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Subject: Re: ACR Calibration



-->Obviously outdoor lighting would make calibration a nightmare at best.

Not really. While the color temperature can vary with the weather, the sun always provides a pretty full spectrum, which is not always the case with artifical lights that use filtration to arrive at the desired color temperature. Decent midrange strobes can vary the color temperature by ±100K. I suspect that the color temperature variance is utterly irrelevant—white balancing should take care of it—but that differences in spectra between brands will cause more variation than drift in color temperature.

My samples were shot under daylight—basically the only light I ever use—and the temperature/tint correx were 5950/-5, 6000/-6, and 6050/-6, respectively. The first set of numbers I posted used exposure correx to get white to 245, then brightness correx to get green in the green patch to 124. The second set just used exposure correx to get green in the green patch to 124, leaving shadows at 5, brightness at 50, contrast at +25. I've found that after white-balancing, I get extremely consistent results from daylight no matter the color temperature.

There are two different causes of unit-to-unit variation. One is the sensitivity of the sensor itself—nominal ISO 100 may be closer to actual ISO 75 or 150. This has an effect on exposure, but doesn't really change the color much. Color variation is from variations in thickness of the filters, which are very thin.

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