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Astrophotography - Film

With CCD imagery grabbing a foothold in all aspects of amateur photography, it seems moot to discuss film. I tend to agree - there serves no purpose to discuss film in astrophotography since even this hobby has be assimilated to use computer technology. The benefits of using CCD's over film are just too great, but there is a certain nostalgia when thinking about film - and to understand photography to its fullest, learning about film is required (in my opinion).

Film, in its most basic form, is simply the formation of Silver Halide crystals residing on a gelatin base. While the gelatin serves no purpose other than to "hold" the image, the Silver Halide crystals provide the magic. These crystals respond to light so that development (chemicals designed to stop the reaction of silver halide to light) locks in the crystals change to light. The result is a culmination of thousands of crystals of various "color" resulting in an overall image. The idea is fascinating but really simple.

Color photography uses died crystals that respond to whatever wavelength to which they correspond, but the idea is the same. Chemicals are used  to extract the final image.

Almost immediately it is evident that several problems exist:

  • crystalline structure of each film will not be exact
  • Chemicals used to extract images are mixed, and variations occur

As such, photography obviously more artistic than exact since every step of photography is not precise: crystal count can vary, percentages of chemicals vary, temperatures of the chemicals can vary, light exposure to the film can vary, and so on...

But dealing with that variability is what separates a professional from an amateur. Experience in chemical use, exposure techniques, film bias, and so on greatly affects the outcome of the image.


To understand film photography in Astronomy, nothing is better than Michael Covington's book Astrophotography for the Amateur (see the reference section) - this book is also good for understanding film in general.

Of the one area of astrophotography does film reign supreme is the capture of star trails:

This image is taken with a 35mm SLR (Single Lens Reflex) camera in a stationary position with the shutter open for an extended period of time.

One of films disadvantage allows for this dramatic image to be captured - that is because film is NOT linear, the overall image does not saturate. The term for this is called reciprocity failure and means that the longer film is exposed, the less sensitive it becomes.

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