In photography, the starburst effect gives the illusion of light rays from strong light sources in an image. The effect happens when the lens’ aperture is not a perfect circle, and it gets more pronounced under narrower aperture.
Flat-Earthers often find themselves looking for any peculiarity —no matter how small or unbelievable— in order to discredit any picture they deem unacceptable to their misguided causes. One of this peculiarity is the starburst effect.
The starburst effect gets more pronounced when a narrow aperture is being used. A narrow opening is usually used to maximize depth of field, to make both close and distant objects equally in focus. Sunrise and sunset photography utilizes this technique, especially if the photographer wants to emphasize nearby objects as well as the distant sun.
The picture on the upper right is an extreme example. The photographer wanted to take time-lapse images of the 24 hours Antarctic midnight sun. The camera is made to focus on both a very distant object (the sun) and a nearby object (his wristwatch). To achieve that, he had no choice but to use a very narrow aperture.
The narrower the aperture, the more pronounced the starburst effect. If it is desired to minimize the effect, the photographer has to use larger opening, or use a higher quality lens with larger, smoother blades. Sometimes this effect is deliberately used for artistic purposes.
Picture © Nicolas Raymond – Solar Sentinel of Point Reyes
In the illustration, a mobile phone flashlight is used as the light source. Then its pictures were taken using four different aperture settings. As we can see, the narrower the opening, the more pronounced the sunburst effect appears.