It is not actually "logarithmic" per se. At a given level of adaptation the relation between luminance and perceived lightness is closer to a square root. But

It is not actually "logarithmic" per se. At a given level of adaptation the relation between luminance and perceived lightness is closer to a square root. But over the course of about 30 minutes there are several different kinds of adaptations (some faster than that) which the eye/brain can make to the current light level, which has an effect of shifting that curve up or down by up to several orders of magnitude.