible distortion of the integrity of the image.
So, the main thing with ISO is to keep the number, or sensitivity, as low as possible without sacrificing an appropriate shutter speed. I was able to shoot the image at the right with a 30" Shutter, so even though it was nearly pitch black out, my tripod, ISO, and shutter speed combination rendered the image quite usable and properly exposed. I used to shoot with a Nikon P80, and as good of a camera as it was, it handled higher ISO noise very horribly, and so I was stuck with having to compensate with a very steady hand- and, of course, that's where a tripod comes in handy. (Also, good Noise Reduction software!!!!)