Tag Archives: light-trails

The lights of Coburg Street and passing cars in a long exposure shot of the street from the traffic lights at the junction with Bridge Street.

This was a 15 second exposure at f/22. The lovely star shape of the street lights is because the opening in the lens was so small (at f/22) and the diaphragms of my lens. It takes a long time to get the exposure but it’s worth it!

This appears to be a beautiful example of Fraunhofer diffraction. It is due to the wave nature of light. The effect depends on the wavelength (that is, the color). It is most pronounced when bright light from a practically infinite distance passes through narrow slits, causing the light to spread perpendicular to the slits. This spreads a point-like beam of light into a pair of streaks.

Using a small aperture creates slit-like situations at the corners formed by adjacent blades. Thus, when you have a combination of relatively intense, pointlike, monochromatic light sources in the image and a narrow aperture, you should see a streak (of the same color) emanating from the points in two directions perpendicular to the blades…

…Finally, length of exposure is related to the occurrence of this effect, as you have observed, but only because exposures with bright points of light are almost always made much longer than needed to record the lights: you’re trying to see the rest of the scene, which is much darker. The brightness of the diffraction streaks decreases so rapidly away from their sources that if you used a sufficiently short exposure to properly expose the lights themselves, the streaks would be practically invisible.