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Photographer Star Spikes / Star Flares

An artifact sometimes called a star burst or star filter and sometimes incorrectly identified as lens flare consists of spikes of light that radiate from bright light source(s) as in Photo 1, below. These sometimes pleasing spikes are not that difficult to achieve without using tools, or filters, or image manipulation. Here we explain how to get these potential eye-pleasers, and also how to avoid them.

Photo 1: Diffraction Spikes in a Night Scene. Taken with a 7-blade aperture at f/14

First, let’s call these spikes what they are. The lines radiating from a bright light source – e.g. the streetlamps in Photo 1 – are diffraction spikes. When light encounters an edge it warps around the edge just as a wave in the ocean can flow around a boat. Where are these edges in my camera? Should I fear for my safety? The sharp edges are inside your lens! The mechanism, sometimes called an iris, that controls your aperture is made from a set of 5 to 15 blades that open and close to change the size of the opening in the lens. This opening controls how much light is allowed to pass through your lens onto to your sensor or film. Usually photographers refer to this opening as their f/stop.

f-stops or f-numbers refer to the size of the opening in the iris as a proportion of the lens diameter and focal length. It is not all that important to know – or even understand what this means, but if you want to dig in deeply, I suggest Matthew Cole’s “A Tedious Explanation of the f/stop“.

If you, like me, are wondering how reflection, refraction and diffraction are related – here is a succinct definition from PhysicsClassroom:

Reflection involves a change in direction of waves when they bounce off a barrier; refraction of waves involves a change in the direction of waves as they pass from one medium to another; and diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path.

So in short, refraction is what your lens is designed to do: pull in light and focus it using multiple lenses. Internal reflection causes flare and is mostly undesirable. Diffraction is also an unintended consequence of lens design – unless you want that star burst. When “stopped down” (larger f/stop numbers) diffraction produces noticeable spikes. To make spikes more prominent increase the f/stop to f/11 or higher. How much depends on the construction of the lens.

The number of spikes created is unique to each lens and depends on the number and shape of the blades in the lens. Spikes always appear in pairs. An even number of blades produces an equal number of spikes. In Photo 2 below you can count 8 spikes. Many lenses have six blades and thus produce six spikes. Lenses with an odd number of blades produce twice as many spikes as blades – so Photo 1 may have been taken with a lens having 14 blades (unlikely), or a lens with 7 blades which is correct for the the Canon 16-35mm 2.8 L II lens.

Of course you can also buy star filters, if you wish. But stopping down is sufficient. There is one more simple way to produce diffraction spikes: place tiny dark threads (or hairs) over your lens. Two threads at right angles will produce 4 spikes.

Note that when you “stop down” (use large f/numbers) the dirt and dust on your sensor will become more apparent as small dark dots or lines. It is also true that the stronger the diffraction the less sharp your image will be overall.

Photo 2: Diffraction spikes from a lighthouse. Notice how many spikes?

What if you do not want those spikes? Answer: keep the aperture as open (wide) as possible. Photo 3, shot at f/5.6, shows almost no diffraction spikes. Do not be confused by the radiating beams from the top of the lighthouse. Those beams are from the Fresnel lens in the Pigeon Point Lighthouse which throws out focused light in 24 directions simultaneously. Notice in Photo 3 that the moon has some vague spikes but the bright lights in the windows and doors show almost no sign of diffraction spikes unlike Photo 2.

Photo 3: This f/5.6 photo reveals no noticeable spikes from the windows and the moon has barely noticeable spikes. The spikes from the lighthouse are from the 24-beam Fresnel lens – not diffraction.

So to recap:

The starburst refraction off the lens blades. if there are 8 blades there are 8 rays. The only way to change the number of rays is by changing the number of blades, i.e., a different lens.

The intensity of the burst can be changed by increasing by increasing the f stop. Lower focal lengths at the same F-Stop creates smaller openings, so you should see more starburst intensity at shorter lengths. The greater intensity may cause the appearance of more starbursts.

1) photo taken with higher (f18) than lower (f2.8) f stop that causes the 2 bright lights to have starburst.
2)a) change lense – this different lense must have a different number of blades that create the aperature size than the first lense – more blades more rays – fewer blades fewer rays
2)b) one could put a starburst filter on lense

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