Sometimes changes in technology lead to new concepts or even newer technologies, and other times new technology brings up the need to reexamine technology that has been around for a while. One piece of technology that was once old and is now new again is the test chart. While many think “digital” takes care of everything, there are many reasons why a test chart should be part of the production process.
In the old days (and by “old” I mean the days of tube cameras), test charts were invaluable. The alignment of three tubes was critical to creating good color images. Each tube captured one of the primary colors: red, green and blue. When you turned on a color camera, the tubes had to be aligned so the image was “registered” on each tube. If this didn’t happen, when the three channels were combined there would be a loss of sharpness. Horizontal and vertical edges exhibited obvious color fringing: red and/or blue ghosts.
The solution was that each time you fired up the camera, you’d let it warm up (a half hour wasn’t uncommon), and then you’d point the camera at a registration chart. A registration chart would be comprised of a grid of fine horizontal and vertical lines. To register the camera, the blue-channel image would be converted into a negative image and overlaid on the green channel. This made it easy to see when the two channels lined up. After that, the red channel would be overlaid with the green. (The green channel was usually left untouched.) The registration chart included a couple of circles. By using the grid of lines and the circles, you could determine if the camera was adjusted properly so that geometric distortions were minimized. This helped ensure that circular objects in the scene remained circles rather than ovals.
FAST-FORWARD TO PRESENT DAY
You may not be able to recognize a registration chart. Many cameras use single imagers that don’t need registration, and those cameras that do use three chips have their registration locked in at the factory. The chips don’t have the electronic “drift” of tubes.
On the other hand, you may be able to recognize a chip chart, and may have used one, even if it was just to set white and black balance on the camera. And even today there are accessible controls in the maintenance menu of modern cameras that allow you to adjust the three channels.
But there’s more to the story than just matching channels. Look at what’s going on with cameras, and you’ll see that people are trying to maximize the image data they’re getting. Besides higher resolution and frame rate, they’re really trying to get the most dynamic range from the electronics. (Dynamic range is the range from dark to bright in the scene where details are still discernable.)
If you follow the work Technicolor did in creating a Picture Style for Canon SLRs, it’s about dynamic range. Or with the ARRI ALEXA and recording in Log C, or Sony’s F3 being able to produce S-Log, it’s all about dynamic range. Simply put, many of today’s cameras can be set up to capture more data about the scene. These setups are less about having a great image on set and more about having a lot of image data to work with in the color-grading suite (or application).