The Orion region, defocused trails-II

Constellation of Orion, defocused trails

Star images are point sources of light of widely varying intensity. Normally, no single photographic exposure on colour film can capture their subtle colours. Those stars whose intensity is just right for their colour to be recorded appear insignificant on the photograph, while anything brighter is overexposed and washed out.

By changing the focus of the lens during a star trail exposure, star images are spread out over the film surface. This effectively changes the amount of light in each star image as well as making it bigger. If this is done in a series of steps, as here, sooner or later all the star images will be recorded with an exposure that will reveal the star colours.

Here we see the stars of the constellation of Orion recorded in an exposure of about 30 minutes, during which time the focus was moved in a series of 10 steps three minutes apart. The effect is to reveal the star colours and that of the vivid red Orion nebula.