D70 Histogram Investigation Part 2
Prepared 2004-12-09 (150/119348) by Bill Claff

The x-axis of the D70 histogram is luminosity.
The histogram display is affected by the tone curve and a gamma correction so it is not obvious how the f-stops are spaced. (it's not powers of 2)
The following chart, derived from the careful measurement of 15 photos that differed only in exposure time, shows f-stop locations at 1/3 EV for the default tone curve in my camera:

So as a first approximation the histogram (for this tone curve) can be interpreted as 5 equal size EV regions.

Let's go a step further and examine the affect of lossy Nef compression.
(See http://www.nikonians.org/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=2971&forum=DCForumID86
for details.)

Consider this chart:

Now

  1. The number of useable f-stops below middle is more obvious.
  2. It's clear that overexposing a little "exposing to the right" improves tone (provided you don't clip )
  3. But you can do even better (if your image doesn't have too much shadow detail) by underexposing by 1 to 1 1/3 EV to get the "sweet spot" centered at about 22%

So, if you're willing to post-process and you understand the D70 histogram, you can get more tonal definition and results that exceed the already excellent performance of the automatic exposure system.

A final reminder, this analysis is for the default tone curve, "your mileage may vary" if you use a custom curve.