Today we're going to look at something that you've seen before, but may not know the name of: diffraction spikes. Let's start with an example. The lights on the left side of this image have strong diffraction spikes, but if you look closely, all of the light sources in the image have them.
First a little background. Some of you may remember from physics class that light behaves like a wave. (If you don't, or if you haven't had that class, that's ok.) Because light acts like a wave, it diffracts (bends) when it passes through a hole. In a circular aperture, like a lens that's fully open, the diffraction is equal in every direction. The stop in the camera is approximately circular, but not quite. Those corners are what creates the spikes.
(For a more information about the topic, check out the Wikipedia article on them. Optics is a big topic, and I really can't do it too much justice in the short form blog entries.)
That said, you don't get spikes automatically every time. Why not? To vastly oversimplify, the light needs to be coming straight into the camera, and there need to be a lot of photons. That means that you need a bright source and/or a long exposure.
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