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Correcting Camera Vignetting

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.

• Get your image into Photodesk and make a copy of it, named "flat" (File>Copy). All the work is done on this copy.
• It can be difficult to see the exact extent of the vignetting, so open the Image processing window, select Equalise and click on the Details and Preview icons.
• Drag the top of the right-hand dotted line halfway to the left. The image should brighten to something like this:
  

  

  Brightened to emphasis the vignetting

• The discolouration is a circle centred on the middle of Orion (not quite in the centre of the image itself). Incidentally, it's worth emphasising here how important it is to set your monitor up properly for this sort of work. Photodesk has a very nice built-in calibration program (Photodesk>Calibrate>Monitor setup...) -- use it!
• Don't apply the brightening, by the way! This is just to get an idea of the extent and gradient for the next section.
• We're going to construct a gradient that's as close as possible to the sky fog, so click on the Paintpot and ensure the mode is set to Colour.
• Click on the Sample tool. Hold down the Alt key, and drag out a square in the middle of Orion, trying not to include any stars. The colour in the top-left box of the Colouring effect window should change to a very dark green.
• Pick up this colour and drop it on the very first box in the palette; you may need to scroll the palette left to see it. The colour should change and the number "0" should appear.
• Now take another sample, this time from one of the dark corners. Remember to hold down the Alt key!
• Scroll to the extreme right of the palette, and drop this colour on the last box (255).
• Scroll the palette back again, and click on colour 0 with Adjust. This will construct a graduated colour range between your two sampled colours, dark green to extremely dark green.
• Now click on the Gradient icon (third from the left). Click on Load palette to make the effect use our newly constructed colour gradient, and ensure Radial is selected.
• Click on Setup. Drag out an arrow starting at the centre of the image and ending in the corner -- your two sample spots, in fact.
• Apply with the Magic brush set to Whole image. This should have replaced the image with a gradient similar to this:
  

  

  The generated flat field

• Open the Calculation window (Ctrl-U), and set up as follows:
  • Source 1:Image - Your original image
  • Source 1:Channel - #0 RGB
  • Source 2:Image - The flat field
  • Source 2:Channel - #0 RGB
  • Destination:Image - New
  • Destination:Channel - New
  • Function: Subtract
  • Offset: 0
  • Gain: 1.5
• Click on Calculate. After a bit of gamma adjustment you should get something like this:
  

  

  The flat field subtracted from the original image (gamma adjusted)

• Depending on your original image, you may need to adjust the Gain factor.
• As you can see, this isn't perfect (it's clearly been "over-enhanced" to emphasis the technique). The sky-fog is not evenly graduated, and our attempt at copying it is off by some way. You can get round this to some extent with the gradient adjustment facility (drag the handles around in the big white box in the Gradient window), and by using the other painting tools. It all depends how much work you want to put in.
• The next section offers another, and much superior, way of fixing this problem....

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