Thanks for the reply. I am familiar with the CiC tutorials, but, as they say, I think it's a bit more complicated than that.
*** nerd alert ***
Another way of asking is "What is the vertical scale?"
The total number of pixels is fixed for any given image, so the Area Under the Curve (auc) is also fixed. However, if I look at different histograms of the same resolution, that is obviously not the case. I can see broad histograms with the auc filling half the space and spikey ones with a much smaller apparent auc.
Similarly, two images of the same or similar scenes show much the same histogram, even if they are taken at very different resolutions. That makes sense - there would not be much sense in having a low res histogram crawling along the bottom of the graph.
So, I it could just scale so that the most intense point of the most intense channel hits the top of the graph. Except it doesn't, because it's easy to find histograms which clearly continue over the top of the visible graph.
It's not really important from a photography point of view, it just bugs me that I don't know.
Incidentally, and I expect this has been pointed out before, the histograms for the same image in LR4 and ACR 7.4 (as in PSE 11) are different.
I think you got it clear Dave. The Y-axis scale of the histogram is arbitrary since it only depends on the software design. This image:
Shows the following histograms in two different programs: left is PS which truncates the Y-axis to make histogram interpretation easier. On the right it is the whole histogram in Histogrammar
using a 256 pixels Y-scale:
Some more curiosities:
1. The following two images have exactly the same histogram. This clearly demonstrates that the histogram has zero spatial information (it's independent of the location of the different tonality levels):
2. The following image doesn't contain useful information itself:
its histogram does:
3. This is what happens to a 16-bit image (X-axis is at max zoom level, i.e. one pixel = 1 tonal level):
when it is opened and saved back in Photoshop without any change made by the user:
It demonstrates that PS is a 15-bit tool, not 16-bit.
4. Find here
the longest histogram in the world
. It corresponds to a 16-bit TIFF image so its X-axis (in vertical position) is made of 65536 values.