Diffraction stars

Yesterday evening, I read an article about diffraction starts in the magazine Popular Photography. (I subscribed through Amazon which costs only $15 for a year.) This evening I found myself with my tripod and camera on a rather boring scene close to Houston’s aquarium. So I thought: “Let’s give it a try with those diffraction stars.” I understood from the article that there would be diffraction stars in my photo with a smaller aperture, so I minimized the aperture to f/20 (focal length 33 mm on a cropped sensor). With an aperture of f/14, nearly identical diffraction stars showed up in the photo. Another precondition is off course the bright light source.



Looking at my photos I noticed that the stars looked different than the ones in the magazine. So now I was wondering: “Why do these diffraction stars show up on my photo?”, and “Why are there different types of stars?”

The diffraction star is caused by the diffracting light when this light passes the aperture blades. As you can see in the photo above, my lens has 6 aperture blades. In case of an uneven number of aperture blades the number of spokes doubles. Shooting at a larger aperture should prevent the stars. It seems to be generally accepted that an aperture larger than f/5.6 will prevent these starts. In the picture below I shot the same scene, but this time with an aperture of f/4.



Understanding when diffraction happens is useful, but the last question is:”Why does it happen?” The problem is that most books and websites discuss the decrease in sharpness at smaller apertures and not the diffraction stars.

There are two types of diffraction a) when light passes the surface between materials with two different densities, and b) when light passes across the edge of a subject. The last type of diffraction causes the stars in the photos. When the light passes the aperture, the light is slightly bend and therefore the arrays of light start to interfere with one another, causing spokes (or reduced sharpness).

I can’t find any information about the reduced ‘sharpness’ of the spokes, but I guess this is caused by the rounded aperture blades. I would have expected more difference between the diffraction stars at the red and green lights of the traffic light, because the average wave length of red is about 30% larger than for green light (701nm vs. 535nm).

The reason for my trip to the aquarium was a fieldtrip from the Meetup group ‘Houston Photography’. The meetup started with an interesting presentation from Dan Zientek at the Hard Rock Café. After the presentation (and us finishing our dinner) we went outside for some night time photography. The other photos I shot were not so successful as the ones above.