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Starburst Effects
Turning the sun into a starburst can add a special something to a photograph. Making it happen though can seem like blind luck, but there is a secret to increasing your odds.
The aperture of a lens is formed by an overlapping arrangement of blades that open and close to control the amount of light reaching the film. A spreading of light known as diffraction along the boundaries between these blades at small openings results in the starburst effect. The smaller the opening, the more pronounced the effect.
As was mentioned last week, a lens's aperture number is a ratio between the diameter of its opening and its focal length. As such, if a lens is set to an aperture of f/22, it has an opening with a diameter that is 1/22 of its focal length. Contrary to you might think at first, if you keep the diameter of the opening the same but increase the focal length, the aperture doesn't stay the same. Take a 50mm lens and set it to f/22. Then take a 100mm lens (twice the focal length) and set it so that its opening is exactly the same size as the 50mm lens. The aperture of the 100mm lens would be a ridiculous f/45 (double that of the 50mm lens), in order to maintain the same ratio (50 divided by 22 is the same as 100 over 45). Since there is no such thing (generally speaking) as an f/45 lens for an SLR, you will likely need to settle for an f/22 aperture on that 100mm lens which would be a bigger hole than would f/22 on that 50mm lens.
 So, in order to get more chance of diffraction, you should use a small lens opening, and you will get a smaller opening if the lens you use is a wide angle lens instead of a telephoto, since by definition this will give you a smaller focal length and therefore a smaller lens opening for a given aperture.
The number of streaks in the starburst depends on the number of blades in the aperture. The diffraction occurs at each point where two blades overlap and spreads in both directions from the center of the lens outwards. Lenses that have an even number of blades will yield starbursts that have the same number of streaks as blades since each streak will have a counterpart from the opposite aperture blade that lines up with it exactly. If a lens has an odd number of aperture blades (as most actually do) you will end up with twice as many streaks as aperture blades since no overlap will occur.
As you probably already know, along with all the other "special effects" filters currently available, you can buy so called "starburst" filters, so all this may sound like more bother than it's worth. In my experience though, these filters rarely yield optimal results. They allow you to create starbursts even when they wouldn't normally occur. And wouldn't really rather have the real thing anyway?
Update 12/12/2004: If you shoot with digital, you might not want to go all the way to f/22. Diffraction though that small of an aperture can start to soften an image more on digital due to the smaller sensor size. You can still get great starbursts at f/16 though.
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