Getting the most from your HD video camera
Calibration and exposure control
By W. Norton
Using the DSC Chart
The great thing about the new Pro HD cameras is their ability to totally adjust the color matrix. Using the DSC color chip chart, tape measure, two equal light sources and Adobe OnLocation, Adobe Premiere Pro or Apple Final Cut Pro's built-in waveform monitors and vectorscopes, we will calibrate our camera to get the most color space possible. Most serious HD cameras have color matrix adjustments - check the user manual for your camera.
Adobe OnLocation is an excellent calibration tool - for use on initial camera matrix calibration as well as for on-set exposure control and monitoring. It is available in CS3, CS4 and CS5 versions. As these are important concepts I will go into some details of both camera matrix adjustment and on-set calibration and monitoring. We'll start with the waveform monitor and the white side of our DSC color chart. First we need to setup our calibration conditions as accurately as possible to get the most reliable results.
For calibration, I find it is best to move the chart to approx. 6 feet from the camera - straight on. Make sure to secure both the camera (locked down on tripod) and the chart so they do not move. I use a tape measure to place 2 identical diffused lights at an equal distance from the chart and aimed to illuminate the face of the chart evenly. If you see a lot of glare from the chart in the camera, tilt the chart slightly forward or backward to eliminate the majority of the glare - the DSC chart holder is designed for this. Be sure to frame the chart in carefully before calibration. The chart has framing registration marks to frame by - zoom (or move your tripod) to place the chart framing references at the edges of your view in your viewfinder. After setting up the lights, turn the chart over to the white side. Your waveform monitor is a great way to gauge your lighting - it should show an almost straight line across the screen when your lighting is even and you have your exposure set properly (the gray bars on the other side of the chart will help us set our exposure correctly a little later). If not, it will show where the lighting is uneven- if there is a droop in the line on the left side of the screen, move the light on the left a little closer and retest - same story on the right side. The waveform monitor represents the light values it "sees" rather than painting a picture that your eyes readily recognize, each color represented by a number position on the waveform grid. This function will help us set our white and black values - a big step in obtaining proper exposure and grabbing all the light from the scene possible. Adjust the line produced by the white chart until it just touches the 100 line - now you have set your maximum white point to 100% - where it belongs (see Example 1).
Example 2 - what you don't want to see!
Once you have your lights setup, before going any further it is necessary to white-balance to your lights (your camera manual will have information on how to white balance - and it should be done at any lighting change). Now it's time to turn the chart around to the color side. Your waveform monitor should now show a pattern similar to the image on the right. Pay close attention to the boxes at the top and bottom of the display - these are the white and black chips on the DSC chart and will guide us in setting up our exposure for the lighting and our bottom black level. The idea is to open the lens (adjust the f-stop or aperture setting) until the white boxes (the 4 boxes across the top of the waveform display) are just touching the 100 line. If after opening the lens all the way and ensuring your shutter is turned off* your white boxes still fall short of the 100 mark you need to add more light and re-adjust. After adjusting the white level check the waveform monitor for the 3 main boxes at the bottom of the "X" (formed by the grayscale chips on the chart), and should be as close to the bottom line as possible. You should adjust your black level or master black level to bring the level to where it belongs. The closer these values are aligned on the waveform and vectorscope monitors, the better will be your color depth and saturation. You will notice a marked improvement in all of the footage you shoot - colors will pop out at you that were somewhat bland looking before and if you use the chart and the scopes before every setup you will always be assured of proper exposure and color control. Example 3 shows a properly exposed DSC chart - notice the white value at 100 and the black at 7.5 - now we have the full range of light in the scene available to record.
Once your master white and black levels are set, it's time to move on to the vectorscope. Here is where we will adjust our color matrix - phase, balance and gain. The vectorscope is set up with 6 boxes as targets - one each for red, magenta, blue, cyan,green and yellow. At the middle of the scope is white. The pattern in the middle of the scope is the incoming signal from the camera of the the DSC color chart and each line emanating from the center represents one of those same basic colors as represented by the vectorscope boxes. The closer the ends of those lines are to being inside their respective color boxes, the closer to calibrated. Looking at Example 4 you see the vectorscope representation of the DSC chart before any adjustment - using a preset on the camera. We want the center of our 'star' pattern pretty much in the middle of the vectorscope when we are done, and the ends of each color line in their respective boxes. This pattern is not too bad, but you can see the star pattern is off-center and the color lines are not extending terribly accurately towards their color box. Now it is time to adjust. Refer to your camera manual and locate the color matrix (correction) adjustments and controls and become familiar with them, they are the key to success! These controls vary drastically from camera-to-camera so the best thing is to get setup with your chart and experiment with each setting while observing the results on the scope. You will see that almost any adjustment you make will have a bearing on all of the other colors as well as the phase (rotation of the star pattern) of the colors. As you adjust for red, your green suffers, and vice-versa. With an appropriate amount of time, and the power of the vectorscope and chart, you should be able to get your colors better on target and much more vibrant with increased saturation by bumping or reducing individual color gains and phases through the camera adjustments. This will require some trial-and-error, but your target goal is clear and obtainable.
Example 5 shows the same setup after a quick tweaking - still not perfect but a lot closer to the targets - with better color depth. Another 1/2 hour worth of tweaking and it will be pretty darn close. There is a point of over-saturation so watch your output monitor in OnLocation for obvious signs of over-doing it. Your chart should look bright, color accurate and rich when your camera is well-calibrated. Your footage will look it's absolute best and provide your efforts in post production with the easiest media to work with.
Why bother? Well, the new HD cameras are nice, but without spending tens of thousands of dollars, they are not quite as good as 35mm film cameras for resolution, and if you want your output to look as good as film you are already handicapped by that fact. In order to get the best quality output you need the best quality input - that means recording as much of the real color space as possible. This is probably the only way to do so. After learning these techniques, my on set calibration and setup time is minimal, my results are predictable and reliable, colors are vibrant and look great - and the footage is easy to work with - the less color correction to do in post production the better! There are many more details to follow related to this subject in following chapters and articles. For now I hope I have given you enough info to get your camera shooting at its best...
* if you do not have a shutter 'off' setting set as follows - for 30fps use 1/60th second shutter speed - for 24fps use 1/48th second shutter speed. Otherwise keep the shutter off unless using for slow-motion or fast action sequences - wn
Copyright 2009 HDCinematics.com -W. Norton
DSC Color Chart
Waveform monitor showing well-balanced lighting - Example 1
Color side of DSC chart showing range of colors - Example 3
Color side of DSC chart uncorrected matrix - Example 4
DSC chart with corrected matrix - Example 5