Monday, September 10, 2012
Overcoming Coma in Starry Night Photography
A night photographer must learn how to control lens aberrations, like coma, in order to produce great photos of a star-filled sky that includes landscape features. (Click on above image for more information and a larger view.)
Those who are familiar with my recent style of NightScape work know that I'm photographing the stars as points of light, rather than as star trials. To do this, I must keep my exposures under 30 seconds, and often as short as eight seconds, depending on the focal length of the lens. In order to get the proper sky exposure, I must use high ISO's (typically above 6400), and large lens apertures, such as f/2.8 or wider.
The problem with shooting a lens wide open is that stars can really show off the faults or aberrations of a lens, especially coma. Comatic aberration causes point sources, such as stars, to appear distorted --appearing to have a tail (coma) like a comet. These coma "tails" are most apparent near the edges of a photo. The good news is that 50% of the distortion goes away when a well-designed lens is stopped down by one stop, and about 80% goes away by two stops. Unfortunately, with my night photography, I can't afford the light loss that comes with stopping down from f/2.8 to f/4.0 or f/5.6!
This is the reason I purchased the Canon EF 24mm f/1.4 L USM II Wide Angle Lens. This lens was used to photographed the above photo of the Milky Way over the Watchman, in Zion National Park. As you can (below) from my own tests on point sources of light, this lens is fairly well-corrected by f/2.8 (click on the image for a larger view):
You'll notice that by closing down two more stops, to f/5.6, does not produce as significant of an improvement! If I get in a pinch, and need more light, I can get acceptable results at f/2.0. But the real beauty of this lens is the bright image it gives you in the viewfinder. You can imagine that composing in near pitch black is made so much easier with this brilliant, f/1.4 lens!
I get similar results with my Canon EF 50mm f/1.4 USM Standard Lens. By stopping down to f/2.8, I've been able to make print enlargements up to 24x36 inches (and 30x40) with very pleasing results. Stopping down to f/2.0 reduces my print quality to about 18x24 inches (or 16x20). Shooting wide open (f/1.4) means I cannot enlarge my prints much beyond 8x10 or 11x14 inches (unless I don't want my name associated with them)!
Royce Bair is the editor of this blog and the photographer of the above images. Here is my gallery of NightScape images. My schedule of workshops, tutorials, and other events is available here.
UPDATE - Inexpensive prime lenses with almost NO coma: Check out part II about the Samyang / Rokinon lenses. These lenses can be used wide open (at f/1.4), and have very little coma!