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Stage to Screen: Goldilocks and the Three Exposure Techniques
Posted Apr 24, 2007 Print Version     Page 1of 1

If exposure is like the three bears' porridge, then Goldilocks was on to something—"just right" is the only way to go. Digital video cameras are very unforgiving when it comes to proper exposure, and this month we'll examine the technically challenging issue of stage lighting and learn three techniques to ensure proper exposure.
 Lighting directors design stage lighting for the live audience experience. They use light and shadows equally to set the mood and tone of a scene. Follow spots and primary lighting are used to draw attention to the foreground and action, while depth is exaggerated by less lighting in the mid-ground and the lack of lighting in the background. This is a practice that lighting directors have been honing in theaters since incandescent lighting replaced open-glass flames in 1885.
 Unfortunately for video producers in the digital era, our cameras cannot see both the shadows in the background and the highlights in the foreground at any single iris setting. An improper iris setting often results in blown highlights or barely visible shadows. This is due to limited exposure latitude. To understand how lighting for video and a live audience are different, you need to look at how the human eye and video cameras react to light and handle extremes in contrast ratio.
 Contrast ratio is determined by how many times the brightest part of the frame is brighter than the darkest. Most digital video cameras can handle a static contrast ratio of 30:1, while the human eye can handle a ratio of 10,000:1. In a high-contrast ratio environment, a video camera can only properly expose a small range of the total image, and this range is the exposure latitude. Everything else is crushed or blown out. Like the human eye, our cameras have an iris which allows more or less light to pass through to the sensor. This does not increase the exposure latitude; it only moves the range up or down.

A larger exposure-latitude range allows for a separation of highlights, mid-tones, and shadows. This separation is a key factor in giving the perception of depth in a two-dimensional environment. Typically, a larger CCD has a greater exposure latitude, and the sampling rate of the analog-to-digital converter and the efficiency of the digital signal processor determine how much detail is available and the range of contrast the camera can handle.

We've all heard the expression "we'll fix it in post," but when it comes to exposure and our MiniDV camcorders, there just isn't much room for error. In the digital world that converts video into a binary code of ones and zeros, an overexposed area has no detail. It is a white blob, and if you try to darken this in post, your white blob becomes a less-white blob, but a blob nonetheless. The same goes for the other extreme; underexposure results in mid-tones that are indistinguishable from shadows.

Regular 35mm film has an exposure latitude of seven stops, and each stop is an exponent on the base of two. So seven stops equals 2^7 or 128:1. Most video cameras are limited to a 30:1 range—and thus incapable of replicating film stock's exposure latitude—but you can reduce the contrast ratio. This allows more of the image to fall into the exposure latitude range, which is desirable because it gives more film-like detail to your highlights and shadows. It also minimizes the blow-outs and crushed blacks if your exposure is off by a stop or two.

The first step in achieving proper exposure is to reduce the contrast ratio. This does not involve your camera, but rather having the lighting director give you flatter lighting. By lowering the light levels in the brightest areas and adding some light to the dark areas, you bring more of the image into the range your camera can handle.

The second step is to use the zebra settings on your camera to find the exposure sweet spot. My PD 170 has two settings: 70 and 100 IRE. I find that 70 is too dark and 100 is too hot, so I like to keep my exposure target one stop above 70 or below 100. My exposure target for stage productions is the talent's faces, not their costumes or the background. I also make sure my setup is set to 0% and not 7.5%. Although 7.5 IRE is the NTSC standard for analog black, when I shoot at 0% and transfer my digital footage into Adobe Premiere Pro 2.0, black is assigned a value of 16 luma. This is the correct digital value the International Telecommunications Union outlined as the eight-bit luminance standard. If I shoot at 7.5%, the camera incorrectly adds the 7.5 setup to the digital signal, which results in blacks that have a value of 32 luma.

Luma is brightness, so the result is that the entire image has a washed-out look. A 7.5 setup is required for analog output to make sure black is compliant; the Sony PD 150/170 and DSR 250, and the Panasonic DVX100 all have a percentage setup that approximates the correct analog output at the expense of the digital signal.

The third strategy is to use the knee and black-stretch settings on your camera. These settings are not available on most prosumer SD cameras and are features reserved for larger cameras and most newer HDV camcorders. You can use the black stretch to brighten the shadows and the knee to compress the highlights. This reduces your contrast ratio without affecting your mid-tones, and gives up to an additional stop of exposure latitude at both ends.

Great video starts with exposure that is just right, and once you've had a taste for proper exposure, you'll never eat cold porridge again.

Shawn Lam is an MPV-accredited videographer and business owner based in Vancouver, BC. He specializes in stage event and corporate video production and presented a seminar on the business of producing dance recital videos at the 4EVER Group's Video 07.

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