Interestingly, on my early 2015 13" MacBook Pro, the very lowest brightness setting seems to have no change in brightness on those last three subdivisions. Once

Interestingly, on my early 2015 13" MacBook Pro, the very lowest brightness setting seems to have no change in brightness on those last three subdivisions. Once I get down to one full "block" of brightness, option-shift-F1 moves the slider down in quarter-block increments but the screen gets no (perceptibly) dimmer until it shuts off entirely. All the other brightness settings have perceptible changes in brightness at every subdivision level.