Last week I got to delve into our Kodak designed sensor in a way I never imagined I could.
Calibration on an analog system, like the one in the D16, is an ongoing process. We will probably still be tweaking settings a long time from now, but I wanted to show you the result of our last intensive calibration session. This is the first time I was able to sit with the engineers (for 3 days straight) and dial in the sensor settings for the different ISOs.
The ISO ratings we were using until now were the default settings in the hardware gain selector. But this selector doesn’t know what kind of sensor you have connected to it so its default settings are, let’s just say, educated guesses.
To start our test, we set up the D16 on a tripod and lit a color chart with a daylight balanced Kino Mini. First we dialed in the 100 ISO settings using reference images of similar charts from other cameras so we could understand where other cameras like the Alexa place their black, white, and middle grey values. 100 ISO is by far the easiest because it uses the least amount of gain; in our case it used zero gain.
Next we tried to balance the other ISOs to match the black, white, and middle grey levels of 100 ISO, compensating with f stop to achieve the same exposure.
The process of balancing such a complex system is extremely difficult. There are matrices of numbers that control each of the processes and malleable aspects of the sensor. There are settings for white clip level, black clip level, overall gain, read out levels for RGB channels, timing settings, and on and on. When you adjust one setting it often throws two or three other settings out of whack. It is a very trying process. The first day I was there I wasn’t sure if we would be able to complete this task in 3 days.
Eventually we developed a method to control the different elements and counteract issues before they arose, so by day two we were speeding along and actually finished the gain and tone calibrations by mid-afternoon on the second day.
The next step was to dial in the color balance using metadata. Again, there were several matrices that all correlate to and effect each other. The first grouping was the actual color matrix, which tells the transcoding software how it’s supposed to interpret the colors coming out of the camera. The color matrix we were using until now we received from True Sense, which I’m pretty sure was intended for scientific use because it made our images very green when you first opened them in programs like Photoshop. In order to correct this hue problem, we were pushing sometimes up to 80 points of magenta into the image, which made pretty nice balanced mid tones, but also created the dreaded magenta highlights we’ve often seen in raw cameras.
The color matrix is a 3 x 3 grid, RGB across the top and RGB down the side. When you change any one number it effects the color balance of the whole image. So you almost always need to change at least 5 of the 9 numbers at a time. And the numbers aren’t linear values, but logarithmic ones, which means to arrive at your next set of numbers to try you are often using a calculator. Very long and tedious work. We worked on this the rest of the second day and pretty late into the night, but by the end of it we had a color set that looked pretty good. Pretty good, that is, when the tint and color temperature were adjusted.
The second color grid is called “Shot As” and controls the way transcoding software interprets tint and color temperature. It’s not a straightforward control, but a group of numbers used to tell the software how each color falls into place.
Here are some test shots before we did the calibration:
Notice how the middle grey changes drastically from ISO to ISO in the before picture, and how stable it is in the after picture. The biggest change, as you can see, is that the color balance of the gray was very green before and now it’s much more even-toned.
Another bonus to adding the proper metadata is that OSX can now create finder icons!
We had to send our newly arrived “Derek” camera back to Toronto to get the calibration implemented because we don’t have a firmware update application yet, but in the future this will be something you can do in 5 minutes with a USB 2 cable.
If you would like to download the new calibration chart files you can do that here.
We can’t wait to shoot footage with this newly calibrated setup and share the results with you!
Thanks for reading, Joe