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Sunday, July 11, 2010

••◊ Feeling techie with Chroma Du Monde (part 1)

Ben Cain over at NegativeSpaces.com sent me a raw picture of a Chroma Du Monde calibration target made for the Red camera. I know what you're thinking about the photo below...big flippin' deal - that has to be a submission for a french high end contemporary art competition, right?. Nope, this target is used to calibrate and align cameras to one standard so you can mix and match cameras and have an easier time in post with color rendition. Like most professional test targets, this is one expensive little piece of cardboard.

The Chroma Du Monde test target has a neutral gradient in the center and calibrated color swatches around the perimeter, which is exactly what we need when using the Picture Style Editor software from Canon. Now before you go thinking that you're going to download this JPEG and do your own calibration...think again. There's a reason I asked Ben to send me a raw file. The color rendition and tone curve depend on which existing picture style you start with. For instance "standard" is vastly different from "faithful" or "neutral." As it turns out, none of the existing picture styles completely align with this chart. That being said; Ben recently showed that the "faithful" picture style with saturation set to -1 comes pretty darn close to meeting the rec.709 (i.e. HDTV) chroma specifications - notice that I didn't say luma, just chroma.

The first place to start when doing any type of calibration is the tone curve since you and I are most sensitive to lightness/brightness. The tone curve essentially sets the "lightness" or luma levels to calibrated steps. To do this I used the center neutral density swatches. By tweaking the white balance in the raw file using the 18% gray border I was able to achieve very near to perfect neutrality in the center swatches (i.e. gray, no remaining color). One thing you may notice is that lighting on the target isn't perfect. The left side of the test target is brighter than the right side. At first I thought, "oh no," but then I remembered that the surrounding 18% gray border is calibrated and consistent from left to right. By measuring the average of the border near each neutral swatch I can calibrate out the lighting change across the target.

The CDM target comes with documentation that shows you where each patch should lie in within IRE levels (i.e. a measure of video signal amplitude). I'm not going to publish the exact values, but I think these pictures show the intent of the patches. Each of the Canon picture styles has a aesthetically designed tone curve applied that tweaks the lightness/brightness of the picture. The first picture below shows the neutral swatches with the "faithful" tone curve applied. The skinny lines are the IRE level reference marks (compensated for light fall off across target) and the fuzzy lines represent the light levels from each of the 11 neutral patches. The Canon profiles all have a traditional S-shaped tone curve originating from the days of film with mids bumped brighter and dark regions slightly crushed. The third picture shows what happens to the tone curve after I tweaked it in the Canon Picture Style Editor software. Ah...much better alignment, although contrast is severely lacking in the picture now, which is good or bad depending on what you're doing. Calibrated doesn't always mean good looking.

The result is a picture of the target using the new custom tone curve. Notice how the neutral swatches are much less contrast-y? In this picture I haven't tweaked the chroma (colors) yet, and yes, they are way off balance. Canon likes to crank up the yellow, red, and green saturation.

So why might this be important? Well, what it means is that you can match the Canon DSLR cameras with just about any other camera on the market, within performance limits, as long as the other camera has footage of the Chroma Du Monde target. If you're shooting with an EX1, HPX170, Red-MX, Alexa, or even Kodak film stock, you can tweak the Canon to match any of those cameras and make the 5D/7D/1D a second unit, P.O.V., or crash cam. It also makes your post production easier. As for getting good looking pictures straight out of the camera...well, not so much. My next entry about this topic will have to deal with tweaking the chroma/color to be more accurate.

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