Monday, August 20, 2007
Essential Processing Techniques
Tools all photographers should know for adjusting exposure, color and sharpening
By Wes Pitts
In the traditional color darkroom, good color starts with neutral color, and the same is true in the digital darkroom. Key adjustments that we make—exposure, white balance and color—are obviously linked, and when your neutral pixels are in balance, color works itself out.
This technique for getting great color is based purely on numbers. With a properly calibrated monitor, there are numerous tools that can help you arrive at a frame-ready image, but this math-based technique will work consistently under any circumstances. You need imaging software that allows you to measure pixel values and adjust levels and curves. I'm using Photoshop CS3 here.
The basic idea is that for accurate color, your whites should be true white, your blacks true black and your grays color-neutral. In digital grayscale, pure white has a value of 255, black is 0, so middle gray is 127.5. I'm going to measure three pixels that I think should be pure white, pure black and close to middle gray and then adjust the image levels and curves accordingly.
The first step is to pick the white, black and gray sample pixels. I scan the image and try to find the whitest pixels, blackest pixels and pixels that should be about middle gray, then use the Color Sampler tool (under the Eyedropper in the Tool palette in Photoshop CS3) to select the three points. Zooming in usually helps the selection process.
Here's the Levels dialogue and Info panel in Photoshop. In the Info panel, you can see the values of my three sampled areas. Note that #1 is perfect-R, G and B values are all 255, which means that spot is absolute white. Sample #2 is close to a perfect middle gray, and the RGB values for the black sample #3 are close, but too high. So I adjust the Levels to make my black point true black.
To do this, I select each channel, one at a time, and drag the left Levels slider toward the right until the color channel's value is 0. Adjusting the channels independently ensures the most accurate results. After doing this, the RGB values for sample #3 are all 0, or absolute black. I'm almost done, and the midtones are pretty good if you check out the values for sample #2, but I'm going to switch to Curves now to fine-tune the midtones.
The RGB values for sample #2 are R=119, G=115 and B=119. That means to get a perfectly neutral gray, I need to increase the green channel by 4. I want to make this adjustment to the midtones only, so I click in the center of the Curves graph to create a moveable anchor point. Now I drag the anchor straight up just slightly, until the green value is also 119.
That's basically all there is to the technique. The difference in color between the before and after images is subtle but very important. There's better saturation, and that translates to a richer print.
To finish the image off, I applied a small amount of sharpening. You need to shoot sharp-blurry images can't be fixed in the computer—but even the sharpest capture first needs a little sharpening.
When shooting in JPEG or TIFF format, the camera processor will do some sharpening. Many cameras have an adjustment for how much sharpening is applied, and it's generally a good idea to keep these settings neutral.
If you want your camera's processor to do absolute minimal processing of each image as it's saved, then shooting RAW is the way to go. The development settings for sharpening, color temperature, exposure, highlights, shadows, midtones, hue, saturation and so on are all done in your RAW conversion software.
What defines optimum for sharpening depends on the photograph and personal preference. There isn't a "right" technique because what type to use and how much to apply is subjective. But as a general rule, subtle improvements are better than obvious ones. Too much sharpening will produce a halo effect along edges, introduce noise or grain, and can make an image look overly pixelated.
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