Extended ISO
ISO settings outside a sensor's native range, marked with an L or H prefix, that are produced by digital manipulation rather than analogue gain.
Measured in
ISO
In depth
An extended high setting is simply the top native value pushed brighter in firmware, so it offers no real sensitivity gain and often loses highlight detail and colour depth. An extended low setting overexposes at base and pulls the result down, which reduces noise slightly but clips highlights a stop early. Both are usually better achieved in raw processing where the trade is visible.
Examples in practice
Shooting ISO 51200 (H1) instead of ISO 25600 gives an image no cleaner than brightening the ISO 25600 raw file by a stop.