When you have a source of light that is significantly brighter than the surrounding environment, the effect is more apparent. The smaller the aperture the more you will exaggerate the rays of light you see when compared to a wider aperture. This is caused by diffraction, or when waves of light you see pass close to an object in this case the diaphragm of the lens. Because a aperture set at f/16 is a smaller opening than compared to wider opening at f/8, you will have a more pronounced effect at f/16 compared to a softer effect at f/8.
You might think I can set it to f/20 or higher and really have a pronounced effect. At higher f/stops you might see a reduced level of clarity, experiment with different apertures to see what you might consider an acceptable level.
Another factor is the number of diaphragm blades and the shape of the aperture. You will have very little control of this this factor depending on what lens you purchase. If the lens has an even number of blades and the aperture shape has an even number of sides you will get a distinct point of the star extending out from each; an eight-blade aperture will give a star with eight points. Lens with odd number of blades will produce double the distinct points, a nine-bladed design will produce a star with 18 points. If the aperture is rounded you will not produce a distinct shape.
When shooting the Sun you will want to try and get the Sun as small as possible, such as close to the horizon or when the Sun is obscured partially by an object.
