A sky full of stars

Moritz Petersen took this wonderful photograph on Flickr:

This sparked a discussion on C.A.F.E. on how he managed to get the wonderful star pattern on his night shot.

I counted 14 points on all the stars. This is because the lens he used, 70-210mm f/4-5.6D, has a straight seven-bladed diaphragm. When you stop down an aperture, point sources of lights get stars due to diffraction. If the aperture is straight-bladed instead of curved, those stars will be more pronounced, but even curved blades will get pointy when stopped down far enough. The Fourier Transform of a delta function and a polygon aperture function is a star pattern with the number of points equal to the number of sides (if even) or twice the number of sides (if odd). A seven-bladed diaphragm produces 14-pointed stars. Some manufacturers prefer odd numbers of blades for this reason.

Why it is rarely seen

You rarely see this in newer lenses because manufacturers such as Nikon have moved to curved nine-bladed diaphragms. Also, many shots are taken with the aperture wide-open in which case the aperture is a circle since the blades are not stopping it down.

If you want to get this effect now, you can buy any one of a bunch of star filters made by Tiffen (mostly for film work), or you can mess with the “Lens Blur” filter added to Adobe Photoshop CS.

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