Stupid question
- This topic has 5 replies, 4 voices, and was last updated Nov 4, 2009 at 9:15 pm by
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Why does the light spot on this image look like a star. I don’t notice it on some of my other lenses as much as I do on this one.
http://www.trophyfishingtn.com/coppermine/albums/userpics/DSC02873.JPG
The “starburst” effect is created by an interference pattern due to diffraction. The light coming into the lens diffracts off the edge of the shutter leaves. Amplitudes of diffracted light which are in phase add together and show up as the bright spokes of the starburst. Amplitudes of diffracted light which are out of phase cancel each other out and this creates the space between the spokes of the starburst.
It does not only have to be diaphragm blades – example: when you clean your lens in “circular”, you may create streaks. These “streaks” would act like a “starburst” filter. Anything that causes diffraction will cause the starburst effect – this includes dust particles, diaphragm blades, shutter blades (leaf shutters), cleaning streaks, glass imperfections etc.. Light spreads out (diffracts) perpendicular to a surface, with star spikes at right angles to flat/straight edges.
So (if you really care) a 5-blade iris will give a 10-point sun star, a symmetical 6 blade iris a 6-point star (because opposite edges are parallel, their points overlap), 7-blade iris a 14-point star, etc…
Since you have noticed the unique starburst pattern in a particular lens we should look at the pattern. Below is a blown up view of your starburst pattern. I count 14 starbursts . . . therefore, without knowing your particular lens, it is likely a 7-blade setup. Furthermore, it is probably different than your other lenses and that is why you are noticing the unique starburst pattern? That’s my bet anyway.
Very interesting Neal thanks for taking the time to explain. The lens is a Carl Zeiss 16-80 and yes it has 7 aperture blades. It’s been my favorite walk around lens so I do use it more then most of my other lenses. Now I’m curious to see what the 70-400 g reflection looks like.
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