Photographing stars as points of light, not star trails
Equipment and film
Sometimes it can be aesthetically pleasing to allow stars to trail across your night photograph as the earth rotates. Other times you might want the stars to appear as crisp dots of light. In this case I recommend that you use a very fast lens (for example f1.2, f1.4, f2) with a focal length of 50mm or wider (28mm or 24mm work well) using 35mm film. The film should have a high ISO rating and be push processed by your lab to maximize the low light you are trying to capture. The resulting transparency may well show lots of stationary stars, but push processing can really lower the density of the shadows in the image. This can be easily fixed for scanned photographs by using Adobe Photoshop's Levels Command:
Simply drag the left-hand Shadow slider away from its origin (shown in my screenshot above as the first/left-hand red line) until it rests underneath the first significant group of dark pixels in your image (represented by the second/right-hand red line). If you need a better explanation of this process then I recommend looking at a good Photoshop manual.
Working out your exposure time
The method I use to show stars as points of light assumes that you have a very basic understanding of the sky. It involves finding 2 key values which you read off on a printable table (available for download from the end of this webpage) to identify the optimum exposure for your image. These key values are:
i) The focal length of your camera lens (in mm)
ii) The astronomical declination of the star (in degrees) which appears nearest to the celestial equator in your intended photograph*
*This is the star which will appear to streak fastest across the final image and therefore the exposure time should be based on it, too short a time will result in a dim image, too long an exposure will show a trail. The star's approximate declination can be looked up in a pocket star atlas.
The table gives an approximate time exposure value in seconds which should be
taken as a basis for your own experiments with 35mm cameras. I say basis as what
exactly constitutes an acceptable amount of movement on film depends on your own
judgment, and a mathematician would probably not agree with the way I have
rounded my figures. If my table recommends a particular time exposure you may
well be happy with the sharpness of the star images produced, if not try shorter
exposures until you achieve the result you want. When you have done all this
experimenting you may want to draw up your own exposure table. Please let me
know how you get on, all I can say is that these figures work for me!
As an example of how to use the table, say you want to photograph a star with a
declination of 55 degrees with a 50mm lens, the suggested exposure time would be
17 seconds.
I have used this table for some years. For the record the table values are calculated using the equation:
where F is the focal length of the lens (in mm) and D is the declination (in degrees) of the star.
Downloading the Table
To download the table, please choose the file in either .gif or .pdf formats by clicking on the relevant icon below [free Acrobat software to read the .pdf version is available from www.adobe.com]:
