Auroras are among the most beautiful and awe-inspiring of natural phenomena. Until the late 18th century, observers were frustrated to try to describe in words or sketches what they had seen. Upon its invention photography promised to better capture the phenomenon, but doing so proved difficult. Early black-and-white films required long exposures (5 to 10 minutes), resulting in blurred, low-contrast pictures. The first photographs to convey some sense of the forms of the aurora were taken in 1892 by the German physicist Martin Brendel. The first color pictures were not taken until about 1950, and Life magazine published color aurora photographs in 1953.
Since the 1950s, faster lenses and color emulsions have passed auroral photography from the hands of scientists to enthusiastic amateur and professional photographers. But while a good auroral display is a visually spectacular phenomenon to the dark-adapted human eye, even the fastest modern color emulsions are not so easily impressed. Auroral photographs require time exposures, with the length of exposure depending on the brightness of the aurora, the speed of the lens, and the ISO rating of the film. During the time required to obtain a satisfactory exposure, the aurora will usually undergo some movement and so cause some blurring of the image. Note, too, that as time exposures increase beyond a few seconds the reciprocity effect on color films reduces the gains to be had by increasing exposure time.
The fastest lenses available for modern 35-millimeter cameras are typically f/1.0 to f/1.2; for medium-format cameras the fastest lenses are usually f/2.8 (though Hasselblad makes a 110-mm, f/2.0 lens). Color film at ISO 800 is readily available, and higher speeds are sold, though they often compromise on color reproduction, black rendition (for dark skies), and graininess. The following table recommends exposure times for various auroral brightness levels (see "An Aurora Watcher's Guide" for an explanation of auroral brightness).
|Recommended Auroral Exposures
(in seconds, with ISO 400 film)
Other practical pointers: A firm tripod and a cable release are essential. If the temperature is extremely cold, film should be wound slowly, and rewound with the camera grounded, to prevent static electricity from leaving spark marks on the film.
The Relative Effectiveness of Camera Lenses
Exposure is only part of the requirement for pleasing auroral pictures. It is always best to have an interesting foreground and a nice star background, so as to give a sense of scale. In particular, a brilliant star background greatly enhances auroral photographs, particularly if the stars are not trailed. So it is well to understand that star exposures do not depend on the f/number of the lens used but rather on the effective aperture of the lens, which is given by the focal length divided by the f/number.
The table below shows the relative effectiveness at capturing auroras and stars of some lenses that might be used for auroral photography. The 50-mm, f/1.0 lens has been used as a point of reference. The table shows a little-appreciated fact: although wide-angle lenses may give a good sense of auroral scale, they are very inefficient at recording stars. The longer-focal-length lenses often provide much better star fields. Of course the longer the focal length, the smaller the area of sky captured on film. Balancing field of view, exposure time, and the richness of the star field is the challenge of auroral photography.
|Relative Effectiveness of Camera
Lenses for Aurora Photography
A final factor to consider is contrast of the aurora in the sky. Moonlight will diminish contrast; a Moon with up to 25 percent illumination gives pleasing results, as the light illuminates the landscape without brightening the sky enough to degrade auroral contrast. However, once the Moon is half lit, auroral contrast will begin to suffer. Auroral photographs taken near full Moon will be very washed out except in the case of extremely bright auroras.
Shooting Auroras with a Digital Camera
The age of the digital camera is here. As prices fall, new models appear, and features improve, these cameras are dramatically changing the way amateur astronomers "photograph" the sky. So does that mean a digital camera can be used to capture images of the aurora? Absolutely, though there are some caveats.
Older cameras, particularly the early point-and-shoot models, may not be useful for aurora photography. But before you start looking either for an old manual camera or a new digital wonder, check to see if your current camera can be set to an ISO of at least 100. Higher is better (up to ISO 400 or even 800) but as the image above shows, if the aurora is bright enough, a low ISO will suffice.
- Take a time exposure of at least 15 seconds. Newer models can expose for up to 60 seconds (which will give you more flexibility), but 15 seconds is okay, particularly if you can set the ISO to a high rating.
- Have its focus set to infinity. Most autofocus cameras, when pointed at the sky, automatically drive the focus to infinity; if yours doesn't you'll have to focus it manually.
If your digital camera can do all these things, then you're ready to shoot an aurora the next time one appears.
Most suggestions for auroral photography apply whether your camera is digital or uses film. These include using a fast (at least f/2.8), wide-angle lens, fast film (or a high ISO setting on a digital camera), a sturdy tripod, and a cable release (if your camera can take one). And don't forget to bracket your exposures — that is, shoot a variety of exposures that can range from 5 to 60 seconds in length.
Some comments, though, are applicable only to the digital domain. For example, when the camera operates in time-exposure mode, noise in the form of bright, randomly spaced pixels can appear in the image. The longer the exposure, the more apparent the noise. (Of course, if the aurora is bright enough, you won’t see the noise!) Most new digital cameras have noise-reduction settings that automatically come into play during time exposures. But be warned: there's a catch. Once the exposure is finished, the camera generates a "black" image that enables it to eliminate the noise in the original image. That "black" image takes just as long to generate as the length of time of your original exposure. So if you take a 60-second shot of the aurora, you'll have to wait another 60 seconds for the camera to "do its thing" before you can shoot again.
Something else to keep in mind is that digital cameras can get warm. A hot camera generates noise, so turn the camera off if you're not using it. Also, turn off any features you don't need, including the flash. And don't be afraid to fill up the memory card (the camera’s image-storage media). The delete button is your friend for getting rid of unwanted images. In fact, the best thing about using a digital camera is that you can see the results immediately, know whether you're getting good images or not, and make adjustments accordingly.