Of course I can’t take credit for this. I learned it from another photographer and I know others who use it as well.ir-leasing.ru
The best way to color correct is of course to use a calibrated monitor, color management, but for many, color management is overly complex. Since we don’t all have calibrated monitors and all our viewers won’t have managed systems anyway, we need to know how to color correct “By the Numbers”.
Color correction “By the Numbers” is a common practice used by image technicians who prepare images for four-color print systems that you see for newspapers and magazines. The inks and papers may vary from run-to-run or from day-to-day. They use a technique of using levels and curves to make sure the image calls for the right amount of inks by sampling areas of the image and working the color channels until they get certain numbers, for example they adjust until skin tones are a certain mix of Cyan, Yellow and Magenta.
Well we can do a similar process for our images. Since most of you are probably outputting photos for web use, or printing at mini-labs, if we are to avoid color management, we need to work in the sRGB color space. Most camera’s will default to sRGB and for those that can use other color spaces, like Adobe RGB, set it to sRGB then set Photoshop (or your image editing tool) to use sRGB and you will have the best bet of having your colors consistent through your whole workflow.
Now most pro’s will not use sRGB since it has a limited color range, but these same pro’s are working with color management. If you want maximum performance, sRGB won’t be for you. Think of hese different color management workflows as the difference between music on a CD and music on vinyl. Most people think CD sound is clearer, though there is a loss of information. For some, vinyl provides a more rich sound, despite the static and noise. sRGB is like the CD, Adobe RGB and Color Management is the vinyl.
This technique works well under most circumstances. Where it becomes problematic is when either you’ve overexposed the photo where there are blowouts or where there isn’t anything “white” in the photo. The technique can be used for these, but you have to be careful. But for this tutorial we are going to keep it simple and assume we have whites to work with.
In our example photo above, we have a photo with a yellow cast to it. There are several candidates for white. The boards should be white. The ice of course should be white and there are several white targets in the two different uniforms. The camera was set to Auto White Balance and under arena lights, things can get off from frame to frame.
To do a quick color correct, use the “Levels” tool. You can bring this up by pressing CTRL-L on a PC or Apple-L on a Mac. Its also under the menu as “Image->Adjustments->Levels”. If you want to do an adjustment level you can as well.
This tool has several things we need to pay attention to. First, at the top of the dialog is a drop-down selector that defaults to RGB. This means that any changes you make will affect all three color channels, Red, Green and Blue the same. This is good if you just want to correct exposure but it won’t remove any color cast.
This tool can also work on each channel individually. You can select the Red channel, make adjustments, select the Green channel, make adjustments, then work on the Blue channel. This is what we are going to do.
The next important part of this dialog is the Histogram. It represents the range of tones in the image. The far left edge of the block represents 0, or the darkest possible values, the right side represents 255 or the brightest values.
An well exposed photo of an average scene will have a nice “bell” shaped curve, with the top of the curve at the center and the curves tapering off to the bottom at the left and right edges. If you see the bulk of the curve on the left side of center, its probably underexposed (which may be what you intended or a natural dark background). Generally if its to the right its over exposed.
Below the histogram curve are three small triangle / drop shaped icons. One is black on the left, one is gray in the middle and one is white on the right. These droppers can be grabbed by your mouse and moved to adjust the tones.
The quick color correct method involves going to the Red channel and sliding the white point dropper left until it meets the point where the histogram bottoms out on the left side. If the histogram touches the left side or spikes back up, leave this channel alone and visit the green channel using the select at the top. You can also use the short cuts of CTRL-1 (red), CTRL-2 (green), CTRL-3 (blue) and CTRL-~ (RGB combined) to quickly move between the channels.
Repeat the process on the green channel, moving the white point left. Then take care of the blue channel. If the photo has a color shift, then you should notice the shape of the histograms will be different for each channel. When its done, the three channel histograms should be similar.
If your photo has specular highlights (like reflections on chrome) there will be a spike at the left/white edge. Since these points are blown out (and are supposed to be), its okay to move the white point past them to the real point where the tones bottom out since they will still be blown out.
After you visit each color channel adjusting the white points, then go back to the combined RGB channel (CTRL-~ or Apple-~) and bring the black point in until it touches the bottom out point on the left side. This will add contrast to the photo (if that’s desired).
Since you’re not looking at the actual colors on the screen, but the values of the histogram, you get reasonably accurate colors even on an un-calibrated monitor. If there are no whites, this won’t work.
If you have Photoshop CS3 or later, an additional tool you can use with levels is to hold down the ALT key while sliding the droppers. The photo will turn black and as soon as you seen any spots of color (it will be red color while on the red channel, green on the green channel etc.) then adjust it to the point where you’re on the border of color spots and no color spots. This will be a little more accurate than just using the histogram.
Click “Okay” when done and voila!
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