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Color Balance Adjustment with Levels

If you have an image that has a slight color cast to it, the Color Balance option in Photoshop is generally your best option to rectify it. It definitely is the easiest option to use. But sometimes you may feel a need to go beyond what it is capable of, and for this there's always Levels. We've looked at the power of Levels before. They're great for optimizing contrast. But when operating on individual channels rather than the image as a whole they can be used to adjust color balance and correct color casts.

Adjusting Green Levels in PhotoshopNormal usage of Levels is pretty straightforward: black on one end and white on the other, with a gray midpoint slider in between the two. If you change the Channel dropdown at the top of the dialog from the default RGB to one of the individual channels though, you enter a new world. Suppose you select Red for instance. Now instead of Black and White on the two ends of the scale, you'll have Cyan and Red. Choose the Green channel and you'll have Magenta and Green endpoints. Blue will give you Yellow and Blue for the two ends. Sound confusing? It's not as bad as it sounds. Each of these pairs of colored endpoints represent opposite colors on the color wheel, just as black and white were opposite ends of the brightness (or luminosity) scale. These might not seem to be opposite colors to some readers since most of us were taught in school that red and green were opposites, not red and cyan. So much for schooling. If you're having trouble keeping these straight, just remember that each letter of RGB pairs up with each letter of CMYK. Well, each letter other than the K of course. The R of RGB pairs with C of CMY so red (R) is the opposite of cyan (C). Going in order then, green (G) must be the opposite of magenta (M), and blue (B) is opposite yellow (Y). Once you've gotten all the colors straightened out, and as long as you keep reminding yourself what color each endpoint slider actually is, Levels for any given channel works much the same as Levels normally does. The Levels dialog is always in black and white, so it's up to you to keep the colors straight.

So let's say you have an image that might benefit from being a tad more green. Perhaps it's an image of a rainforest — they're popular here in the Northwest. Create a new Levels adjustment layer and change the Channel dropdown to Green. This means that your blackpoint indicator is really magenta, and your whitepoint is really green. Rather than being middle gray as in regular levels, the graypoint indicator shows where the cutover between magenta and green is. If you want your image to have more green in it, you want more of the spectrum to be on the green side of your middle point, which means you have to move the midpoint slider left, towards magenta. This seems backwards but actually makes sense once you think about it. This is how regular Levels works too of course: if you want your image to be brighter you move the gray midpoint slider towards the left — towards the blackpoint — so that more of the spectrum is on the white end. You move it away from the end point you want more of.

In Levels you can move the two endpoints themselves too of course, but when correcting color by adjusting an individual channel this will often create too much of that color so be careful. As with normal Levels, you can hold down the Alt key (Option key on Mac) to check for clipping.

The Hoh Rain Forrest
The Hoh Rain Forrest
The Hoh Rain Forrest, a Bit Greener
The Hoh Rain Forrest, a Bit Greener

In RGB, color isn't really separate from brightness. Suppose you adjust the composite RGB levels by moving the midpoint slider left from its starting point of 1.00 to 1.10. That will make the overall image appear brighter since more of the range is now to the right of this point than was to begin with. If instead you do this the Red channel your image will turn somewhat more red. If you then do the same on the Green and Blue channels, your image will also get somewhat greener and bluer. But if your image is now simultaneously redder, bluer, and green, isn't it really just that much brighter. The color shift from each of the individual channels will have been equalized and therefore neutralized, leaving you in the exact same place as if you had adjusted only the composite RGB channel. In other words, equal changes to Red, Green and Blue channels are equivalent to the same change to the RGB channel.

So adjusting color affects brightness. The Color Balance dialog deals with this by providing a checkbox for "Preserve Luminosity." But Levels has none. If you used an adjustment layer to tweak Levels (and you did use an adjustment layer now didn't you?) you can accomplish the same thing by changing the Blending Mode for the Levels layer from the default of "Normal" to "Hue." This makes it so that only hue differences (the color itself, not how bright it is) affects the underlying image layer, and thus luminosity is preserved.

The cardinal rule when adjusting color via individual channel Levels is "easy does it." A small amount of change can work wonders. A change too big will create a horrible colored mess.


Date posted: May 25, 2008

 

Copyright © 2008 Bob Johnson, Earthbound Light - all rights reserved.
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Previous tip: Tweaking Color Balance in Photoshop Return to archives menu Next tip: Color Balance Adjustment with Curves

Related articles:
The 1-2-3 of Photoshop Levels
Tweaking Color Balance in Photoshop
Color Balance Adjustment with Curves
 

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