Linear, and the scale isn't in stops. It's arbitrary.
Michael
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Well, that depends on what is being plotted on the horizontal axis. The histogram is linear if one is plotting the pixel value of the gamma corrected image on the horizontal axis. If you want to relate the plotted value to the scene luminance, then the histogram is not linear but gamma corrected as explained in a post by Jonathin Wienke:
[a href=\"http://luminous-landscape.com/forum/index.php?showtopic=4723&pid=39033&mode=threaded&show=&st=&#entry39033]http://luminous-landscape.com/forum/index....st=entry39033[/url]
In a white paper on the Adobe site Bruce Fraser shows a linear histogram
http://www.adobe.com/digitalimag/pdfs/linear_gamma.pdfTo make sense of the differences between linear, log, and gamma corrected, consider what is being plotted on the horizontal axis according to the following functions. In a linear histogram, the plotted value is linearly related to scene luminance. If the value plotted was according to log base 2, the plotted value would be in f/stops. If the gamma corrected value is being plotted, this represents a power function.
linear: f(x) = ax
log: f(x) = log(x)
power f(x) = a ^ x