Noise is variation that appears as grain or colour speckles rather than useful subject detail. Some comes from the randomness of captured light and some from the camera’s electronics. It is often most visible in dark areas that are brightened later.
Keep the comparison fair
An ISO test can answer different questions. At fixed aperture and shutter time, increasing ISO changes image brightness. If you change shutter time to keep that brightness similar, you also change the amount of captured light. Those are different experiments.
Try it
Use a stationary subject in a dim room with a tripod. Make a fixed-aperture, fixed-shutter ISO series, then compare equally bright processed images at the same size. Next allow a longer shutter time at a lower ISO and observe how recording more light affects a still subject.
By Joseph Hubbard, photographer and artist at Hubbard.ART.
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