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One of the industry's most respected specialists offers some tips on how to use camera calibration charts and vectorscopes to make your camera images the best they can be.
Setting up your camera properly requires more than just turning the camera on and doing a white balance. The vast majority of cameras that have HD-SDI out allow you to customize a camera's look. This can be everything from subtly higher color saturation and contrast, to extreme manipulations to represent an alien world.
The ability to modify a camera's color or "photometric" matrix -- also called "re-sectioning" a camera -- was designed originally into broadcast cameras to allow them to be "color matched" so that there were not any color or density shift when cutting between multiple cameras during live broadcasts. In spite of being "factory" calibrated, most cameras are given a common universal setup profile as part of the manufacturing process, thus allowing for often wide variances in hue, chroma and saturation from device to device using the same settings.
You have surely seen some variation of a test pattern or calibration chart, which is designed to serve as a common reference point for multiple cameras. You can see why this is so important when shooting multicam.
Mismatched colors create a huge amount of extra work.
For digital cinema, adding a color chart for calibration is a also necessity when we are talking about shooting as RAW or in LogC, where the DP's onset looks and settings may not be able to accurately relayed when that image needs to be corrected by someone else when the shooter is out of touch. And of course, when very different cameras are mixed -- a Sony EX3 and an F35, for example, or cameras from different manufacturers -- calibration is one way to ensure consistency as well as accuracy.
Needless to say, using charts for precise matching between cameras in stereoscopic shooting is absolutely essential.
Charts are also essential for cameras that cannot make these adjustments. Shooting a few frames at the beginning of each set-up is vital, and will allow a colorist to properly match across shots in post.
CHARTS AND CHARTS
There are, of course, many companies who make outstanding charts, but on a trip with my wife to Toronto on our wedding anniversary, I made made a detour to visit with my friend Michael Wiegand at DSC Labs, to get a peek at some of the new charts that they were working on. (Happy Anniversary, honey!)
DSC Labs is the brain child of David and Susan Corley, but DSC Labs is more than just a small team of people working behind the negative air barrier and tack mats at every entrance to keep the dust and detritus from marring your chart during manufacture. It is not about the aircraft-grade aluminum sheets that every chart is mounted on to maintain flatness. It is about a company whose attention to those minutiae creates the specificity of trust that keeps DSC Labs, and the ChromaDuMonde chart in particular, as the standard in the industry.
Housed for more than 22 years in a small building 10 minutes from Toronto's Pearson Airport, DSC Labs has moved from being an actual film processing lab for motion and still film, into the leading developer and manufacturer of technical and calibration charts for the film and television community. All from what can best be described by what my wife called "a surgically-clean facility." It was in fact cleaner than a couple of medical facilities I have been through in my life.
For those needing the absolutely most critical luma and chroma calibration, DSC Labs has the Ambi-Combi series of transparent, back-lit charts that offer the highest level of calibration available. For my recent tests on the Arri Alexa, DSC Labs was able to supply me with an AmbiCombi chart having a 102 db range, with over 17 stops of proven, repeatable latitude to test the camera's capability. Yet the test charts do not stop there, DSC makes specific charts for a wide variety of uses. There are charts for SD, HD, resolution, focus, skin tone accuracy, to assist with VFX work, and even 3D specific charts to aid with color accuracy and critical focus and alignment between stereoscopic camera setups.
The DSC Labs AmbiCombi back lit chart. Photo through the courtesy of DSC Labs.
Other charts are designed to be immersed underwater, or to calibrate images in zero gravity on the International Space Station, or their "T" series Medical charts that are used as a forensic guideline, such as in cases of physical abuse.
One may ask, why all the different types of charts, why not one-size-fits-all? The answer is simple: everyone's needs are different. Much like the way camera manufacturers have multiple models for differing needs, the wide variety of DSC charts offers users charts designed for their specific needs.
CAMERAS AND COLOR SPACE
Before I get into the nitty-gritty, let's talk about how cameras record light.
Light is RGB. RGB defined in our world as one pixel for each of the primary colors, Red, Green and Blue. (Green is of course only a primary color in additive color space, which is where light operates. Ink, paint, crayons, etc. all work in subtractive color space, where green is obviously a secondary color.)
Cameras for the most part record in the Y' Cr' Cb' color space, where chroma and density are separated into individual components: Y = luma, and Cr and Cb represent the red and blue chroma values.
To oversimplify and skip over quite a bit of math, light and color separated provided the most efficient method of reducing overall signal bandwidth for TV transmission.
It had a secondary purpose, allowing TVs not able to see color to discard that part of the signal, showing essentially a black and white image onscreen instead of a color one.
With that in mind, let's take a look at one specific chart from DSC Labs chart to understand what you'll be looking for as you begin to calibrate your camera. I will go over the setup and tools needed to do this type of calibration properly with your camera.
The DSC Labs ChromaDuMonde +24 chart seen in image below offers a number of tools that allow you to both control and diagnostic tools on the chart. Notice that the chart has a neutral 18% gray background, precise framing guidelines for both 4x3 and 16x9 aspect ratios, and resolution "trumpets" that allow the user to determine sharpness and resolution of their cameras.
The ChromaDuMonde chart from DSC Labs
The opposing gray scales in the middle of the chart are useful for determining your camera's gamma (knee) settings for luma, while the 24 color chips plus four fleshtone colors are used to define the chroma settings.
The large number of color samples allows you to precisely calibrate your camera for the most accurate color reproduction by using your camera's color matrix to align the cameras signal to correctly match the color swatches on a vectorscope. The same processes can be used to modify the settings to create a customized look for your project.
Matching your camera on a shot by shot basis is not usually possible with most shooting setups. Instead, shoot a couple of seconds of your specific chart with every setup. This allows you to then match correctly in post, thereby both saving time on set, and avoiding secondary color correction hassles later on.
Reminder: when using a chart for calibration, always remember that correct setup is vital. Lighting on the chart should be adjusted so that there is less than a 0.1 stop difference across the face to ensure chroma and luma accuracy.