Saturday, July 4, 2015

Why I Make Composites for My Night Sky Photos

I enjoy making photos of the night sky, especially when the Milky Way is clearly visible. I usually try to shoot on clear moonless nights because I find that a moon produces too much light. Even a quarter moon lights up the sky and, as a result, dims the stars. Of course, the problem with shooting the night sky with so little light is that in order to light up the foreground, a long exposure is required. An exposure long enough to light up the foreground is too long to capture pinpoint stars.

Left: Exposure for the foreground.

Right: Exposure for the sky.

I photograph the night sky with a 14mm, 21mm or 24mm lens which means that I usually keep my exposures to 25 seconds or less. Anything longer than that will produce unacceptable star trails. Now star trails can make very nice photos but the exposure needs to be more like two and a half hours. A dark foreground can’t be captured satisfactorily in 25 seconds. Using Photoshop to increase the exposure for the foreground can work, but there is simply too much noise to make the shot acceptable.

 Either before or after I expose for the sky, I shoot the foreground. I’ve found that I like to have three stops of light greater than the exposure for the sky. Getting three stops requires about four minutes. (A one minute exposure doubles the 25 second exposure, two minutes doubles it again and a four minute exposure doubles it again for a total of three stops.)
Compositing the two shots in Photoshop is relatively easy so long as the foreground isn’t too complicated.

Here is he final composited image.