>So think of it as iso 50 is the same as iso 100 with exposure >reduced in camera by one stop... Now the Hi1 and Hi2 (iso >12800, 25600) are boosted iso settings from iso 6400.... > Reduced by one stop and then the brightness adjusted to make the image correctly exposed IIRC
Nikon has the below to say about ISO hi settings:
ISO sensitivity in photography is a measure of how sensitive a digital sensor or film is to light. Correct exposure is gained as a combination of shutter speed, aperture and ISO sensitivity. These sensitivity settings are used to ensure that all sensor or film sensitivities are calibrated to the same ISO standards regardless of the manufacturer of the camera or film type. All ISO 200 rated film or camera sensors will respond to light in the same way, regardless of the manufacturer.
The ISO sensitivity of a digital sensor works in a slightly different way to film. A film rated at 200 ISO is physically different to a film rated at 800 ISO, whereas a digital sensor is the same regardless of the ISO setting used in the camera. All digital sensors have a base ISO setting but to achieve higher ISO settings the sensitivity of the sensor is increased.
The sensitivity of a camera sensor is increased in steps which correspond to ISO settings i.e. ISO 100, ISO 200, ISO 400, ISO 800 etc. The difference between ISO 100 and ISO 200 is that ISO 200 is twice as sensitive as ISO 100 (or 1 stop difference). This is also the same between ISO 200 and ISO 400 (1 stop difference).
When increasing a digital camera's ISO sensitivity by a large amount the sensor may perform slightly differently to the ISO standard for that ISO speed. Due to this we rename these settings as Hi 1, Hi 2 or Hi 3 instead of actually naming the sensors ISO setting. Whilst the camera is often performing at levels far past available film speeds, the performance of the sensor, in relation to how much light is needed to expose correctly, may differ by a very small amount to the level expected at that ISO setting. This would be shown as very slight under or overexposure compared to how the sensor should perform if it was to follow the predetermined ISO rating for that ISO speed. It is important to note that these differences are very, very small and the differences are unlikely to be noticed on the final image.
For accuracy we rename the cameras highest ISO settings as Hi 1, Hi 2 or Hi 3 but in terms determining the correct exposure using these Hi settings the equivalent ISO setting for the camera should be used. For example with the D3S the Hi 1 setting equates to ISO 16,000 and the Hi 3 setting equates to ISO 102,400.