I think this has come up before, but I can't remember where. Attached is an image that has a background layer only. I checked the histogram (luminance), and the histogram (levels). Nothing was done to the image, but the histograms are a lot different. Why????

Ed

It looks like the primary difference is simply of scale. It doesn't change the relative values.

Thanks for the reply Doug. For some reason, I expected it would be you who answered. :) I'll take a close look.

Ed

It looks like the primary difference is simply of scale. It doesn't change the relative values.
Disagree here, Doug.
Both histograms are telling us something about the overall grayscale image - independent of color.
Making a grayscale representation of a colored image is a big subject matter discussed in many threads around here.
Here basically each histogram used a different method:
- Histogram (luminance) is showing the luminosity
- Levels histogram is simply adding up the color values (desaturate).

Here's a quick way to test it by forcing the grayscale conversion:
1) Take your photo and desaturate it - now both histograms look like the "Levels" version;
2) Go back to the original colored photo and get the luminosity (put in a new empty layer, fill with black and set to blending to color) - now both histograms will look like the "Luminance" version.

Hope this helps out.

Rô

I shot from the hip, and didn't give a fair answer.

According to Adobe: "The Luminosity histogram in the histogram palette/dialog is calculated as the approximate gray value of the image (Gray = 0.3 * Red + 0.59 * Green + 0.11 * Blue)" whereas "The RGB histogram in the Levels dialog is the total of the individual channel histograms".

So I was incorrect by saying "scale", when I should have said "weight". But it still doesn't change relative values as long as you're comparing them simultaneously.

Thanks Ro and Doug. It's a little more clear now. :thumbsup:

Ed