Often you will find that photos you must use have color casts in them. Man times these color shifts can be adjusted with a simple color balance correction -- finding and correcting the neutral grays. For this miniskill we will use a shot from White Sands. The gypsum dunes are brilliant pure white tending slightly toward the blue.
The first thing you need to understand is that neutral grays in RGB have identical numbers in the RG&B channels. Many photos can be corrected simply by finding the neutral grays and making sure that the RGB colors are all nearly equal. For example, Red 77 Green 77 Blue 77 would be an almost exact 70% gray. If you can globally adjust the RGB based upon the color shift in the grays, your picture will instantly become much closer to being balanced.
As you can see below, the shot
of the sands is obviously much too yellow. At this point we will
not worry about the equally obvious darkness overall. What we
need to do is open the info palette and use the Eyedropper tool
to check out the areas you think are the most certainly neutral.
Click here to download the 110K
high-res JPEG
Open the JPEG in Photoshop and
do the thing with the Eyedropper and Info Palette just mentioned.
Remember, the sand is pure white. The only color would be the blue replection
on the sand of the brilliant blue New Mexico sky. On my screen, as I moved
the cursor around, I found that the Blue was always 20 to 30 numbers (levels
of gray) lower that the Red and Green. The Red was normally 2-3 levels higher
than the green, but that is on the edge of nitpicking. A sample reading is
shown below:
The easiest way to fix this is to use the Image>> Adjust>>
Curves dialog box. Curves will allow us to lighten the blue 20+
levels and darken the Red 2 levels or so. Here are captures of
setting the blue channel:
Do the same thing with the Red channel moving the 3 to 0 and the
255 to 253. When you click OK you will see that the sand now looks
white. Then Adjust>. Curves normally and sharpen appropriately.
You can really see the difference.
Is this perfect? No. As you move your cursor around the image, you will find many areas where the RGB levels are not identical. You can nitpick this one to death and try to get a little closer, but in virtually all cases this is a waste of time and money. The main thing is that the iamge now looks like white sand — and it is!
Save it as a PSD and attach it to an email for grading.