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Star Streak Tutorial
The actual _area_ of the lens is the determining factor.
Thus a 200/4 has as much light-gathering area as a 50mm f1.0 !
So, I had problems recording good star trails on a 45mm f4.0; no wonder !
this has only a 11.25mm effective aperture... that 200f4 has a 50mm aperture !
thus, a shorter lens will need to be faster, or use faster film.
Bear
-- Barry Rowland, April 29, 1997
You recommend a manual camera, because of battery consumtion and this is true for most of the modern
cameras, but the old Canon EOS cameras (600-series) feature a mechanical lock of the shutter and draw
nearly no current (25 microamps, this means a fresh 2CR5 battery will last for 50.000 houres ...)
-- Helmut Faugel, June 17, 1997
Concerning astrophotography and the recording of stars, there seems to be some confusion about fratios,
aperture, focal lengths, etc. For point sources such as stars, it is the focal length, not the physical
aperture, that determines the limits of what will be recorded on film. This is because the amount of
background sky included in the picture varies with focal length and thus the amount of magnitudelimiting
sky fog goes up as the focal length decreases. Longer lenses include less of the sky and
therefore less of the sky fog. Since the stars are points, their light is not spread out as focal length
(magnification) increases. This effect results in an increase in the ratio of starlight (point source) to
skylight (non-point source) as focal length increases, and fainter stars are recorded before being limited
by the sky fog. This light-source ratio is not affected by the f-ratio or physical aperture of the lens. For
example, a 50mm lens at a dark site has a limiting photographic magnitude of about 11.5. A 500mm lens
has a limiting magnitude of about 16. The magnitude scale is a way of estimating the brightness of an
object, with each successive magnitude number being about 2.5 times brighter than the next one
(magnitude 1 is 2.5X brighter than magnitude 2). The f-ratio does determine how fast the sky fog limit is
reached. Exposures longer than that needed to reach the sky fog limit will not record fainter stars.
For star trails, you should pick an f-ratio that will give you a decent star exposure for the faintest stars
you want to record. For a given exposure time, too low a ratio will cause a fast sky fog build up with
little contrast between stars and sky. Too high an f-ratio will result in fewer stars against a darker
background. The f-ratio you choose will depend on the local sky conditions and the focal length of the
lens for the reasons stated above.
In visual astronomy, the physical aperture determines the limiting magnitudes of stars. At the same
http://www.photo.net/astro/star-streak (3 of 9)7/3/2005 2:15:29 AM