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.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.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.
The Contest Results
Here was the two-part question we asked in the Trivia Contest.
- What caused the starburst effect seen (in photo 2)?
- Without resorting to photo editing is it possible to get a different number of spikes? If so, how?
More than seven contestants provided answers to the trivia challenge. We scored each answer as follows: Full credit was awarded if both questions were answered completely. a score of 50% was achieved by clearly identifying stopping down and diffraction as the cause of the starburst effect. An additional 50% was awarded if the answer mentioned changing lenses having a different blade configuration as the means for changing the number of spikes. Partial answers got partial credit, so for example one answer to “How do you get a different number of spikes” was “stop down further” – that scored zero points – stopping down further may make the spikes more pronounced but does not change the number of spikes. The answer “The number of spikes is based on the number of blades in the lens” we scored at full credit even though it doesn’t mention swapping lenses as we felt that switching lenses was implied. Final scores ranged from 40% to 90% correct. For the purpose of this trivia contest any score over 65% was deemed correct.
Those answers scoring 66% or higher in the order they answered were:
- Brian Wong [2011/03/29 at 12:16 pm (emailed to me at 8:15 am)]
- Jack Tasoff [2011/03/29 at 12:59 pm]
- Deborah Zajac [2011/04/05 at 6:34 pm]
Congratulations to Brian who is signed up for the November Star Circle Workshop in Lone Pine, CA. If Jack or Deborah attend they will receive a $25 rebate – and that is in addition to the current early sign up discount.
Discount registration expires on April 30, 2011 – Sign up soon to save yourself $100.