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  • Photoshop Tutorial: Curves white balance

    2011 - 01.28

    Curves!  Probably the most useful and basic tool in Photoshop for making adjustments.  You can find it under image > adjustments > curves.  For the uninitiated, a common use of curves is to increase contrast in a photo.  If you make the good ol’ “S” curve using 3 points, you can add contrast to any image.  Just like you see below!  I’ve added a small change to my S curve: the middle point is raised above center, instead of being dead center.  This has the effect of brightening the overall image as well as adding contrast.  How does that work, you ask?  Read on!

    Basically, if you think about the labels on each axis here, you can understand what’s happening.  You have a point with a certain brightness, let’s call it “50” out of 100, smack in the very center.  That’s your “input”.  The way the photo is, already.  Your “output” is what you want to do to it, i.e. make it brighter or darker.  So for “50” which should be exactly in the center of the grid, we’ve raised it a bit, meaning it’s now brighter, maybe a 58 or so.  With the S curve shown, our darks are just a touch darker, our midtones are a touch brighter, and our brights got a noticeable boost even brighter.

    The photoshop trick that’s really been growing on me lately is using curves to tweak white balance. Even if you had the right white balance selected when you took a given photo, there’s often a subtle color cast. Messing around with this color cast can sometimes give a real nice feel.  Or, if you’re like me and you sometimes forget to set your camera back to the right setting when you walk from indoors to out (and auto-WB never seems to get tungsten right), you can use this technique to correct photos that are wayyy off.

    There are three eyedroppers in here that aren’t really labeled. I put a nice bright red box around them in the screenshot so you’ll notice them–I overlooked them myself for a long time. You can use them to pick the black, gray, and white points for a photo, assuming your picture has those three colors. (Most do.) It’s pretty simple. Picking the white point usually yields the most dramatic effect. Click the white eyedropper in the Curves window, then start clicking on places that are completely saturated, or the pixels which are immediately adjacent to a saturated area.  In the image below, I picked a bright white spot on one of the apples, reflected from the lights above.  Now this point is the “new white”.  Viola-instant white balance correction!  Note that you can lose detail here by selecting something darker than absolute white, because whatever lightness you click on now BECOMES absolute white with the resulting new curves. There are some really interesting changes in the color cast of the photo when you start clicking around.

    Try a lot of points, and notice that your color cast is the opposite of whatever you click on. For example, if you click on an area that’s blue-ish, your resulting white balance will shift the colors toward red-ish. It’s trying to compensate against whatever your clicking on. If you click on a “cool” area, it will “warm” the photo. The inverse is also true.

    You can do the same process with the black and the gray eyedropper, but a lot of times I find that the black point really doesn’t do much to affect the colors, and it’s hard to really get a good shade of gray in your average color photo. Maybe my technique is still maturing in this territory though.   If you do have a shot with a nice even gray in it, like concrete, definitely try using the gray eyedropper on that area.  It can produce some nice changes in the shades of your colors.

    Finally, advanced students will note that if you wanted to, you could achieve the same thing by manually moving the individual color curves.  Sometimes, for examples like the picture below, where your camera was completely on the wrong white balance setting when you took the shot, you might want to apply the curves by hand to get it just so.  Go give it a try!

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