It seems that some of us including myself are missing that all important basic information when it comes to HD video production so I have decided to run a series of blogs called “Back to basics”.
Once a week I hope to explain some basic information about HD video production from audio basics like choosing PCM or Dolby Digital, why 10 bit is better than 8 bit but to start the ball rolling I am going or should I say Tom is going to explain how to use and set up zebras.
Most semi pro and pro camcorders have zebra as a choice in the camera setup menu as well as a choice of settings.
Rather than me prattling on I would like to introduce you to a complete stranger, Tom Tanquary, I have no idea who Tom is but his posting about zebras is very simple to follow and works like a charm…
Tom “I’ve been shooting video since 1977 and the introduction of the zebra pattern for exposure has been a total blessing. But, from the discussion above it seems not many people know how to use them. Zebras indicate a specific exposure level for a reflective surface at that iris opening. Overall exposure of any scene is determined by many other factors. Like a spot meter to a still photographer, the zebras can be very helpful if you use them on a known reference surface to determin proper exposure. And proper exposure simply means the “look” you want to get.
Virtually all professional video cameras come with the zebras set at 70 which means 70% of a full signal. Even the pre-set on my PD-150 has only 100 and 70 as its zebra choices. There’s a reason for that. While each camera system will have a different dynamic range between 7.5 (black) and 100 (pure white), certain constants will always be true. The number 70 was picked for 2 basic reasons: it’s the proper exposure for a white sheet of paper while still seeing detail on it (such as writing, or it’s texture) and it’s the level of typical skin exposure where detail begins to be lost.
Skin tones usually fall between 55 and 65% for a very natural, well detailed, chroma-rich look. At 70% skin will start to “shine.” At 75% you are loosing detail, and at 80% most all the detail is gone. That maybe fine if you’re trying to make an old actress look 20 years younger, but in general it means that the person’s face is bland, pale and washed out. If the 70% zebras just barely appear on the most reflective portions of a face, like a cheek or upper forehead, then the rest of face falls into that 55 to 65 range. It’s a very simple way to maintain a consistant good look.
But many times the scene is wider than a single face and other exposure factors may take over. Wide shots of people directly lit by the sun may have their faces at more than 70% to acchieve an overall good exposure. A washed out face in this case is hardly noticeable and that screen area is too small for much detail anyway. It’s at this point that the dynamic range of the camera comes into play. Which is more important: the shadows or the highlights?
Setting zebras at 100 has never made sense to me. 100 is pure white, no detail. So what? There are so many things in a scene that will be over-exposed long before you get to 100. The white paper is a good example. At 90% that paper is blooming big time, at 100 it’s gone. Most all of your exposure decisions are made well under the 100% video level. Which is why professional videographers have always used 70 as a benchmark. Most all surfaces in the upper third of reflective quality (reflect the most light) will start to loose detail above 70%. Whether it’s human skin, a white piece of paper, or a sun lit concrete wall, those zebras let you know how much detail you’ll get at that iris.
Trusting a tiny viewfinder, that may be out of adjustment, who’s picture quality is determined by the angle of your eye to it, under less than ideal viewing conditions is a dangerous way to make decisions on exposure.
And there’s this little switch on the back of the camera that can easily turn off the zebras once your exposure is set.”
Tom Tanquary (From DVInfo.net)
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