When light is forced through a tight aperture, each edge will create a diffraction spike... and the spike will go both inward and outward (perpendicular) to that edge.
If a lens has an even number of aperture blades -- say 8 so the aperture has an octogan shape like a "Stop" sign -- then aperture blades on opposite sides of the lens are parallel and this causes the diffractions pikes that pair blades to be overlaid. You will only 8 spikes.
If the lens has an odd number of blades (say 7), then there aren't any parallel blade edges and that means the and you'd see 14 spikes from a 7 blade aperture (or 18 spikes from a 9 blade aperture.)
The spikes are technically always present but they get longer as the aperture size gets smaller... so they're only really obvious when you use very high f-stops (e.g. f/16 will show them... but f/22 will show them much stronger... f/32 would be even stronger yet.)
Note that it does require a somewhat bright point of light to create the spikes.
If you wanted to photograph, say, holiday lighting... and you wanted every miniature light to create a "star" then you'd probably have an easier time doing it with a cross-screen filter. I have a couple of these... a B+W brand 4x cross-screen filter as well as a 6x cross-screen filter (they make them in 8x as well.) That's a glass filter that threads onto the front of your lens but it has etched lines it it (like micro-prisms) to create the diffraction spikes. Some versions of this filter have very thin wires embedded in the glass.
Tim Campbell
5D III, 5D IV, 60Da
