D70 Histogram Investigation –
Prepared 2004-12-09 (150/119348) by Bill Claff
- The x-axis represents the D70
- The y-axis represents 12-bit
raw CCD readings
(The curve is the tone/gamma curve)
- The circular symbols are
spaced at 1/3 EV intervals
- The diamond symbol represents
the middle of the automatic exposure range
(Where Auto chose to expose.)
- The chart shows about 2 1/2
stops above the middle before clipping
- The chart shows 2 1/2 to 3
1/2 EV below the middle before detail is lost
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.
Consider this chart:
- The y-axis now represents
the number of compressed datapoints for a given 1/3 EV interval
- The number of useable
f-stops below middle is more obvious.
- It's clear that
overexposing a little "exposing to the right" improves tone
(provided you don't clip )
- 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.