Monitoring the all-important Blue channel — with help from Blackmagic Design’s UltraScope.

By Tyson Birmann

Working as a Digital Image Technician (DIT) in the Los Angeles film industry, I find myself dealing with the overall exposure of digital cinema cameras on a daily basis. I've noticed that the use of video has made shooters reliant on the monitors as a way of judging light levels. The idea being, if it looks good on the monitor, all is well.

This isn't always true. Quite often it is what the eye doesn't see that can cause the most problems later. Many of these problems can be avoided by paying attention to the video's Blue channel. The blue channel is where video hides all of its ugly little secrets. Secrets that don't really show up until you start to play with color in post.

In RGB (Red, Green, Blue) color processing, the sensor builds an image by combining those three primary colors to create all of the colors in the video spectrum. Without getting into the science of light and color, it is easiest to just say blue light is the weakest of the three. It is the channel that requires the most light for proper exposure. If it doesn't have enough, we begin to see “noise.” The problem is, when you are looking at the image on a regular monitor you may not even see this issue because the other channels are masking them. It isn't until you get to the post process and start to play with the colors a bit that you see problems. By manipulating them, the “noise” starts to bleed through to the overall picture and degrades the quality of the entire image.

Think of the color channels as three students in a classroom. The red channel is the fastest learner and requires the least information to have a “clear picture” of the subject. Green is the middle level. And Blue represents the slowest learner. This is the student that needs the most time and information to get that same “clear picture.” The trick becomes giving all three the same amount of information, and make sure it is enough for the slowest learner, but still not so much that the fastest student gets overwhelmed with stuff they don't need.

So how do we know what “lesson plan” (light level) will accomplish this? The only way to truly know what you are getting is through the use of a waveform monitor. The use of these tools have become as common on set as a light meter. In a way that is exactly what they are. The problem is, people aren't using them in the same way. More often than not the lighting is done before hand and then the waveform is used to simply set the exposure.

I suggest the opposite. Set your exposure to a level that you know will give you a clean image and optimum latitude (in many cases this is somewhere around a f/5.6 to an f/8 stop.) Now you watch your waveform and light until you have the levels that you need. This establishes a mid point and allows you to adjust the brightest and darkest areas so that they fall within the range of the image sensor's ability to capture a clean picture on all channels, including blue.

In the past, waveform monitors were all but un-affordable for most of us. Not so anymore. I have been using the UltraScope by Blackmagic Design:

This surprisingly affordable computer hardware has allowed me to have real time monitoring that provides many of the tools that were once only possible through the use of separate components costing many thousands of dollars.

The UltraScope waveform tool allows simultaneous viewing of both standard overlay mode and parade view. Parade view lets you see each channel's level as a separate display. It also offers a “blue only” view within the image window. A simple mouse click allows you to view the blue channel without the red or green masking the issues that may be there. When the image is underexposed, you will start to see a tremendous amount of “noise” (much like film grain) that may otherwise go unnoticed.

I have used the Ultrascope on five feature films now and don't know what I would do without it. Now, Blackmagic Design has recently introduced the “Pocket Ultrascope.” It has all of the same features and functionality in a external piece of hardware the size of a small TV remote. It allows full HD monitoring and waveform functions on a laptop via USB 3.0.

So what if you can't control the light to get “enough”? We often come across this. There are times that budget or time or even simple location limitations keep us from having as much light as we might need. In these cases you have to pay attention to certain things more carefully. Since we are going to be limited in the amount of color correction we can do, it is best to be sure to minimize the correction that is needed. In this case I suggest using a color temperature or white balance that is slightly “cooler” than you might normally use. This way, most of your color correction will be done by removing blue levels and as a result, some of the “noise” within it. Of course this isn't an ideal solution, production rarely gives us much that is ideal. In the end, it is important to remember that a video image is a sum of its parts working together as a team. Like any team, it is often only as good as it's weakest link. The secret is identifying that weakness and giving it the proper attention and compensating for it. With the proper tools and a bit of attention to detail, the blue channel can become a great place to look to assure proper exposure and a quality image.