Photography 101: Diffractive Starburst effects on sunlight taken with Nikon D40 without starburst filter

To achieve a starburst effect, without using any special filter:

1. Source of the light must be small.
   
2. Set the aperture of your camera to f/16 or smaller (Small aperture = small opening = BIG F number).
   
3. Depends on the image you are taking, if it's too dark due to the object casting a shadow, you may want to set to Spot metering on your camera. After setting to Spot metering, focus or your object such that it will not cast shadow (optional step).
   
4. Look into your view finder, move around a bit, see the starburst, take a shot.
   
5. Good luck.
   

The below are my 3 attempts in sequence.. the last 2 with some orangey dots/spots/blobs are causing sun glare / lens flare.. so-called undesirable and can be prevented using hood.. but i hate my hood.. so i didn't bring it out.. :P.. The long explanation on star effects are quoted below.. read them if you want to understand on what cause this star effect..

There are two ways that light is caused to be bent. One is refraction where the light passes across the surface between transmitting media of two different densities and the other is diffraction where light bends when it passes across the edge of an object.

Refraction occurs in a lens as light, moving through air, enters the glass material where it bends toward the perpendicular to the surface. When it exits the denser material, it bends away from the perpendicular. This effect can be seen in prisms and is a very important property of carefully designed lenses.

Diffraction, on the other hand, not only bends the light, but the light is usually scattered some because of the microscopic imperfections in the object's edge.

A lens opening of f/8 is considered the optimal aperture for a normal 50mm lens. As apertures become smaller, diffraction becomes more pronounced and begins to degrade the image significantly (although depth of field will increase). This is a physical limitation and is caused by reducing the ratio of the area of the aperture (that affects refraction) to the aperture circumference (that affects diffraction). Diffraction begins degrading the image at apertures smaller than f/8 for a 50mm lens, but diffraction does not achieve the same effect for a longer focal length lens because the physical size of the f/8 aperture is much larger. For example, the critical aperture for a 200mm lens would be f/22 and smaller.

The effect of diffraction is noticeable when photographing point light sources in a dark background, or even just very bright light sources. Most obvious is the starburst effect caused by diffraction. This may occur even at the larger apertures because of the high contrast between the light source and the rest of the scene. The starburst is caused by the fact that the aperture is not perfectly circular, but is made up of several blades, each having a straight or nearly straight edge. Thus, at the corners of the aperture, the light that is bent and scattered by one blade gets reinforced by the light that is bent and scattered by the adjacent blade to increase the exposure in that area and form the point of the star.

The smaller the aperture, the more accentuated the starburst will become. A special filter is not necessary to create the starburst effect, selecting a smaller aperture will accomplish the same result. In high contrast situations, you may not be able to eliminate the starburst even when using the widest aperture available on the lens.

Note: Small aperture = small opening = BIG F number

Reference:
http://www.photo-artiste.com/existinglightguide.html