In 1939-40 the pioneering photographers Ansel Adams and Fred Archer developed (no pun intended) an exposure system based on this fact, a system that is still used today. It is called the Zone system, and is quite possibly the most useful piece of photographic knowledge youâ€™ll ever learn. There are several variations on the original system, but Iâ€™ll go with the one that is easiest to understand.
Starting with 18 per cent grey as the mid-point, the system divides all the tones between black and white into 11 zones, numbered 0-10. Zone 0 is featureless black with no details visible, which in your image editor would have an RGB value of 0,0,0. Zone 10 is pure white with no details visible, and an RBG value of 255,255,255. The mid-tone 18 per cent grey is zone 5, and should have an RGB value of about 127,127,127.
Theses zones represent exposure values, or EV. The difference between one zone and the next is equivalent to the difference between one exposure setting and another one exactly one stop higher or lower.
This is probably a good time to take a slight detour and explain how exposure is controlled.