How does color space affect in-camera JPEG histogram?

JPEGs are by definition 8-bit. Even though Adobe RGB has a wider gamut, the JPEG is limited to 8-bit color depth. Why and in what way does the color space you select in camera affect the JPEG histogram that you use to guide your exposure settings? Here is an example of what I'm trying to understand. This is an animated GIF file of two histograms of the same image. I took a RAW file and exported it as two separate JPEGs, the first in sRGB and the second in Adobe RGB. Why does the histogram shift from one version to the other?

What you are seeing is the difference in the color gamut between sRGB and Adobe RGB color space if you would export a third JPEG in the Pro color space it will also have a different histogram compared to the other two. More specifically your animation is showing how the sRGB color space has a limited ability to render red colors then the Adobe color space.

Thanks Don. But I thought Adobe RGB was almost the same as sRGB in the reds; it's the greens and blues where they differ.

I always notice it in the reds and yellows when converting to sRGB for posting on the web I see it a lot on my landscape images. When I get in front of my computer I will post a link showing the different gamuts between the spaces

In the initial pane there was peripheral mention of a JPEG being 8 bit. However, bit depth and color gamut are different things and the relevant one here is gamut.

Your question was about the in-camera histogram, but the ones you showed were from exports from a RAW converter, which involved conversion to the output color spaces and subsequent viewing. I’m wondering how relevant those histograms are to the question about what the camera shows.

I’m curious what RAW converter you used, and how you subsequently viewed the histograms. And do you see that same difference on the back of the camera?

To try to get my head around this, I shot two identical images (tripod for same composition, same lighting with a preset WB), one with the camera (5D Mk II) set to sRGB and one to Adobe RGB. On the back of the camera the histograms look slightly but not remarkably different. And of course coming into LR, both RAW files have the same histogram.

Then I exported from LR to JPEGs in sRGB and ProPhoto RGB (to maximize the differences). I can view these JPEGs in several different ways in order to see a histogram, most easily either in PS or by importing them back into LR. Viewing them in LR, both histograms are identical. But when I open them in PS, because the color space doesn’t match my working space, I have the choice to convert the sRGB image to the working space (ProPhoto) or leave it as is. If I convert to ProPhoto (even though it can’t invent colors that weren’t there after I clipped the gamut in conversion) the histograms are identical. If I choose leave as is, the histograms are quite different, even though the appearance doesn’t change perceptibly. (Caveat: I am still using an older monitor that is sRGB.)

Here is the article comparing the two color spaces. http://www.cambridgeincolour.com/tut...obeRGB1998.htm
Diane, to really see the effect open an image in PS that has a lot of reds like a sunrise or sunset in either the Pro or Adobe color space make sure your histogram is set to show colors and go to Edit>convert to profile, chose sRGB and watch the histogram as you convert to the new color profile, more then likely you will clip the red channel as long as the colors do not shift I will ignore the clipping when I prepare my images for web viewing.

OK -- understood that the idea behind converting to a different profile is that the color appearance (in a color-managed application) will not change -- unless colors in the initial image were well out of gamut for the destination color space, and then you will see some well-mannered changes. What will change are the color numbers (that specify each color) in the image, but they will be interpreted by the ICC-managed application using that profile so the appearance will be preserved (within reason).

So -- dumb question -- in that simple conversion example, why does the histogram change? It is not representing the color managed image? Is that to make any out-of-gamut issues more obvious?


If you output two JPEGs from the same RAW image, one in sRGB and one in ProPhoto, in LR are the two histograms are very close to the same, presumably because each is interpreted in its correct color space? And the same holds true in PS when you answer the profile mismatch question with "Convert to the working space." (That must be in effect what LR is doing.) But when you answer in PS with "Use the embedded profile instead of converting to the working space" you see a very different histogram, but the same colors.

Diane, I will have to do more research but my uneducated guess is since the clipping point for the color in the new color space is different it is reflected in the histogram. Again uneducated guess

I'm guessing, too -- but the histogram isn't just stretched, it's different. Is it displaying the altered color numbers in the file? (They have been altered in order to display each color the same in a different gamut.)

If so -- why? What useful information does that provide?

Doug -- still wondering where your two histograms came from -- as in what steps to get them. I'm curious because of the subtle difference in that one fairly light tonal range that gave the red bump. When I tried something similar, I got more dramatic changes throughout the histogram, although that could change from image to image.

What happens if you do the same thing with a very different image?

Quote:

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Originally Posted by Diane Miller

I'm guessing, too -- but the histogram isn't just stretched, it's different. Is it displaying the altered color numbers in the file? (They have been altered in order to display each color the same in a different gamut.)

If so -- why? What useful information does that provide?

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OK -- got the answer to my question -- yes, PS displays the histogram representing the actual color numbers, not their interpretation in any color space.

If you assign a profile the color numbers (and histogram) don't change, but the image's appearance will, according to how different that profile is from the working space. But if you convert to a profile, the color numbers change (and thus the histogram) by the amount needed to preserve the appearance in the current working space (within the limitations of taming out of gamut colors).

Sorry. I'm away in Costa Rica leading a bird photography workshop. Internet connection is spotty. I'll revisit the thread when I get home this weekend. Thanks for all your help!

Doug, Diane, Don,

Greetings. I just wanted you to know that I was once a reasonably sane person. But that was before I started to try and understand histograms, color spaces, why the histogram in my camera was different from the histogram in PS, etc., which drove me absolutely crazy! So now on a good day you can see me stumbling around humming (Kodachrome, you give us those nice bright colors...) while mumbling So! if I take my photo with my camera set on Adobe RGB and bring it up in Lightroom (which I know has a histogram based on a ProPhoto color space but that isn't exactly ProPhoto) make an adjustment then open it in Photoshop which converts to real ProPhoto (was that with perceptual intent?) after processing there is it better for me to convert to 8-bit RGB (still in ProPhoto) then convert to sRGB for the web or vice versa? Wait where exactly do I reduce the image size? And is black point compensation the best idea? Will it make a difference if I sharpen before I convert to sRGB after I reduce image size (before?).

And what exactly does it mean that I'm editing in 16-bit RGB/ProPhoto when my monitor is showing me 8-bit RGB/sRGB?

So my advice is... just don't go there unless you really need to (you'll know it if you really need to... my condolences). And stop comparing those histograms... it will drive you crazy.

Cheers,

-Michael-

I have SO been there, and concluded (several times) that it don't really make no differ'ce anyhow. Chill out and enjoy editing!

But might not be a bad idea to make sure PS is telling you the truth about profile mismatches (Edit > Color settings) so you can decide on your own. (The defaults won't so that.) Lightroom has mercifully taken the burden on itself.