Neutral gray is normally used for exposure. The theory behind modern light metering is that if you average the "brightness" of every pixel (or the average dens

Neutral gray is normally used for exposure. The theory behind modern light metering is that if you average the "brightness" of every pixel (or the average density of crystals in the case of film) is should "visually" be around 50% brightness or density, or neutral grey. Spot metering on something that's already neutral gray sort of short cuts that process