India Point Park

India Point Park
by matt-ball                

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See for example this photo:

Example photo

From my experience, the longer the exposure the stronger can this effect be observed. Is this correct? Are there any other factors that influence the creation of these stars (is there a better word for it, by the way?) And what exactly does technically happen?

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Is there a way to avoid this effect with filters or something else? – Luis Carlos Aug 12 '12 at 18:41
@Luis: According to the answers, I suppose you can just open up your aperture. – eWolf Aug 12 '12 at 20:30
up vote 51 down vote accepted

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. When your diaphragm is formed by straight blades, this will cause there to be twice as many streaks as blades. However, the streaks for parallel blades will coincide. Thus, for a diaphragm with an odd number of blades (where no two blades are parallel) there will be twice as many radial streaks as blades but for a diaphragm with an even number of blades (where opposite blades are parallel) the streaks overlap in pairs, giving the same number of streaks as blades (but each streak is twice as bright).

A classic example is shown in the first image in the Wikipedia article on diffraction, for Fraunhofer diffraction through a square aperture. You see four well-defined streaks.

The theory is further explained here. This explanation was published in 1967 by CA Padgham. Ken Rockwell mentions it in his discussion of bokeh.

We should expect a certain amount of diffraction always to be present. It is usually slight and averaged out in most pictures: it just contributes a tiny amount to the blurriness that is present in any image when looked at closely enough. Only in images that bring together several factors--points of intense monochromatic light, small apertures, straight diaphragm blades--will it become prominent. This information shows how you can make the stars more prominent or how you can suppress them, by altering these factors for your exposure (to the extent you can).

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. For instance, there are dimmer but still prominent light sources in your background: they look like windows in the far distance. They, too, must have their own streaks, but those streaks are too dim to see. (Appropriate software filtering might be able to bring them out.)

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This is clearly the most detailed answer. Thank you! – eWolf Jan 9 '11 at 1:34

It's due to diffraction where the aperture blades meet as stated by John and Pearsonartphoto. It's a neat way to test how many aperture blades you have!

To answer your second question, the length of the exposure doesn't directly affect the effect. There are two main factors, the first is the size of the aperture (it needs to be small), and long exposures tend to go with a small aperture. The second factor is you need to be shooting into the lightsource. This tends to only happen at night with artificial light, so again people tend to wind up using long exposures.

Here's an example (not mine!) of the effect with a very short exposure to demonstrate the point:

(c) photogeek133

Ok I lied it was a long exposure with moving flashes set to strobe, but each light was on for a very brief period. The other two ingredients - shooting into the strobes, and small aperture (f/14) are what is producing the star patterns.

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That certainly is an impressive photo! I've heard the term diffraction before. It was mentioned as a problem - does (and how does) diffraction appear anywhere besides from shots like these (directly into the light source)? From my understanding, it shouldn't be a problem usually. – eWolf Jan 8 '11 at 16:45
In simple terms (see whuber's answer for a detailed analysis!) diffraction causes light to spread out. This is obviously a problem if all points of light spread out, as this will give a blurry image. Diffraction happens all the time, it's just the spreading is not noticeable for big gaps or dim lights. What we have here is a very small gap, and a bright source, so the star pattern which would usually be too dim is clearly visible. – Matt Grum Jan 8 '11 at 17:06

What you are seeing is the result of the shape of the aperture in your camera. If you put, say, a heart shape or other "filter" on the front of your camera, you'd see a different shape in place of those lights.

You're almost correct in your guess that the longer the exposure, the stronger this affect can be observed. What's actually happening is the smaller your aperture, the more this affect will appear.

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There are filters, starlight, that are designed to do this, but without a filter, the effect is usually seen with tighter apertures on lenses that have straighter aperture blades. The straighter the blades, the more pronounced the effect.

So, what is happening is these bright and stationary light sources are having their light bent by the aperture of your lens and the star pattern is being created by the sharp points being defined by the hexagon from the six blades of your aperture. You'll notice that the star rays are all going in the same direction for the lights, that's because of the aperture blades.

By the way, I like the shot.

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LOL, I answered at almost the same time, so... – PearsonArtPhoto Jan 8 '11 at 16:02

Why do light sources appear as stars sometimes? Well, I changed my opinion and share now the prevalent one that the stars come from diffraction effects. The strongest argument for favoring diffraction over reflection comes from the symmetry properties of the star pattern, namely, if N is odd, then N iris blades generate 2*N spikes.

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I believe that you will find the answer to your questions under

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Stephan, we'd like answers to the questions rather than pointers to other places people can go research. It looks to be a helpful link, but could you summarise what it says here, as it applies to the question? – MikeW Aug 10 '14 at 18:52

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