An Introduction to Waveforms, Scopes, and Exposure

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Before the rise of the anarchy of digital video, analog video was ruled by engineers at virtually every step of production, post production, and delivery. There were rules that had to be adhered to very strictly to ensure that your footage met the technical requirements of broadcasters. Remember, NTSC was instituted in the 1940s, (PAL appeared about a decade later). So, making sure that your video would display properly on every television required a fair amount of control. Additionally, there were limitations within the capability of analog and digital standard-definition recording technology, as anyone who has severely overexposed parts of their images and suffered the horrific clip to nauseating yellow can sadly attest.

This necessity for rigorous control of exposure and color required the use of various pieces of gear—most notably the waveform monitor and the vectorscope. Don’t feel as though you have to run out and purchase one; many monitors—studio, field, and on-camera, have these displays, as well as histogram functionality, built in. You will also be able to find these scopes and displays in many NLE systems.

Waveform Monitor

The waveform monitor is used to evaluate the brightness of your image, regardless of the color. The scale of the waveform monitor is 0 to 100 IRE. IRE represents the scale invented by the International Radio Engineers society. Essentially, it is designed to match the capabilities of early televisions to display an image. Anything at 0 is completely black, with no detail, and anything above 100 will be clipped and white, with no detail. With modern televisions and displays, especially with HDR displays, it is possible to exceed the 100 IRE threshold and not clip your highlights; luminance values found in this region are often called super whites. However, the clip at 100 situation still exists, and be aware that unless you are sure your post production and delivery path can handle images with areas over 100 without clipping, literally cutting off any information in that part of the image, it is safer to keep your images in the 0 to 100 IRE range.


 This waveform shows severe clipping

 This waveform shows a corrected level

Of interest to note about a waveform monitor display, as opposed to a vectorscope or histogram, is that the waveform produces an abstract version of your image; viewing the waveform from left to right mirrors viewing the image from left to right, making it easy to reference the brightness of specific areas of your image/frame quickly.  

Vectorscope

The vectorscope displays six color targets essentially fixed into an odd-shaped pattern on a grid. In fact, each color is represented by two targets, but what is important about the vectorscope is that it displays color information that the waveform monitor does not. In the old days of analog, the vectorscope, along with the waveform monitor, were tools used to align multiple cameras, so that their colors and brightness matched when shooting the same set. Specific color charts were shot, and the technician would adjust the cameras to make sure that red on one camera was the same red on another. This would be done by manipulating the camera electronically to make sure that the known color landed in the center of the target. Even in the digital age, or perhaps especially in the digital age, with productions using a wide variety of cameras with different sensors from different manufacturers, being able to read your camera on a vectorscope is precisely what you need. Getting your colors to align accurately when shooting will save you significant time in post, even if you are only working with one camera.  

Vectorscope image with uncentered chroma

As far as reading the vectorscope itself, there are two targets for each color (red, magenta, blue, cyan, green, and yellow—the primary and secondary colors) and you can set your scope to display at 100 or 75%. What this means is that if your Vectorscope is set at 75%, then the target represents 75% saturation, and the farther away from the center of the display, the more saturated that color. At 100% setting, the target box represents 100%. Generally speaking, going above 75% saturation can be dangerous in terms of displaying colors that are outside broadcast safety and not reproducible by all monitors. So knowing your entire post path, including final display format, is important; otherwise, maybe it is best to err on the side of caution with both exposure and color.


Histogram

Histogram is a display that analyzes your image and displays the percentage of luminance bit values across the screen. So, while you are scratching your head and trying to figure out what that sentence means, let me try to explain it in simple terms. Unlike the waveform, which displays brightness vertically from zero to 1 volt, the histogram displays luminance horizontally, darkest as in 0 at the left, and brightest as in 100 to the right. If you are working at 8-bit, the display would essentially be 0 to 255 values. However—and this is important—it isn’t really displaying the real values; what it is showing you is the percentage of the pixels in your image that are at the values, and rather than use a pie chart, it uses something akin to a bar chart display. The weakness of this display is that it does not show you which parts of your image are dark or bright, just how much of the total image is at each value.

If you look at a histogram and the majority of the display is all the way over to the left or all the way over to the right, this indicates that most of your image is either very dark or very bright. How does this differ from a waveform monitor? A waveform monitor will allow you to see which part of your image is above 100, and thus clipped, allowing you to identify hot spots or exposure issues. The Histogram just shows a rough percentage of where your image is exposed. To get the most out of your histogram, you really will have to consider the shot itself. Is it supposed to be mostly dark with a few highlights, as in a dark scene—night, perhaps? If so, then having the majority of your pixels over to the left may make sense. Is your image mostly bright, say an intentionally bright desert shot? Then it would make sense that the majority of the pixels were over to the right. In either of those scenarios, it might make sense to adjust your exposure some and leave yourself room in post to make adjustments. So, using the histogram to help you determine exposure and then, adjusting your exposure to make sure that most of your information isn’t either clipped at the right, or lost in the shadows to the left, can be a valuable procedure.

While we are discussing exposure, remember that even in a dark scene, having strong highlights or backlighting can help define the shapes in the image, and can even help bring a round, natural-looking depth to the composition, enhancing the dramatic effect of your image. Also, having a good bright and dark reference is comforting to the eye, and it helps keep the eye-brain combo from making the image wash out to medium gray.

I hope you have found this brief introduction to waveform monitors, vectorscopes, and histograms useful. For more detailed info about reading histograms, check out our article, How to Read Your Camera’s Histogram.

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