Tracking the Light Presents—Cumbres Pass by Moonlight.

It may surprise some readers, but the full moon provides the same light color balance as the sun, just at a significantly lower intensity.

This is one of the many differences between the way a human eye interprets a scene and a photographic image.

Where to the eye moonlight—even the full moon, appears dark, a camera can capture a moonlit scene so that it has the same color and illumination ratios as daylight. The difference isn’t with the scene, it is with our perception of it.

Complicating matters are the ways cameras collect light. Back when I used Kodachrome 25, very long time exposures rarely turned out as I hope they would. The film’s ISO rating in very low light was irrelevant.

The primary reason for this condition was a combination of the film’s long reciprocity curve and its very poor sensitivity in extremely low light. Essentially in moonlight, the film lost its effective sensitivity.

Basically this meant that Kodachrome was optimized for daylight situations and its ratio of sensitivity was not proportional in very low light (and effectively lost its ability to record images). So, when I left the shutter open for hours in moonlight, the film was incapable of recording an accurate image.

Even when I’d calculated the theoretical correct exposure. All I’d get back from the lab was a fairly dark slide with a few overexposed specular highlights (bright spots). Not very inspiring.

In the mid-1990s, Mel Patrick encouraged me to experiment with Fuji Provia 100F in moonlight. This was a modern film. Not only was it vastly more sensitive in low light, but it provided a much better color rendition in night situations. While the film still suffered from the reciprocity effect, I found it possible to calculate and compensate for this failing.

Yet, even with this improved film, moonlight exposures still required very long exposures, sometimes up to an hour or more.

Mel had made some outstanding railway landscapes by moonlight, and I tried to emulate his successes.

Cumbres Pass by the light of the moon. A long time exposure made using the full-moon as the primary light source. Does this look like daylight? The star trails in the sky are the tip-off.

A difficulty was finding situations where there was virtually no man made light, since mercury, sodium, and incandescent lights not only tend to be much brighter than moonlight, but cause objectionable color casts and harsh contrast (a topic for another day).

While there are relatively few places in the eastern United States that are completely free from man-made light (even in the wee hours), in the West, clear open skies (and a dearth of population and objectionable light) abound.

I made this image at the summit of Cumbres Pass, Colorado in September 1998. My car and my tent can be seen in the distance. I don’t recall offhand, but my exposure was about 40 minutes at f2.8 using a Nikon F3T with Nikkor 24mm lens. My camera was firmly mounted on a Bogen tripod.

Tight crop of the same Fujichrome colorslide. I don't think this image is critically sharp, but keep in mind the shutter was open for more than half and hour, and even a gentle breeze will create a slight vibration.
Tight crop of the same Fujichrome colorslide. I don’t think this image is critically sharp, but keep in mind the shutter was open for more than half an hour, and even a gentle breeze will create a slight vibration.

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Tomorrow: Big engines in Oregon!


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