Why is the term Hi-1 or Lo-1 used for describing ISO sensitivities?

Why is the term Hi-1 or Lo-1 used for describing ISO sensitivities?

ISO sensitivity in photography is a measure of how sensitive a digital sensor or film is to light. The higher the ISO sensitivity the less light needed to make an exposure allowing faster shutter speeds or smaller apertures.

When expanding a digital camera's ISO sensitivity beyond its base range the camera may perform slightly differently compared to the normal ISO standard for that ISO speed.

Sensitivities that fall outside the cameras base ISO range are given names instead of ISO numbers. i.e. the base ISO range for a D800 is 100 - 6,400, but its expanded ISO range goes from Lo1 (equivalent to ISO 50) up to Hi 2 (equivalent to ISO 25600). The specific ISO range and equivalent ISO value will vary depending on the camera.

Hi 0.3-Hi 4
The settings Hi 0.3 through Hi 4 correspond to ISO sensitivities 0.3-4 EV over the base ISO range. i.e. on a D800 with an ISO setting of Hi 2, this would be the equivalent of using ISO 25600, which would be helpful in low light conditions.  Pictures taken at these settings are more likely to be subject to noise (randomly-spaced bright pixels, fog, or lines).

Lo 0.3-Lo 1
The settings Lo 0.3 through Lo 1 correspond to ISO sensitivities 0.3-1 EV below the base ISO range i.e. on a D800 with an ISO setting of Lo 1, this would be the equivalent of using ISO 50, which would be helpful if shooting with larger apertures in bright conditions. Contrast is slightly higher than normal with this option in most cases, if this is an issue we would recommend using one of the base ISO settings on the camera.

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