Subject: Re: Exposure to the right and tone placement
Very perceptive questions. I will try to answer according to what I know and perhaps Bruce will jump in with additional details.
The camera histogram is calculated from the jpeg preview that the camera generates for each raw file. This takes into account the color space and other camera settings such as contrast, etc. Most cameras do not have settings for ProPhotoRGB, which is my preferred space, so there is no way to get a camera histogram similar to the ACR histogram if you are using this space (which has a gamma of 1.8 as compared to 2.2 with sRGB and aRGB as well as a larger gamut).
Cameras that show only one histogram usually show a luminance histogram, which weights the channels according to the sensitivity of the eye and gives preference to green. The composite histogram may not show clipping in the blue or red channels, and this gamut clipping can sometimes be controlled by use of a wider space such as ProPhotoRGB. To the best of my knowledge,a wider space will not increase the overall dynamic range of the image.
ACR shows a composite histogram with all three channels displayed, similar to the RGB histogram in PS. PS does have a luminance histogram similar to what is shown on the camera. Shown below are examples of an ACR histogram with a blown red channel (mostly elimated with ProPhotoRGB, not shown). If you want the writing to be legible, you must view the images at 100%. As you can see the blown channel is not apparent on the PS luminosity histogram, which appears similar to what I saw on the camera.
Aside from channel clipping, I have found the blinking highlight display and histogram on my Nikon D70 to be quite useful. If you find the warnings to be too conservative, you can address this by uploading a custom curve to the camera to boost the highlights to where they will give a warning as shown by a few tests. The camera histogram will take this curve into account. ACR ignores such custom curves and other camera settings except for white balance.
As Bruce has pointed out previously, rendering of a raw image into a color space involves more than just application of a gamma curve and demosaicing operation. Some converters try to shoe horn the gamut into the working space, while ACR seems to take more of a minimalist approach and leave these decisions to the user.
The higher end Canons and Nikons now show histograms of all three channels so you can detect channel clipping. If the camera shows only one histogram, it would seem to me it should be similar to the RGB histogram of PS, rather than a luminosity histogram.
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Exposure to the right and tone placement =>
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