Digital Photography Tips and Techniques

Where the streets have no name by Smaku

You probably have seen these sparkles (stars). Many people like the effect in their night photos but some think those are distractions. What causes these? How to enhance or reduce the effect? Read on to find out the answer.

Apparently this is caused by a physics phenomenon called diffraction. Diffraction occurs when light hits an opaque edge in the light path, such as the aperture blades in the lens. Diffraction causes the light to be spread out in a plane that is perpendicular to the edge from where the diffraction occurs. So you get two points of sparkle from one edge. If there are odd number of aperture blades in the lens, you get 2X the number of points for the sparkle. If the lens has even number of aperture blades, you get the same number of points as the number of the blades due to over-lapping points of sparkle.

The diffraction is a fundamental physics phenomenon of the light. You cannot eliminate or reduce the diffraction but you can certainly reduce (or enhance) the effect of diffraction.

Small aperture, like f/22, combined with a high contrast scene like the above is a perfect combination to make the the effect of diffraction more prominent in night photos. There are several reasons for that:

• The lens aperture looks more circular when wide open but looks like regular polygon when stopped down. There is also diffraction with perfectly circular aperture, but there will be no star effect because there are infinite number of points so they are blended out and not clearly visible.
• With small aperture, the exposure time will be longer, allowing the diffracted lights to accumulate the exposure over where the dark background areas are.
• The ratio of diffracted light intensity to the incoming light intensity at a given point on the sensor becomes higher when the aperture becomes smaller.

If you like this effect, you can simply take a long exposure at a small aperture. If you’d rather to reduce the effect, you will need to open up the aperture and shorten the exposure time. Since aperture also affects the depth of field (DOF), sometimes it becomes a trade-off between maximizing DOF and minimizing the sparkle effect.