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Tuesday, August 23, 2011

Solutions: How To Shoot Star Trails

Follow these tips and tricks for shooting in the dark

Resist the temptation to use a high ISO. Keep your camera at ISO 100 or 200. You want to be sure not to generate noise in the shot. Noise in a pure-black sky comes out like colorful pixels. It looks horrible.

Because stars are pinpoints of light, exposure can be confusing unless you’re an amateur astronomer. Instead of writing a treatise about focal ratios and limiting magnitudes, suffice it to say that you want to shoot wide open. A faster focal ratio will let you capture more stars than a slower ratio, but since you probably want to capture as many stars as possible, just go ahead and shoot wide open. Shutter speed is determined by how much of a streak you want to have and how long your DSLR can record a single image before the sensor begins generating noise.

The Earth rotates 360º every 24 hours. To create a star trail that goes 90º (a quarter of a circle in the sky), you want a shutter speed of six hours. Most photographers keep the maximum length of any individual trail to about 15º, which means an exposure of about one hour. Depending on your camera, this may be too long for a clean image. Experiment to see how your camera does.

If an hour is too long, you can shoot a series of shorter exposures and combine them in Photoshop. Since your DSLR is locked down, it’s a fairly straightforward process. You can shoot a series of 10- or 15-minute exposures and let the sensor rest between. Be sure to keep the rest period to less than 30 seconds between exposures or you’ll create gaps in your trails.

In these extremely low-light situations, the key to good results is to experiment. Go out on a clear night and give it a try. You’ll quickly get a feel for how your camera behaves and what the limitations are as far as shutter speed. Star trails are fun to shoot, and they always make an impression.


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