I am often asked how I create a starburst when it appears in one of my photos. Often people think it is something that is done in Photoshop but nothing could be further from the truth. So how do you create this effect without using a special program? Like many things in photography knowing the hows and whys can help you to maximize or minimize this effect. For every photographer who loves the starburst effect there's another who does not and are constantly working to avoid it. It's really just a matter of aesthetics and preference.
Lens and Aperture: It is the phenomenon of diffraction that creates the star effect on distant single-point light sources and, sometimes, on specular highlights on an object in the frame. The size, shape, and characteristics of the star effect are a function of the size of the aperture opening (diffraction of light passing through an opening) and the number of aperture blades on the aperture diaphragm (diffraction of light passing an object). The design of the aperture diaphragm has a profound effect on the star effect. Sometimes, your gear isn’t critical to a successful image, but when you are dealing with star effects, you will find that lenses have vastly different characteristics when it comes to this type of diffraction. If the aperture blades form a perfect circle, you will not get the star effect and you will, instead, have distant highlights producing Airy Discs on your image. In general, the circles emanating from the disc will be so small and faint, depending on the light source’s distance and intensity, that you will not see the rings. Several modern lenses feature “rounded aperture blades” to help form a circle for the light to pass through.
Use smaller apertures: When it comes to star effects, in general, the smaller the aperture, the more pronounced the effect will be (there are other considerations that I will discuss later). However, just as you are increasing the diffractive effect from distant light sources, you will also be increasing diffraction throughout the image. There is a trade-off. When the aperture diaphragm forms a polygon instead of a circle, we get the star effect. The light streaks or star points of the effect extend from the vertices of the polygon formed by the blades.This is where it gets cool. With diaphragms that form a polygon, if you have an even number of aperture blades, you will get one point per blade extending from the vertex of each intersecting blade. The light streak will continue across the opening where it intersects the opposite vertex. Therefore, an even number of blades will create one star point per blade. A 6-blade aperture produces a star with 6 spikes. If you have an aperture with an odd number of blades, the diffraction extends from the vertex across the opening where it does not intersect with another vertex. Therefore, an odd number of blades creates two star points per blade - a 7 blade produces a star with 14 spikes.
Your exposure affects the intensity of the star effect. The longer the exposure, the more star effect you will see, until the point at which the entire image is overexposed. The brighter the highlights in a photo, the more star effect. In night photography, burning out highlights, such as street lamps and other artificial light sources, is sometimes unavoidable. The amount you let those highlights burn is directly related to the size and intensity of the star effect. How much you see of the star effects is dependent on the contrast in the scene.
Love them or hate them? Cool trick or cheesy distraction? Star effects are a creative part of photography.