Bair Art Edition's Tutorial on:
Adjusting Color & Exposure
In Adobe Photoshop


Solving Color and White Balance Problems: Using Levels


Levels can be a useful tool for color correction in general, and is my favorite for correcting White Balance. For basic information on levels, and how to create a levels adjustment layer, see the Exposure Module: Using Levels.

How the Channels Work

In order to comprehend where you need to go with an image, and what channels you need to use in order to get there, you must first understand the relationship between RGB and CMYK color. I've written up some of those basics under How Light and Color Work, in the introduction to this module.

    

In the levels control panel, you'll see a pull-down menu labeled "channel." This allows you to address specific color values. When you select a channel, a histogram much like that in the full RGB channel comes up.

In the RGB mode, we were using the graph to show how much of the image fell into the tonal ranges from pure black to pure white. In this case the "black" and "white points" become colors.


In the Red channel it becomes "Cyan-point" to "Red-point"


In the Green channel it becomes "Magenta-point to "Green-point"


In the Blue channel it becomes "Yellow-point" to "Blue-point"

This is because, in the Red channel for example, the "white" area is getting closer and closer to contributing pure red; and in the "black" area, it is getting closer and closer to contributing no red whatsoever. In RGB, the lack of Red results in Cyan.

Is this confusing?
If it is, you might want to see another way to envision it




Correcting White Balance

We'll start with this more specific application of levels, because it teaches much of the basics; and because it should be the first adjustment to the image, if it is to be done at all.

I'm going to use an image that I shot, which isn't particularly great, but it does show a poor white balance.



As you can see, the image has a very distinct yellow-cast to it. This was disturbing to me, as I had shot it using what seemed to be the correct White Balance setting on my Fuji S2-Pro, "Shady/Bright."

With an image such as this you select the Blue channel in levels, and shouldn't be surprised to see that all of the information is weighted in the "Yellow" half of the channel. By pulling the white, or "Blue," pointer to the point on the graph were you begin to see information, the values will even out across the scale, and the image will lose it's yellow cast.



Before Blue adjustment
After adjustment


Is this confusing?
If it is, you might want to see another way to envision it


Correcting Color Balance

Now that I've made a major correction in the white balance, the image still has some residual problems in the color. To correct that, you probably won't ever want to move the "black" and "white" points around in the specific channels. To do so would "clip off" detail and values in the image. When using levels, only move the black and white points to clip off portions of the graph where there is no information.

Instead, you will be using the mid-point in order to "stretch" and "compress" the color values. Remember, when you are using the mid-point slider, you are defining the "half-way" point. All of the detail and information to the left of the mid point after it is moved, will be on the black half. With regards to individual channels, it will be in the Cyan, Magenta, or Yellow half.

All of the information to the right of the mid-point slider will be interpreted to be in the white half. And in the case of channels, that is the Red, Green, and Blue half.

With this image, I think it is still too much on the warm side. I just need to take away some warmth.

I could do so by moving mid-points of the Blue channel towards "black"/Yellow (this would place more information on the Blue/"white" half of the graph).

Or I could move the Green channel mid-point towards "black"/Magenta (this would place more information on the Green/"white" half of the graph).

Or I could move the Red channel mid-point towards "white"/Red (this would place more information on the Cyan/"black" half of the graph).

In the end I decided to move the Green mid-point towards Magenta/"black." I could do so by using the slider, or type a value greater than 1.00 in the middle box. I took the gamma up to 1.10, and this is the result:



Before Green adjustment
After adjustment
Is this confusing?
If it is, you might want to see another way to envision it



Now the image on the left may look more pleasing to you (depending on the calibration of your monitor), and it may even print better if the paper/ink combination has a slight cool cast. In any case it is apparent that they are different.

After Thoughts

The Levels tool is probably the most powerful way to correct White Balance, because you can see where information begins. It can also be used for Color Balance, but isn't as powerful as the Color Balance tool, or the Curves tool, as it can't isolate shadows, midtones, and highlights. Instead it does an overall adjustment. This global adjustment may be desired, however, since you must visit three windows in the Color Balance tool in order to do the same thing.

The following are the other sections of this module:

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