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How does the Photoshop Color Balance Command Work?

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  #1  
Old 05-27-2011, 11:28 AM
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AKMac AKMac is offline
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How does the Photoshop Color Balance Command Work?

I'm pretty sure I'm not the only one who uses the Photoshop Color Balance Command a lot but still finds it difficult to get my head round exactly how it works.

I use it in RGB, and tweak the sliders till I get what I am looking for, but I feel I could use it far more efficiently if I fully understood the interactivity of the controls.

I do have a sound understanding of additive colour theory - so the question isn't pitched at that basic level.

Was it originally designed to work in Lab mode and then modified to accommodate RGB? Why are the luminosity results so hard to predict? How does the interactivity between the Shadow, Mid, Highlight targets
and the Preserve Luminosity function work? Etc.

AK
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Old 05-29-2011, 07:07 PM
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John Wheeler John Wheeler is offline
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Re: How does the Photoshop Color Balance Command W

Hi AK
I think I have some answers for you on this one and also a solution to the luminosity issue with the Color Balance Adjustment Layer. I discovered some of the same issues you are mentioning when I was doing so probing of what the heck goes on inside PS a few years ago. It does not always do what the names in their options imply.

The most accurate way to describe what is going on is with RGB transfer curves (same as Curves Adj Layer) yet unfortunately that is the hardest to describe with just text. So I am going to just use a parallel with the Levels Adjustment Layer.

First I will describe what is going on with the Red/Cyan Slider with the Luminosity checkbox turned off and only adjusting the midtones. Adjusting the Color Balance Slider from Max Red and then to Max Cyan is virtually identical match to moving the Gamma (mid) slider for the Red Channel on the Levels Adj Layer from 1.4 to .7 (on a Curves Adj Layer this is the same as a slight curve up of the Red Channel to a slight curve down while maintaining the end points). Note: Only the red values in an RGB image are being adjusted with these options and therefore Luminosity will slightly increase and decrease respectively


Now what happens if you click the Preserve Luminosity Checkbox and again move the Red/Cyan slider as above for the midtones from Max Red to Max Cyan. In this case, the near equivalent on a Levels Adj Layer is moving the Gamma from 1.18 to .84 (narrower range) while the Green and Blue Channels have their Gamma move from .84 to 1.18 respectively. (On a Curves Adj Layer this is moving the Red Channel Curve up just a little while the Green and Blue Channels are bent down just a little - as you move from Red to Cyan, the Red swaps places with the Green and Blue Curves. The attempt in PS is to presever Luminosity by increasing the Color of Red while decreasing the Green and Blue colors proportionately. Makes some sense. This may be a little complicated without graphs yet that is what PS is doing.

For the above two paragraphs, for the Red/Cyan slider to the Red Channel adjustments mentioned, it is a parallel and not repeated here equivalent of the Green/Magenta slider to the Green Channel adjustments and the Blue/Yellow slider to the Blue Channel adjustments.

So what happens when you adjust the highlights or the shadows is a little more complicated yet the big big issue is that there "IS NO" effort to preserve luminosity with the shadows or highlights when you check Preserve Luminosity and that is why it is hard to predict what is going on. The color balance adjustment layer also clips the colors which is usually not a good thing to do. Yet there is an easy alternative that you should try:

To preserve Luminosity while using the Color Balance Adj Layer
1) Make sure the Preserve Luminosity checkbox is turned off (yep, turned off)
2) Change the blend mode of the Color Balance Adj Layer to Color.

Now that may sound odd but changing the blend mode to Color will capture the color change you desired from the Color Balance Adjustment Layer while using the Luminosity from the Layers below. Now that is really preserving Luminosity. The behavior will be different than what you have seen yet should match the intent much more closely.

The truth is, you could leave the "Preserve Luminosity" checkbox on. The Color Blend compensates and still preserves luminosity. The behavior of what changes in highlights, midtones, and shadows changes a bit though not to my liking yet you could give it a try.

This would be a lot easier in a tutorial with graphs or a screencast. If I get too many "Sorry John, I just don't get it", I will put that on my to do list.

Well, there are more details yet this is probably way more than you ever wanted right

Hope this helps
John Wheeler
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Old 05-30-2011, 04:07 AM
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Re: How does the Photoshop Color Balance Command W

Thanks John,

I get the gist of what you are saying, and find your explanation very helpful (although, where you say the effect is similar to a gamma range from 1.4 to 0.7, I find the similar gamma range to be approx. 2.00 to 0.5)
I suppose it's ironic that, while I would automatically consider working with a layer in Color Blend Mode mode if wanting to adjust colour but preserve luminosity, the very fact that the Color Balance Command ostensibly "commands" those parameters has blinded me to that glaringly obvious workaround.

From now on I will be working in Color Blend Mode when using Color Balance. Suddenly my life has become less complicated.

regards

AK
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Old 05-30-2011, 07:50 AM
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John Wheeler John Wheeler is offline
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Re: How does the Photoshop Color Balance Command W

Hi AK
You are welcome. That Color Blend comes in real handy when you want to preserve Luminosity (that always sounds funny to say ).

As far as you experiencing a match at a different Gamma number with the Levels Adj (LA) Layer, that got me curious. The reason you see a difference may be helpful to me to know the cause (we may want to move this to a PM discussion).

As it turns out, the adjustments of the Highlights, Midtones, and Shadows are highly overlapped with the Color Balance (CB) Adj Layer. The only way I was able to see a match with a substituted Levels Adj Layer to the Gamma numbers you experienced was if I had swung both the Mid-tones and the Highlights both to their extremes (e.g. on the Red Channel). Or it could be a difference in our approach. Did you fully reset the CB Adj Layer and only change mid-tones of one color, then disable CB Layer and matched with a reset LA Layer, and only move the Gamma slider? If yes and you still get the range of Gamma you mentioned, I would be interested in seeing your test image and also giving it a try. I may have something to learn here that I did not expect.

Thanks in advance if you have any more time to spend on this.
John Wheeler
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Old 05-31-2011, 02:40 PM
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John Wheeler John Wheeler is offline
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Re: How does the Photoshop Color Balance Command W

Discrepancy resolved.
AK your range of numbers were correct for matching with the Levels Adj Layer.
I had accidentally had a 50% gray tone Layer mask on the CB Layer giving a different reading. So the correct equivalent Gamma range using the Levels Adj Curve is from 2.0 to .5 to simulate the full swing of the Mid-tone Red-Cyan slider on CB Adj Layer from max Red to Max Cyan.
The rest of the post was correct.
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