... blog post:
Sometimes you can add a little extra sparkle to your photos by including sunstars in them; sunstars are caused by diffraction spikes due to non-circular apertures in lenses when photographing light from a bright 'point' source that is being partially obstructed, like the sun.
To get sunstars you need two things: the sun (or equivalent other bright 'point' light source) partially obstructed, as previously mentioned, plus a wide angle lens well stopped down to a small aperture. Popular groupthink has it that you need a wide angle lens of 28mm or wider and an aperture of f16 or smaller to get sunstars, plus you cannot avoid getting lens flare artefacts in your picture (even if you use a 'quality' lens) which you will need to remove by some form of post processing.
Well, there is quality and then there is quality. The above images were made with my Distagon 35mm f1.4 ZM at f8 on my M10-R full bore into the sun in one case and into the reflection of the sun on the water in other. In neither case was there any flare nor was there any need to stop down to any smaller than the optimal aperture of f8.
As you can see from the pictures that there are ten double points in each of the stars in each image (though the second one is a double star) which is in direct proportion to the number of blades in the aperture (iris) of this particular lens. So, there you go adding a little sparkle to your photographs.
Then again these days you could insert completely false sunstars into your images in post processing using a software tool, though often the end results look 'uncomfortable' in the scene.
Finally, you could also use a star filter on the front of your lens like those from Hoya to add stars to point sources of light in your images. These have lines etched into the surface of their high-quality optical glass to create the flare needed to create the star effect. They come in 4, 6 and 8 cross patterns for 4, 6 and 8 star effects. However, these star effects are single point stars caused by deliberately inducing flare, not the proper double points of true sunstars created by using the diffraction spike technique described above.