To APERTURE & SHUTTER SPEED
by Flo Deems
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ISO is the "speed" of the film or the sensitivity of the digital camera's sensor to the amount of light available. An ISO of 50 or 100 is usually the slowest ISO of modern digital cameras. An ISO of 50 is twice as slow as ISO 100, and 100 is twice as fast as 50. This means that if you set your camera's ISO to 50, the sensor will need twice as much light as an ISO 100 setting. ISOs are also read as "stops," just like apertures of lenses and shutter speeds.
Like shutter speeds in which we multiply by 2 to reduce the light (increases the number of the ISO) or divide by 2 to increase the light (decrease the number of the ISO), ISO readings as full stops also use the factor of 2. But some cameras have third or half-stop ISO settings in addition to the full stop settings.
Suppose you need a shutter speed of 1/800th of a second to stop an action (so the focus looks sharp). But your ISO setting is too low to allow you to use the 1/800th setting. So you must increase the ISO's sensitivity to the light. Just for an example, suppose you have the ISO set to 200, but the fastest shutter speed you can get is 1/400th with the aperture you want to use. You don't want to change the aperture. So your other option is to change the ISO one full stop, from 200 to 400. This will allow you to use a shutter speed of 1/800th.
All this may sound confusing. So the only way to really understand this well enough to use it is - you guessed it - practice, practice, practice! So go shooting and try out different ISO settings with the same aperture setting and see how this affects what shutter speeds you have available at each ISO setting.
At night and in low light situations, you must raise the ISO to 1000 or perhaps even higher. But remember that the higher the ISO, the "noisier" your images will become. "Noise" means that the colors tend to separate into tiny but visible dots in the darker areas of the scene. So in these situations, you must stabilize the camera so you can use very slow shutter speeds. This allows you to use lower ISO settings. But here again, too slow a shutter speed can mean "noise" appearing.
If you have a point and shoot digital camera, the ISO may not go very high. But in higher end cameras, the ISO can go all the way to 64,000! Most cameras will make the best image qualities at ISOs under 1000 ISO. The more recent the camera, the better its image quality will be at higher ISOs.
For more info on the relationship of ISO, shutter speed and aperture, please read this Digital Photography School's article. There are also several articles linked to at the bottom of this article containing more info about ISO, shutter speeds and apertures that you may find interesting.
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