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Camera shading basics

Dec 1, 2008 12:00 PM, By Tony Manard

Making your cameras match is both art and science.

Charts and test patterns

Although it is best to perform these alignments with a set of dedicated test charts, it is not always practical. For the purpose of the kind of on-the-fly setups we are discussing here, an 11-step grayscale chip chart and a back-focus chart will do nicely. Also be aware that white cards are produced that offer corrections for warmer skin tones and difficult lighting situations. If you find yourself in a variety of lighting situations, a few of these application specific cards would be a good addition to your arsenal.

If no chart is available or the placement of the cameras makes charting them impractical, find a source of white, such as a billboard or canvas, and balance all of the cameras to that. The key is consistency so that each camera reproduces the same scene with the same colors, even if it is not 100 percent accurate to what the eye sees.

Setting up the cameras

Setting up the cameras properly is more than half the battle of camera shading. Use a white card or, better yet, an 11-step grayscale chip chart. Line up a full frame shot of the card on each camera. With each camera pointed at the chart, close the camera iris, and set the master black level at the 7.5 IRE or pedestal level on the waveform monitor. Open up the iris until the white card or highest luminance level of the chip chart is at about 90 percent video. We set this at less than 100 percent to avoid any effect on the signal from the auto knee circuitry. Perform an auto black balance and an auto white balance from the CCU. Although sometimes the black balance is skipped, it is a good idea to perform this step because many cameras use this procedure to map around any dead pixels on the CCD.

Figure 3. Using the grayscale chip chart as the source, note the gamma crossings in the lower right of the figure.
Click to enlarge

If the auto black and white procedures complete satisfactorily, look at the output signal on your vectorscope. The dot should be at the center of the circle for each camera because the white card or chip chart contains no chroma information. If you only have a waveform monitor, this can be checked by using the luminance filter. Because there is no chroma information, the waveform should look the same with the chroma turned on or off. If you still have chroma information or the dot does not paint to the center, then you may have to switch to a manual white balance and use the CCU's red and blue color adjustments to paint out any chroma. Most CCUs allow for color adjustment for both black and white, so check this again with the iris almost completely closed.

Using the 11-step grayscale chip chart, you can check the gamma crossing. Because the camera CCDs are not subject to the same limitations of our eyes and display devices, gamma correction must be applied to the image. The 11 steps of the chart are logarithmic, but become linear when gamma correction is applied. This is to ensure linear color reproduction through the range of luminance values. The signal represents a stepped X on the waveform monitor. The steps should be linear with the crossing taking place in the middle of the X. (See Figure 3.)

Back focus

Although back focus adjustments are not necessarily considered camera shading, they are critical to getting clear pictures. A back focus check should be performed anytime the cameras are moved, a lens is changed or removed, or whenever it is suspect. A back focus chart comes with most broadcast lenses, but can be performed with common objects on location.

Focus on an object that will show details (like a sign with lettering) at the farthest point of the lens zoom. Pull the zoom back; the focus should remain constant throughout the travel of the zoom. If it does not, loosen the screw at the ring closest to where the lens mounts to the camera body (often this is the ring with the macro), and adjust this ring for focus. Zoom back in to the original object, focus again, and then zoom out again. Perform this procedure until focus remains consistent through the zoom travel. Back focus should be performed with the iris all the way open, for minimum depth of field. In bright light environments, a neutral density filter may have to be employed.

Shading the scene

With the camera properly matched for white balance and gamma, it is time to start shading. In the case of people's faces, the general convention is to set the iris where the face is at about 75 IRE to 80 IRE. This is variable because of other elements in the scene.

Examine the black levels. The master black level functions somewhat like a contrast control on a monitor. If it is set too low, details in dark areas will be lost. This is known as crushing black. If it is too high, it will provide poor contrast to the higher levels of luminance, and blacks will appear gray.

A good starting point is to place any black items in the scene at or just below the 7.5 IRE video pedestal and adjust from there. A word of advice: Set a black level for all of the cameras, and stick to it. When switching between cameras differences in blacks are obvious. This requires looking at the scope and the calibrated monitor to arrive at a happy medium.