What are sunstars?
Sunstars are bright points of light in your scene, usually street lights or even the sun itself, that create a wonderful star pattern effect in your images. They have the benefit of adding sparkle and drama to your images.
How do I create sunstars in my photos?
They're easy to achieve, however your results may vary depending on what lens you are using.
All you need to get sunstars in your photos is a small aperture setting - the smaller the diaphram opening, the better your sunstar will look (the "points" will be more elongated and finer in detail). Anything smaller than f/12 will get you decent results, although you have to remember that past f/8 the overall sharpness of your image may drop due to light diffraction. I like to shoot at around f/14 - f/16 when going for sunstars as it will give me a nice balance between sharpness and sunstar goodness.
If shooting in the evening or night, you'll definitely want a tripod as the shutter speed will be quite slow due to the smaller aperture setting (a small aperture opening lets less light onto the sensor).
Setting the aperture on your camera is done by putting it into Aperture Priority mode (usually labelled A or Av) and then dialling in the desired value, if your camera doesn't have that functionality, then throw it out and upgrade it.
Different lenses produces different sunstars
To complicate things just a tiny bit, the lens you use will affect how the sunstar will turn out.
The number of aperture blades will determine the number of points on the star - a lens with an even number of blades will have the same amount of points as the blade count. For example, a lens with a 6 aperture blades will have a 6 point sunstar (see the second photo above in this article). A lens with an odd number of aperture blades will have TWICE as many points as the blade count. I don't know the physics behind it, but it just does. The diaphram of my Canon 24-70 f2.8L II contains 9 blades, therefore it will produce lovely 18 pointed sunstars as seen in the photo at the start of this article.
Now apparently lenses with rounded aperture blades will churn out sloppy and inferior sunstars compared to those with straight edge blades. Historically this may have been the case, but two of Canons newer lenses which I happen to own have circular aperture blades and they both produce kick ass sunstars - I'm talking about the Canon 40mm f/2.8 and the Canon 24-70 f/2.8L II. Zoom lenses also apparently aren't as good at producing sunstars as primes, but my new 24-70 has the best sunstars out of any lens I own or have owned! Props to Canon for working their magic here.
The next time you're out in the evening snapping away, think about how you can bring out the best in those shiny lights in the shot.