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Here's the article, and the URL: http://www.railfan.com/boyd1288.html This is pre-EOS, remember. "Filtering Vapor Lights" by Mike Thomas Railfan & Railroad, Camera Bag - December 1988 ------------------------------------------------------------------------ Night photography is a technique which has gained wide acceptance by the railfan community Once a skill which was mastered by only a few experts, the growing popularity of the technique is evidenced by the many fine night photographs which appear in print and the popularity of the night photography sessions offered at the various railfan outings. The technique which is widely used today is to use flashbulbs or modern xenon strobe units to "paint" the subject with light while keeping the camera shutter open. Another interesting method which is often overlooked and can provide dramatic results is the use of natural light to expose the photographs. It seems simple enough. Put the camera on a tripod and open the shutter -- some of today's cameras will even automatically expose the photo when taking a time exposure. Why isn't it used more often? The simple fact is that most illumination in railroad yards, stations and engine terminals does not lend itself easily to color night shots. As anyone who has waited for those dynamite night shots to come back and found the slides with a green or orange cast from a nearby light knows, today's modern street lights do not provide correct illumination for color photoraphy. The days of incandescent lighting on railroad property are all but gone. With the recent modernizations of railroad facilities the incandescent lamps have been universally replaced with gas discharge lamps. These lamps are far more efficient and last far longer than the older incandescent bulbs. Discharge lamps produce light by passing an electric current through a gas contained within a quartz envelope. The electric current excites the gas molecules and causes them to give off light. Illumination from thus type of source consists of many narrow spectral spikes whose particular wavelength (color) depends on the type of gas in the envelope. Incandescent bulbs, on the other hand, produce light by heating a filament. This filament glows and gives off light which is continuous across the visible spectrum. This illumination is similar to the sun and is the reason incandescent lights can be easily filtered to give pleasing results on typical color film. The electric discharge lamps are tailored so that their discrete spectral lines mix and look white to your eye. Here is where the problems lie: film does not respond the way your eye does to this type of illumination! Where your eye can mix the colors, film responds only to the sharp lines, causing the photograph to be the color of the predominant spectral line of the source. This effect is compounded by the fact that long exposure times, usually greater than five seconds, are required. The two main gasses used in outdoor lighting are mercury and sodium. Mercury has a very strong line in the green region of the spectrum turning your slides green, and sodium has a number of lines in the yellow-orange region of the spectrum. So much for the physics class. Now, how do we correct for this type of light and get our photographs to look on film like they look to our eye? The proper combination of color correction filters can "fool" the film into thinking it is being illuminated by the sun. After many phone calls to Kodak's customer service department, a check with the street light manufacturers and a few calculations, the proper color correction filter combination was put together. To correct for mercury illumination (lights which appear whitish or bluish to your eve) a 40M and 20Y (40 magenta and 20 yellow) filter should be used. The lens should be opened one stop to make up for the light lost in the filters. To correct for high pressure sodium lights (appear yellowish-orange to the eye) use an 80B and 20C (80 blue and 20 cyan) while opening up 2.5 stops. With this combination of filters your K-64 will respond as if illuminated by the sun. The above mentioned filters are available from Kodak, however they must be custom ordered from most photo shops. The 80B is available from Cokin as a hard plastic filter. An easy and inexpensive way to find the needed color filters is to purchase a set of Cibachrome color correction filters for color printing applications. These filters, known as Wratten filters, are soft plastic which scratch and fingerprint easily, so some type of mounting must be done to make them reusable, especially under the typical rigors of night photography I used a Cokin frame and cut the Wratten filters to fit into the frame. For the sodium filter, the hard plastic 80B was used and a Wratten filter was cut and taped to it along the edges. This provided support for the soft filter. Cokin makes a filter attachment which allows the filters to be slid in front of the lens for the photographs. I made the mistake of trymg to put the holder on my lens in the dark and wound up simply holding the filter over the lens while exposing the shot. As you can see from the two CN photos on page 18, the filters performed precisely as calculated. The before and after shots show the interior of CN's Gordon Diesel Shop in Moncton being lit by a large bank of mercury lights. Notice the difference in the "reds" on the nose and how the color was made more lifelike. I have gotten similar results with the sodium filter pack. These filters provide a simple and inexpensive way of getting back into natural light photography Remember, they have been optimized for K-64. Satisfactory photographs may result with K-25, but using K-200 might present problems. If you use these other films it is best to experiment. Another point is that these filters will not remove those pesky colors from flashbulb shots. If the filter is left on while flashing bulbs, it will compensate in the opposite direction, since bulbs and strobes are made to emulate the spectral characteristics of the sun. One final note, remember when taking night shots around an engme terminal, be careful of moving equipment. EDITOR'S NOTE: Mike Thomas is a laser engineer from Nashua, New Hampshire, who deals with color spectrums in his professional work -- J.B. ======================================================= -> SPORRS: 'Serious Photographers Of Railroad Related Subjects' -> Web Site: http://www.anet-stl.com/acphotog/sporrs/ -> Message © 1998 SPORRS® - All Rights Reserved =======================================================
