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What causes this starbust effect at night?

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Thread started 21 Dec 2008 (Sunday) 13:12   
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curiousgeorge
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Is it a characteristic of the lens? I never had it with my 17-40.

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Post #1, Dec 21, 2008 13:12:41


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ed ­ rader
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curiousgeorge wrote in post #6920980external link
Is it a characteristic of the lens? I never had it with my 17-40.

stopping down and long exposure have always given me the starburst effect with any lens i have used.

ed rader

Post #2, Dec 21, 2008 13:18:14


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sugarzebra
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as Ed says the starburst effect is a function of the aperture more than the lens. The smaller the aperture (i.e. f/22) the more pronounced the effect.

Post #3, Dec 21, 2008 13:20:21


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curiousgeorge
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Thanks. This was at f14 but I was also getting them at f10.

Post #4, Dec 21, 2008 13:29:44


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ed ­ rader
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curiousgeorge wrote in post #6921038external link
Thanks. This was at f14 but I was also getting them at f10.


me too :D.

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Post #5, Dec 21, 2008 13:35:55


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curiousgeorge
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Quite nice aren't they!

Post #6, Dec 21, 2008 13:50:12


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ed ­ rader
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curiousgeorge wrote in post #6921113external link
Quite nice aren't they!

oh yeah and they were free ;).

ed rader

Post #7, Dec 21, 2008 13:51:42


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KarlosDaJackal
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if your aperture has an even number of blades, say 8 you should get 8 points, if it has an odd number of blades like 9 usually you will get double that number of points so 18.

edit, so ed is using some lens with 7 blade aperture for his shots ;)

Post #8, Dec 21, 2008 14:00:57 as a reply to ed rader's post 9 minutes earlier.


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Jpatten
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Why Double on Odd numbered apertures?

Post #9, Dec 21, 2008 14:19:42


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Eyelikedurt
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Im sure theres more science to it than that. Why wouldn't it work on the moon in that last shot? I reckon it has something to do with wavelengths of light. Maybe the fact the lights are man made. Or some other kind of juju!

Post #10, Dec 21, 2008 14:22:31


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Jon
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I believe it's because you get one ray from each blade from each side of the light source. With an even number of blades (let's use 8, numbered 1-8) the rays from blades 1 and 5 would be on top of each other. With 7 blades, the rays from blade 1 would be between the rays for blades 4 and 5. It's essentially a diffraction effect off the edge of the diaphragm. The shorter each edge of the diaphragm opening is, the stronger the effect will be.

Eyelikedurt wrote in post #6921256external link
Im sure theres more science to it than that. Why wouldn't it work on the moon in that last shot? I reckon it has something to do with wavelengths of light. Maybe the fact the lights are man made. Or some other kind of juju!

The moon's not all that strong a light source in this instance; it's merely a diffuse reflector of sunlight back at us. You're seeing the starbursts off of emissive light sources, which are bright enough to leave a visible trail of diffracted light. If you overexposed the moon enough, you'd see some off it as well, but the sheer size of the subject would make it less pronounced than a point source of the same brightness, let alone a much brighter source.

Post #11, Dec 21, 2008 14:28:36


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Madweasel
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The reason it is exacerbated at small apertures is that the inside corners of the shape of the aperture are sharper. You'll not get it at all at full aperture because the opening is circular. It becomes less circular in practically all lenses as you stop down. As others have said it is a diffraction effect. As Jon said in his answer about the moon, it is more pronounced where the light source is pointlike.

Post #12, Dec 21, 2008 15:00:50


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Eyelikedurt
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Jon wrote in post #6921282external link
I believe it's because you get one ray from each blade from each side of the light source. With an even number of blades (let's use 8, numbered 1-8) the rays from blades 1 and 5 would be on top of each other. With 7 blades, the rays from blade 1 would be between the rays for blades 4 and 5. It's essentially a diffraction effect off the edge of the diaphragm. The shorter each edge of the diaphragm opening is, the stronger the effect will be.

The moon's not all that strong a light source in this instance; it's merely a diffuse reflector of sunlight back at us. You're seeing the starbursts off of emissive light sources, which are bright enough to leave a visible trail of diffracted light. If you overexposed the moon enough, you'd see some off it as well, but the sheer size of the subject would make it less pronounced than a point source of the same brightness, let alone a much brighter source.

see? science!

Post #13, Dec 21, 2008 15:05:15


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donaldjl
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Eyelikedurt wrote in post #6921448external link
see? science!

Hah! This thread is now Bill Nye Approved! :D

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Post #14, Dec 21, 2008 15:57:12


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Jpatten
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That makes sense

Post #15, Dec 21, 2008 21:45:57


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