Photophys.com: The Science of Photography

Years ago I enjoyed astrophotography with an Olympus OM-1 35mm camera mounted on a 5” Celestron telescope, but only recently have I realized the accessibility of great astrophotography to anyone with a DSLR and a tripod. Night sky photography is so much easier and better now with digital cameras. If you have not discovered the wonders of the sky at night, I suggest you look at Manel Soria’s article on Landscape Astrophotography at Luninous-Landscape.com. Also Phil Hart’s eBook, Shooting Stars, provides an excellent introduction.

Last fall we traveled to southern Utah for landscape photography, and I packed my Sigma 10mm f/2.8 fisheye lens along with my Canon 10-22mm lens with night sky photography in mind. One clear light the Milk Way was clearly visible, and I aimed my Canon 60D at the sky for a few trial shots. This was, of course, greatly facilitated by the articulated LCD screen. Then I realized how difficult it is to manually focus on faint stars with a wide angle lens. Of course, the infinity marks are not good enough, and one must find something bright to focus on. A flashlight at least 15 meters (50 feet) away will work with wide angle lenses, but that solution is not always practical.

My solution to this problem is to determine the focusing settings for all lenses of interest during the daytime and to document the settings with photographs. In more detail, I focus on a distant object, e.g. the moon, in the daytime sky and make a set of photographs of the focusing scales. A set of such photos now resides on my iPod for easy reference. For example, I have the Sigma 10mm fisheye and the Canon 10-22mm at 10mm. The settings depend on particular lens and camera combinations. I have found some deviations to be very small as with my Sigma 105mm macro lens while the deviation is large with my Sigma 15mm fisheye lens when used on my Canon 60D.

In my first few days of night sky photography I was pleased to get this shot at Bryce Canyon National Park and this view of the Milky Way at Capitol Reef. (Standard exposures for photos of this type are 30 sec, f/2.8, and ISO 3200.) A little success provides a lot of motivation for additional work.

Those who are more interested in stars and less interested in landscapes will be anxious to use their telephoto lenses. In addition to magnification a telephoto lens also collects more light. It is interesting to note that the exposure of a point source like a star depends on the area of aperture of the lens rather than the F stop. A 400mm, f/2.8 lens has an aperture of 143mm while a 10mm, f/2.8 lens provides only 3.6mm of aperture. The area is, of course, proportional to the square of the aperture, so the 400mm lens collects 1600 times more light than the 10mm lens!

The advantages of telephoto lenses bring along complications that the amateur photographer will have to address. For example, even fairly short exposures yield star trails. Solutions involve hardware for star tracking and software for image alignment and stacking. Gary Seronik has described a simple tracker that is fine for exposures of 30 minutes or so,¹ and Michael Covington provides extensive technical information about amateur DSLR astrophotography.² In addition there are numerous web sites devoted to astrophotography. See for example Astropix.

In upcoming trips I plan to practice landscape astrophotography with single shots, star trails, and especially time lapse sequences.

As always I invite your comments and suggestions.

1.Gary Seronik, “A simple_hinge tracker: you can make this mount even if you’ve never built anything more complicated than IKEA furniture,” Sky and Telescope. Aug. 2011, p.64.

2.Michael A. Covington, Digital Astrophotography, Cambridge, UK, Cambridge University Press, 2007

© 2012 Charles S. Johnson, Jr.