jamesw wrote:it was my vague understanding that noise was caused by excess energy/heat through the sensor. like i said. very vague.
Not quite. The sensor always works the same way. When the ISO is increased, the camera's internals boost the output from the sensor.
Think of it like an amplifier playing music. The source is the same, but volume is increased by turning up the the amplifier - if you keep turning it up, you'll get to the stage where there is distortion.
When you increase the ISO of your camera, you are amplifying the signal that comes from the sensor, and if you keep increasing it you will eventually get distortion (noise).