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updated : mon 16 sep 2002 3:11am bst
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Correcting Camera Vignetting


Star trails

This photograph, taken in November 2001 during our brief but pleasant stay at COAA in the Algarve, is a time exposure of the constellation Orion. Camera Olympus OM-1; 28-80mm Hanimex zoom lens at 50mm; Fuji Superia 800 film; exposure time not noted but we think it was about 15 minutes -- one day we'll measure the length of the trails and do some sums to work it out.

The trails are caused by the rotation of the Earth, of course. The bright object at the very top of the frame is Jupiter, the orange star in the centre is Betelgeuse, the three stars below it are Orion's Belt, with the Orion Nebula visible as a reddish streak below the Belt. We didn't know about the telegraph pole -- well, it was very dark....

The greenish glow is sky fog, and will appear on any time exposure taken at night. It's nearly entirely man-made, and of course it's much worse in most of the UK, where it takes on a distinct orange tint for obvious reasons. The particular film we used seems to have affected the colour -- a film's spectral sensitivity at very low light levels is not guaranteed.

Sky fog is the bane of astrophotography. Much effort is expended on minimising it, from using specialised filters and films to living on top of a mountain to putting the entire telescope into space.

What's also interesting is the vignetting effect visible, with the glow concentrated in the centre of the image and fading towards the edges. This is caused by the camera and lens in conjunction with the chosen aperture -- for astronomical photography you tend to use a wide aperture to gather as much light as possible -- which concentrates light in the centre of the film plane. To get rid of it completely is impossible, unless you build a spherically-symmetrical camera with appropriately curved film (this is actually done in a Schmidt camera).

All cameras and lenses exhibit vignetting to some extent, though with normal daytime shots it's completely unnoticeable. The effect is also accentuated in wide-angle lenses, which unfortunately also amplify the sky fog (sky fog is constant per unit area of sky, so it's proportionally less of a problem when you're taking photos through a telescope or telephoto lens with a far smaller field of view).

Can these twin problems be fixed? Yes, to an extent. We'll demonstrate two approaches here: this section shows how to construct a 'flat field' which is then subtracted from the image, and in Lifting the Fog there's a demonstration (on a much worse picture) of using the image itself as the flat field.