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Thursday, May 31, 2007

Calibrate Your Camera

To get complete predictability, consistency and control, it's important to calibrate your entire system. We used to do it with film, now we do it a little differently with digital gear.

Once you're in Photoshop, use the eyedropper to select a point and you can see the hue, saturation and brightness values. Then you can compare values in your real-world and your LAB reference file and get an immediate comparison.

The thumbnail previews will change, and you'll most likely find that the expected E.I. now looks too dark.

As I mentioned before, most of my cameras look best exposed one ƒ-stop higher than normal, meaning that I have to rate it at E.I. 50 instead of 100. At this point, all of the thumbnails visible in the panel at the left will change to reflect the new zero slider settings, and you can get a good feel for which exposure is closest to the ideal with minimal processing intervention. Select the best exposure to display it in the big preview window.

Evaluating In Lightroom

At the time this article was written, Lightroom was still in beta, but the calibration functionality isn't likely to change between now and its release. If you're experimenting with it, you can achieve results similar to ACR because the two products share the same underlying color engine. To zero out the sliders, select the linear tone curve from the presets.

Calibrate Your Camera Adjusting For Color Calibration

The next step is to determine custom adjustment settings for the best color rendition of a gray-balanced shot. ACR has a unique method to adjust for subtle color bias in the Calibrate tab under the Settings menu. Before doing anything else, however, you must use the White Balance tool (the gray eyedropper in the row of icons at the upper left) to neutralize the color.

Select the White Balance tool and click on the light gray patch. The Settings drop-down will change from As Shot to Custom and the Temperature and Tint sliders will change to reflect the new white balance.

Click on the Calibrate tab, select the Color Sampler tool (eyedropper with target) and place a sampler in the light trap black hole. If you're lucky, the black point will be fairly balanced (R=G=B); if not, you can use the Shadow Tint slider to balance the numbers. Moving to the right will shift toward magenta, to the left toward green. Don't beat yourself up if the numbers are within two points of each other. Look out if one channel ends up being five to 10 units higher!

Now you can begin to adjust the Red, Green and Blue sliders to get the patches on the ColorChecker to match the hue and saturation of the ideal values. Lightroom has an almost identical set of Camera Calibration sliders in the develop module, so you use the same procedure to set shadow tint and RGB hue and saturation.

At the moment, the only wrinkle is there doesn't appear to be a way of setting fixed samplers, so you have to move the cursor into the image to read RGB numbers for the shadow values. Neither ACR nor Lightroom give us Hue, Saturation and Brightness (HSB) numbers, only RGB, unfortunately, so the easiest way to figure out how to set the sliders is to process out the file and read HSB numbers in Photoshop.

First, set up a crop with the Crop tool; aim for a crop size close to 800 x 600. Click on Open to bring the image into Photoshop. The idea is to compare the real-world ColorChecker against an artificially generated "ideal" chart. Bruce Lindbloom has developed a LAB file for the color values of the ColorChecker by measuring a large number of real ColorCheckers with a spectrophotometer and averaging the values. You can download the file by clicking right here.

There's also a wealth of information on this website that's worth exploring.


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