histogram inconsistency?

JB
Posted By
jannes_bolten
Jul 10, 2007
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465
Replies
4
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Closed
I have just converted an image from my Nikon D200 with Adobe Camera Raw to a 16-bit RGB file and opened it in Photoshop. I chose Adobe RGB (1998) in ACR for the conversion and my color settings specify Adobe RGB (1998) as my working space.

After this operation, the histogram shows that enormous amount of pixels have been clipped on the shadow side of the histogram (mostly in the green and blue channels) compared to the histogram that I saw in ACR when I judged the conversion settings. In ACR the only channel that showed mild clipping in the shadows was the green channel. It looks as if somewhere during the conversion a significant color shift occurred between ACR and Photoshop.

I notice a similar discrepancy in the histograms between Phaseone’s C1 Pro raw conversion software and photoshop. This surprises me, because I always take great care not to make any mistakes in my color management settings (both in the raw converter and in photoshop).

Anyone else experience this phenomenon? Any solutions that make the histograms in the raw conversion software and in photoshop resemble each other more?
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JB
jannes_bolten
Jul 10, 2007
Follow up:

I just noticed that, in Photoshop, the seeming inaccuracy of the histogram occurs only after doing an uncached refresh by clicking the warning triangle or the refresh icons in the histogram dialog.

Initially (I mean: right after opening, without touching anything), the histogram is – or seems – accurate: no clipping on the shadow side for any channel. This initial representation is in accordance with an examination of the image with the color sampler in the Info dialog: none of the darkest portions of the histogram are anywhere close to level 0. The darkest areas I can find anywhere in the image in the Info dialog and the color sampler are around level 25. This finding is backed up by using the threshold command: if I move the slider to threshold level 1, there is purely white screen – no clipping at all. All this is done on the background layer with no editing whatsoever perpetrated.

But after an uncache refresh, the entire histogram violently shifts to the left and all 3 channels show clipping (green and blue the heaviest, as stated in my first post above).

I thought that uncache refresh was supposed to show a more accurate histogram.
MD
Michael_D_Sullivan
Jul 11, 2007
Refreshing the cache does show a more accurate histogram. The difference between the histogram in ACR and in PS is that the ACR histogram is linear, based on the actual sensor data, before a gamma curve is applied; the histogram in PS has a gamma curve applied, and is in RGB space — there are 256 buckets across the x axis, one for each R, G, B, or Luminosity value. Because this is essentially a logarithmic scale with respect to the sensor data, the left end of the ACR histogram gets squished together in the PS histogram, making the clipping seem particularly bad.
JB
jannes_bolten
Jul 11, 2007
Thanks Michael – that explains part of my problem.

But what about the inconsistency between what I see in the info palette (when hovering the color picker over the darkest areas of the image) and the (uncached) histogram? Judging by the uncached histogram the darkest areas of my image should be a smooth, featureless black (level 0). But the info palette tells me the lowest value in that area is around level 25 – just where I thought it would be judging by the cached histogram!

I do get the feeling – judging by (a) what I see on the screen and (b) in the info palette and (c) in the Threshold dialog – that the cached version of the histogram is more accurate than the uncached one. Or are (a), (b), and (c) fooling me in some way?
MD
Michael_D_Sullivan
Jul 11, 2007
Jannes, are you viewing at 100% zoom (actual pixels)? If you are viewing a zoomed-out image (say, 50% or 25%), you are actually seeing a cached version of the file corrsponding to the cached histogram. At 100%, you are seeing the actual pixels, represented by cache level 1 (refreshed). This might explain the values in the info palette.

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