You are changing exposure. The previous comment was about keeping the same exposure (shutter speed / aperture) while changing ISO.
Yes, a good point.
But if you set the ISO to a higher figure and keep the exposure the same then the resulting file will have higher tonal values thus clipping the highlights. By way of example, if you expose a grey card at 200 ISO to produce a mid-grey (nominally, the correct exposure) and then increase the ISO to 800, say, but keep the exposure the same, then you have effectively pushed the grey-card exposure into the highlights at +3 stops. So, I would expect to start to see some clipping.
However, I think we may be missing something in our understanding.
THE SENSOR
In analogue terms, before the signal is sent to the ADC, setting a higher ISO would in theory give the sensor (before the signal is amplified) more headroom since the exposure, as advised by the meter, is reduced. So, in theory, increasing the ISO by 5 stops, as in my test, would give an extra 5 stops more headroom above mid-grey (assuming that the ‘base’ ISO is 200). But my results show identical tonal compression and loss of colour fidelity (clipping) at the same amounts of over-exposure for both ISO 200 and 6400.
At the moment I don’t really know why this should be. It could be down to the ADC or some other digital processing. I need to think about this some more and perhaps do some more tests.
As I understand it, at the moment, on the analogue side of things the sensor is limited by the maximum amount of volts it can produce. Exposing it to more light will not increase its output voltage (full bucket). This varying voltage or signal is produced when the sensor is ‘read’ and all those buckets full of electrons are emptied
.
SETTING ISO
I believe the ISO setting on the camera, or more properly the Exposure Index (E.I.), does at least three things. It changes:
• the sensitivity of the camera’s exposure meter
• the analogue amplification the signal sent to the ADC
• the ISO flag in the EXIF data to the ISO selected.
I suspect that it may do more than this. Possibly it may introduce analogue noise reduction at higher ISOs or introduce some processing of the resulting digital RAW file. Current evidence from some modern DSLRs and higher-end mirrorless cameras suggest this may be the case.