WHEN THE ISO is set to a higher number, like ISO 1600 or ISO 3200, the camera will capture an image with less light coming into the camera. This lower light level is then amplified so that the final image is of a normal brightness level. This amplification process produces artifacts; these are most easily seen in the dark regions of the photograph, and areas of low contrast. These artifacts can manifest as errant red and blue pixels that result in a blotchy or grainy look.

Setting an ISO for something higher than the base or native setting (100 or 200) should be done if you need a faster shutter speed that would otherwise not be possible. ...

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