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In over 30 years of making optical filters, Ira Tiffen created the Pro-Mist, Soft/FX, Ultra Contrast, GlimmerGlass, and others, netting him both a Technical Achievement Award from the Academy of Motion Picture Arts and Sciences and a Prime-Time Emmy Award. Elected a Fellow of the SMPTE in 2002, he is also an Associate member of the ASC, and the author of the filter section of the American Cinematographer Manual.

It happens that we sometimes need to capture action that occurs at night. The general lack of light, and the need to somehow provide more, becomes especially difficult when the area in view is too large to readily illuminate artificially. Long ago (in cinematic years) a technique called Day-for-Night (DFN) was conceived to assist in these situations.

The idea for DFN centers on the fact that it is dark at night and that this can be visually approximated by underexposing about two stops. From there, different paths emerge. Today, we will address one that has seen use in many productions. It involves using a DFN filter with a blue hue to it.

Here�s how it works. The underexposure reduces the luminance level to simulate night. Now, we are psychologically inclined to consider nighttime, after the warm sun goes down, as having a relatively cooler temperature than daytime. In addition, we are also so inclined to consider �blue� suggestive of a cooler temperature than any other hue. So, by connecting the two inclinations, you can see how we can successfully simulate moonlight with both underexposure and the introduction of blue.

Effective DFN depends on other things to keep in mind. The features of the image should also fit our conception of what you can (and can�t) see at night. An important example: unless you can significantly darken the sky with a polarizer or a graduated filter, leave it out of your image � a bright sky is a sure way to destroy the effect.

The light source should preferably emulate moonlight, or sometimes streetlamps or other night-friendly sources. For moonlight, the more frequently-used approach, you want to have a single point of primary light, as if it was coming from that small white disc in the sky. Fortunately, bright sunlight can do this quite well.

Use the light to provide enough detail, as in the bark on the tree trunks in the DFN image shown, to give depth and positioning cues to the audience, but without revealing more than would seem appropriate with nighttime; there are dense featureless pools of darkness as well. One benefit of DFN is that there is light available over a wide area, but don�t show too much of it � just enough is just right.

The two images were taken moments apart, and the filter, which absorbs about two stops by itself, was not compensated for, thus providing the underexposure needed.

There are other roads to effective DFN; we�ll get to them another time.

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Check out this article in the April 2008 print edition of StudentFilmmakers magazine, page 6.

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