Photo Commentary #27: Starbursts and Fraunhofer Diffraction

Photo Commentary #27: Starbursts and Fraunhofer Diffraction

Photo Commentary #27: Starbursts and Fraunhofer Diffraction 1024 683 varunvisuals
Starbursts in Nightime Urban Scene

This gorgeous photo taken by Xerwyn Flores serves as a great example of starbursts in an urban environment. Notice that the lights further back in the photo (and thus less in focus) have weaker starbursts.

One of the most common characteristics you will notice in urban photographs is a lighting phenomenon colloquially known as “starbursts”. This, of course, is in reference to the the star like appearances of lights within an image due to the outward direction of their rays. However, the true explanation of this effect is a little bit more complicated – here goes nothing!

Before I explain the effect in the world of photography, we have to first take a slight detour through the world of physics. The “starburst” effect you notice in many photographs is due to a concept known as the Fraunhofer Diffraction. Fraunhofer Diffraction refers to the phenomenon by which light waves behave when travelling from an infinite distance through a slit or narrow opening. The reason we use infinite as the ideal distance is because it theoretically results in straight parallel light rays which cumulatively create what is known as a plane wave.

Fig 1.) Think of the plane waves as a 3D picture of the "incoming waves" in fig 4. A billion parallel incoming waves theoretically create a "plane". The arrow would point to the slit and screen.
Fig 1.) Think of the plane waves as a 3D picture of the “incoming waves” in fig 4. A billion parallel incoming waves theoretically create a “plane”. The arrow would point to the slit and screen.

In summary:

1.) Infinite distance between light source and slit produces parallel waves.
2.) Parallel waves cumulatively form a plane wave.
3.) All the individual parallel waves within the plane wave enter the slit or aperture at the same phase

Phase Changes and Velocity Measurement. Fig 2.) Phase refers to a point on the light's wavelength
Fig 2.) Phase refers to a point on the light’s wavelength

4.) Fraunhofer Diffraction occurs and light rays meet at a common point (light is focused)

Fig 3.) Light rays diffract and meet at a common point, creating a focused image
Fig 3.) Light rays diffract and meet at a common point, creating a focused image


5.) If light is focused, we can then assume an infinite distance to the light source

Fig 4.) A 2D depiction of parallel light waves entering a slit to create Fraunhofer diffraction. The first figure is a 3D view of the same incoming waves.
Fig 4.) A 2D depiction of parallel light waves entering a slit to create Fraunhofer diffraction. The first figure is a 3D view of the same incoming waves.

When the incoming light wave passes through the slit or opening, some of the rays diffract or bend according to several variables including: size of the hole, shape of the hole, and the distance between the screen, slit, and light source.

Now back to photography. In the beautiful image of the city skyline, we will take a look at the first street lamp on the top right corner – this will be the source of our light. The diaphragm or aperture of the camera lens will be the “slit”, and the light sensor of the camera will be the “screen” (or in other words, the “photo” we see). Let’s start with the light source.

The light source has an impact on the diffraction produced due to its distance from our camera lens as well as its other properties such as wavelength (color) and intensity. We can assume the distance from the lamp to our camera is infinity. Reason being, the street light is in focus. If you look at lights in the background or in the buildings, they have reduced focus, hence less diffraction and weaker starbursts occur.

The aperture of the camera is the slit in the diagram. The narrower the slit or aperture, the more diffraction occurs, and thus, the greater the diffraction. In other words, f/16 would create MORE diffraction than f/11. However, things don’t end there. Another aspect of the aperture is its actual shape – this is where the magic happens.

Fig 5.) As the blades of the camera's shutter increases in number, the more circular the aperture's opening becomes and less diffraction occurs
Fig 5.) As the blades of the camera’s shutter increases in number, the more circular the aperture’s opening becomes and less diffraction occurs

Camera apertures are not perfectly circular. Rather, they are comprised of numerous blades to form various shapes, depending on the number of blades. The more circular the hole, the less the diffraction. This also means, the more the blades in your camera’s aperture, the less diffraction you will see. The shape of your camera’s aperture is also the reason the starbursts get their shape. Fun fact – if your camera has an even number of aperture blades, that will be the number of points in the starbursts you create. If it has an odd number of blades however, the starburst will have twice as many tips. In the main photo, I counted an average of 14 rays in the starburst. This could mean one of two things – the camera has an aperture with either 7 blades, or 14 blades.

Fig 6.) In order to theoretically achieve any Fraunhofer Diffraction, the equation above must be less than one. However, to be seen, the equation must be much smaller than 1. 
W = aperture size
L = Distance from light source to aperture
λ = Wavelength of light
Fig 6.) In order to theoretically achieve any Fraunhofer Diffraction, the equation above must be less than one. However, to be seen, the equation must be much smaller than 1.
W = aperture size
L = Distance from light source to aperture
λ = Wavelength of light


So all in all – if you want an epic starburst, get a camera with 100 aperture blades, get a light source in focus, and make sure it is red in color (red has the longest wavelength)!

Oh and big thanks to Xerwyn Flores for taking this beautiful photo!

Photographer: Xerwyn Flores
https://www.eyeem.com/u/xeriez

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