I believe that Michael's informative article on histograms inadvertently short-changes one of photography's most difficult challenges, namely exposure latitude. Michael states that the range of brightnesses is about 10 stops, but in fact it's at least twenty.
My Pentax digital spotmeter shows "EV" numbers, each increment of which indicates a change of one stop. EV 8 is twice as much "brightness" as EV 7. The scale on the meter runs from 1 to 20.
I know from experience that reading the brightness of the brightest objects in nature (the reflection of sunlight on water or ice) results in readings over EV 19. Blue sky at the zenith usually reads very close to EV 13. Similarly, the meter will accurately measure brightnesses down to EV 1 or EV 2.
Twenty stops is 2 to the twentieth power, a very large number. If my math is correct, it's four million, meaning that the brightest objects we can expect photograph are at least four million times brighter than the darkest.
Not that we would expect a single scene to exhibit the entire range of 20 stops, of course, but it in my experience, natural scenes can exceed 10 stops of brightness range.