Reading histograms

This shot courtesy Bill Janes is the back LCD of D200; the subject is T4110 transmissive Stouffer grey step wedge composed of equally spread 41 steps, density step = 0.10 (1/3eV), maximum density = 4.05, more about the product here: http://www.stouffer.net/TransPage.htm#transmission step

The whitest part of the wedge corresponds to the second (massive) peak from the right. The rightmost small peak is due to light leak.



This is not a scientific test, but a demonstration how Nikon calibrate their default tone curve in cameras; it also allows to match histogram with exposure compensation (for green channel only of course, if you do not use UniWB).

 

This means that first three rectangles cover about 5 eV, and if, for example, your highlights are in the middle of the first rectangle from the right, you have to increase exposure by about 1 stop.

 

Dear Dao Dang,

The scale is not a calibrated one, if you make a little more controlled experiment then you will see that with contrast set to "normal" each of first three rectangles from the right cover 1 2/3 stops, 5 stops in total. The last rectangle contains the rest of the dynamic range, exact amount depending on camera model/ISO/WB/noise. For jpgs, from my experience, compensation from the centre of the first rectangle to the right is close to 1 stop (5/6eV, but we can't set it that way in camera). I do not mind tiny specks in the image to go pure white. Tiny white specs will disapper on the print because of ink smearing and dot gain. Because raw has more headroom centre of the first rectangle allows me to increase exposure by 1.5 to 2 stops, depending on the raw converter mode.

 

Spot-/centre-metering from snow and compensating plus 2-3eV is such an old knowledge I won't associate Thom's name with it.   I use 2.5 eV, it works with our converter well enough.

By default ACR interprets exposure differently from my practice. To see that and to match what you see on Nikon's in-camera histogram you can shoot some grey step wedge.

 

Grey step wedges are important tonality control tools; they also help to adjust white balance linearly across the whole tonal range, from highlights to shadows. For a list of available grey step wedges, see here:
http://imatest.com/docs/q13.html

For everyday use I have Kodak/Tiffen Q13, Jessops, and Stouffer charts. Those charts have known densities of grey, and flat colour response. When a shot of such a chart is opened in processing software it is easy to determine if the processing achieved correct dynamic range and contrast/tonal response. Charts are also useful to correct for any colour/light/lens/processing inconsistencies,.