Both cameras and video assistance equipment on set come with many tools to help you work in and speed up all types of production, especially those that require speed and precision. Tools such as the zebra to monitor the highlights and avoid overexposure; or the false color to control at a glance the exposure and contrast in the picture. All have their uses and it is convenient to know them thoroughly in order to avoid human errors that we all make by quickly judging a frame. Although, if we had to stay with only one tool, without a doubt, this would be the Waveform Monitor.
A Waveform Monitor (WFM) is essentially an oscilloscope that analyzes the luminance of a video signal. In other words, it is a tool that tells us the luminance line by line and represents it in a graph. One of the biggest advantages is that it gives this information in real time, so we can see a representation at the moment of what our camera is capturing. Broadly speaking, the left of the WFM corresponds to the left part of the frame, just as the bottom of the WFM corresponds to the blacks and the top to the high lights.
In addition to getting a correct exposure, the WFM is also used for matching cameras in a multi-camera set and spare a lot of post-production work to compensate for uneven video signals. With the same idea, it is also one of the most useful tools in color correction.
The luminance in an WFM can be seen mostly in IRE scale, where 0-2.5 is the absolute black and 100 is white (in more modern cameras we find a 100 to 109 range of super-whites since they can retain more information before burning the highlights). Although it is a scale that was born from analog signals, it is still used as a unit of measurement.
Before continuing, we will take a moment to talk about exposure. Exposing correctly is not only capturing the image we want to represent it, but that the capture is what we want to see on monitors, screens and final reproduction media. Exposing a scene so that it is "well lit" does not mean that at the end of the workflow it comes out as we had initially conceived it, at least not without having problems in post-production.
Exposing correctly in video is a bit more delicate than in still photography, especially when we enter the television broadcast. In photography you have to take into account also how the image will look once printed and also monitor the color spaces. But as color spaces and logarithmic curves work (as they represent the captured image and the dynamic range each of the cameras) we can make more errors if we do not know well the material with which we are working.
In video we have 4 main color spaces nowadays:
- Recommendation 709, which can represent about 6 stops of dynamic range and corresponds to the standard for HD video broadcast.
- Recommendation 2020, an evolution of the 709 for UHD with approximately 15 stops of dynamic range.
- DCI designed for cinema.
- ACES, the standard space created for digital cinema that allows a maximum theoretical representation of 25 stops. It is designed as a space that allows to unify systems and improve the workflow when using images from different sources, from analogue scans to digital cinema cameras.
Knowing the curve in which the camera works and the destination curve (if our project will be for film or television) will help us to know at what point to locate the medium gray and the skins. It will also give us information on where we can take the image to the extreme because we will retain more information (there are sensors that work better on high lights and sensors that have very good blacks).
The best option is to look for the manufacturer information, mainly where the middle grey is placed in their logarithmic curves (in Rec.709 for example it is usually placed in 45 IRE). If you cannot find the information, you can always do camera tests with a color card.
Table with middle greys in most common log curves:
S-Log 2 (Sony)
32 - 33 IRE
39 - 41 IRE
42 - 46 IRE
Once we know where to place our middle grey or our skin-tones, it is as easy as looking at the WFM while adjusting the exposure in camera until we put the image range where the middle grey or skin-tones are at the desired point of the WFM graph (in case of Rec. 709, 45 IRE). If we have correctly exposed, then in the grading it is much easier to adjust that middle grey to the standard where we are going to emit our footage and thus make sure that the image delivered is as faithful as possible to what the DoP and the project client wanted.
In the following example images, we can see how the skin-tones are slightly overexposed (around 80 IRE) but it was already the artistic intention in this case, being our point of reference for the correct exposure the briefcase.
Initially it may seem like a complicated tool to use, but with a little practice it is very easy to get used to. In the end, the more we use it, the more we will know the limits of our camera, which will allow us to gain efficiency. In addition, we will have the certainty that the final product will be exposed exactly as we want, whatever the destination platform of our production.