What does the so-called "18% neutral gray" mean in photography photometry and exposure?

Where the value of 18% comes from is really impossible to verify. According to some technical documents in 1950s and 1960s, the average reflectivity of the earth is 18%, while the reflectivity of general objects is 18%.

The gray with reflectivity of 18% becomes medium gray, which is indeed the midpoint of reflectivity geometry (the geometry here is the geometry of geometric series).

The reflectivity of white objects is about 70%, and that of black objects is about 4% (pure white and pure black objects do not exist). Why isn't their midpoint around 33%? Because at this time, we can think that the brightness of an object with a reflectivity of 70% is about 1 times that of an object with a reflectivity of 33%, but the brightness of an object with a reflectivity of 33% is 8 times that of an object with a reflectivity of 4%. It can be seen that 33% is not the scientific median (average reflectivity).

Now let's consider the case of 18%. At this time, we can think that the brightness of an object with a reflectivity of 70% is about four times that of an object with a reflectivity of 18% (two stops), while the brightness of an object with a reflectivity of 18% is four times that of an object with a reflectivity of 4%. This happens to be an intermediate point, a geometric intermediate point. In photography, the exposure value (EV) formed by aperture value and shutter value changes in geometric series.

There is a median between 70%, 18% and 4%. Specifically: 72% (white), 36% (light gray), 18% (neutral gray), 9% (light black) and 4.5% (black).

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