wrote in message
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I am new to reading digital photo histograms. I've been told that the
horizontal scale of luminosity is logarithmic.

Right, just like film curves and all the other exposure scales you've seen.
Equal distances correspond to equal numbers of f-stops.

That the histogram is
divided into 8 segments (not visually differentiated) of equal width.

No. It is more or less continuous, or divided into hundreds of segments.

The reason for the log scale is that we perceive light logarithmically. The
shutter speeds on your camera are logarithmic (1/1000, 1/500, 1/250... not
1/1000, 2/1000, 3/1000...). The f-stops on your camera lens produce
logarithmically scaled intensities. One "stop" is a factor of 2, not an
increment of 2.

In short: Don't panic. Photographic measurements have been logarithmic all
along. What you think of as "midtones" are halfway along a logarithmic
scale, not halfway along a linear scale.

If the histogram were not logarithmic, the highlights would take up far too
much of it.