How to Create (and Avoid) Starburst Highlights

We’ve all experienced it.  You snap a perfect picture of the setting Sun just as it drops over the horizon.  As you glance at the result though, the perfect orange ball has turned into a huge starburst pattern.  It doesn’t ruin the picture.  In fact, it adds some artistic flare.  But is a curious phenomena.  Why does it happen?  Read on to find out.
An adjustable aperture is a piece of mechanical magic.  In mere thousandths of a second, the razor thin blades snap shut to just the right dimension for your exposure.  Then they spring back to their default (wide open) position.  Ideally, apertures close down to perfectly circular holes.  Based on the number and shape of the blades, though, the actual aperture opening can end up looking more like a polygon.  This effect is accentuated as the aperture closes down to its smallest settings.  At small apertures (high f-stops), the individual blades form hard edges around the aperture opening.
Apertures do a great job of funneling light through a precisely sized hole, but light is particularly difficult to harness.  Because of its wave nature, light has a way of bending around the hard edges that attempt to contain it.  When apertures close down and form hard edges, diffraction waves from each aperture blade edge can combine to form diffraction patterns.  The optical result we see is a starburst pattern.
Although diffraction is present in all circumstances, it is particularly apparent for intensely bright light sources, such as the setting Sun or a Christmas tree light. The best way to visualize the diffraction effect is to use a diffraction simulator.  For an aperture with six blades, the aperture shape will be a hexagon.  The resultant diffraction pattern will then resemble a “starburst” pattern with six points.  For each of the edges of the hexagon, the diffracted light will combine and cancel to produce the effect.
For all apertures with even numbers of blades, the effect will be similar (n blades = n points on the starburst).  For odd numbers of aperture blades, the effect is slightly different.  Unlike the even numbered case, apertures with odd numbers of blades are not symmetrical.  Since opposite sides of the aperture do not line up,  the diffraction pattern will have twice the number of points as blades.  It is this ability to produce such dramatic starbursts that drives demand for apertures with odd numbers of blades.
What conditions are necessary for starbursts?
*  Small aperture (high f-stop) – small aperture holes are harder to form into perfect circles with aperture blades.  Circular apertures begin to take more polygonal shapes at f/8 and higher.  Also, diffraction is accentuated by small aperture holes since a greater percentage of the passing light is affected by the diffraction edges.
*  An intensely bright and stationary light in the field of view – the setting Sun, a Christmas tree light or a lamp post will do nicely.  A flickering candle, however, will not produce a starburst since the movement of the flame will result in a blur.
How do you avoid starbursts?
*  Use a larger aperture (low f-stop value) – Wide open apertures are much more circular and unlikely to produce starburst diffraction patterns.  Unfortunately for landscape photographers, this is rarely an option since a higher f-stop is required for proper depth of field.
*  Place the bright object in a bright background. This will cause the starburst diffraction to be washed out by the background.
The better question may be why would you want to avoid them.  Used correctly, starbursts can add a dramatic artistic effect to an image.  Go ahead.  Close down that aperture and have at it.

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