Originally posted by GUB I use the histograms a lot and sort of understand them.
But if the y axis represents the number of pixels then wouldn't the area under the line be always equal?
Glancing at my camera that would not seem to be the case although it is hard to tell. And certainly doing the curves in gimp the area changes.
May be the scale changes on the y axis to fit things in.?
I think this is the key. Some software, and even cameras, automatically scale the y-axis to fit things in to some extent.
As an example, when you are clipping an image, the end points of the histogram will be very high leaving the remaining portion of the histogram looking flat. As a result it's all relative.
In reality, I think you really only have to know it's relative and understand what the shape of the histogram means roughly and make sense of it.
For instance, the earlier post about a histogram that was essentially flat. Did this make sense for what the image looked like? When you end up with spikes at the ends when things are clipped, does that make sense? When you see a more typical curved histogram, where is the peak?
To me, the y-axis numbers for a histogram and the general shape of the histogram mostly serves as a qualitative guide. On the camera it helps you to readjust and reshoot if the exposure is too high or too low. In PP it helps for guiding adjustment of the brightness, contrast, curves, etc. I almost never look at specific numbers or worry about them.