Bright stars stand out better in a picture if they are surrounded by diffraction spikes. Normally, a high-quality lens, wide open, will not produce diffraction spikes because its aperture is circular. If you close it down one stop, you may be rewarded with a dramatic pattern. Figure 7.6 shows an example, from a Canon lens whose aperture is, roughly speaking, an octagon with curved sides.
Another way to get diffraction spikes is to add crosshairs in front of the lens (Figure 7.7). The crosshairs can be made of wire or thread; they should be opaque,
7.4 Diffraction spikes around the stars
Figure 7.6. Dramatic diffraction patterns from a Canon 300-mm f /4 EF L (non-IS) lens at f /5.6.
thin, and straight. Figure 7.8 shows the result. A piece of window screen, serving as multiple crosshairs, would produce a similar but much stronger effect.
Whatever their origin, diffraction spikes are a stiff test of focusing accuracy. The sharper the focus, the more brightly they show up. For that reason, many people find crosshairs or window screen useful as a focusing aid even if they don't use them for the actual picture.
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