Digital Video - July 2008 - (Page 48) DV 101 BY JAY HOLBEN ART OF DARKNESS SHOOTING IN LOW-LIGHT CONDITIONS IS A TALENT UNTO ITSELF. Model Brittney Siebers is barely exposed. Using a mini Maglite flashlight beside the camera to just bring up her face, doesn't help the image much. With the same mini Maglite on the wall, I got better use of the same limited source by separating Brittney from the wall in a cleaner silhouette. The best use of the small source, I've positioned it as a 3⁄4 backlight to edge her face and still give the feeling of darkness while being able to clearly see her features. T he subject of shooting in low-light conditions is timely for me because I recently completed shooting the feature Two Million Stupid Women, using the Panasonic AJHPX3000 in nearly available light in bars, clubs and cars in and around Los Angeles — all at night. However, this doesn’t mean the audience will have to struggle to see poorly lit images. The art of shooting this project was in choosing our locations and, more specifically, the locations within the locations, very carefully to optimize the existing lighting. THE SCIENCE STUFF All cameras have a base sensitivity to light. This is defined in different ways, but I use the International Standards Organization (ISO) method of base sensitivity, which is the minimum amount of light at which a “proper” exposure can be achieved. “Proper” exposure for a digital video camera is defined as an 18 percent neutral gray card exposed at 50-55 IRE on a waveform. (I’ll explain more about this and how to find your camera’s ISO in one of this month’s DV101 Blog entries at www.dv.com.) Most digital camera manufacturers don’t readily offer their cameras’ ISO, as this number can vary slightly from situation to situation. Instead they most commonly use lux, which is a metric version of footcandles. One lux is one lumen over a one-metersquare area (1 lux = 10.76 fc; see the blog for more). They’ll typically represent the minimum lux required to make an exposure at a particular f-stop. Whichever system of measurement you prefer, it all equates to nearly the same thing — how much light you need. FUMBLING IN THE DARK When you’re underexposing an image, that means you are 48 dv july 2008 shooting in less available light than is required to achieve a “proper” exposure. Sometimes this is done deliberately for aesthetic reasons, but sometimes this is done because of a lack of light. At proper exposure, most light values and colors within a certain range will be represented as they are seen by the human eye. When you are underexposing an image, the camera is struggling to translate light photons into electronic signals and the direct result is a compromise in the image. In an underexposure situation, you begin to lose contrast (difference between highlight and shadow) and color saturation. The image will become monochromatic as you get deeper and deeper into underexposure. Digital cameras have a gain function that artificially amplifies the light passing through the camera’s imager to achieve more exposure. This is akin to an audio amplifier that takes an electronic signal and boosts it, but these tools can only boost so much. Gain is measured in decibels (dB), a relative logarithmic scale of amplification based on the values of illumination before and after the gain is applied. Gain is typically represented in steps of + or -3dB, and with each successive application of gain, you are amplifying the signal more and more. As gain is a relative scale, there is no direct correlation between exposure stops and gain increase, but it is approximately one stop increase per +6dB of gain. A byproduct of this amplification is noise, a distortion of the image with what looks like static. The more gain you apply, the noisier the image will become. Most cameras cannot amplify beyond +18dB. Although the noise that results from over-amplification of the image may be part of your aesthetic choice, it is generally considered objectionable. Some shooters feel that even the slight amount of noise generated at +3dB is objectionable; some find +12dB to be completely acceptable. It all depends on your projwww.dv.com http://www.dv.com http://www.dv.com
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