Stars in my lens

June 9, 2009 7:11 AM

When a camera is focused on a bright light, there is a "star" flare around that light. I've noticed that in different situations, that flare has a varying number of points... What decides how many point the lens flare has? BONUS QUESTION: I've only ever seen even point counts... Can odd happen?
posted by frwagon to grab bag (8 comments total)

It depends on the number of aperture blades the lense incorporates.
posted by sanka at 7:11 AM on June 9

Some reading about lens flare.
posted by wg at 7:16 AM on June 9

Yep, aperture blades. And there's always an even number because each blade's midpoint makes a point, and the corners between two blades makes a point.
posted by notsnot at 7:21 AM on June 9

previously
posted by scruss at 7:45 AM on June 9

There are in fact lenses with odd numbers of aperture blades. Some SLR lenses have 7- or 9-blade apertures.
posted by kindall at 7:58 AM on June 9

Yes, but as notsnot points out, each blade creates two points. So there can only be an even number of points.
posted by HFSH at 8:37 AM on June 9

Each blade creates two points? Huh? Bokeh can be any shape. Check out the Lensbabygallery under the creative aperture kit.

I'd deep link to the 5 point star orchid picture but web 2.0 doesn't like that.
posted by chairface at 11:47 AM on June 9

Yep, aperture blades. And there's always an even number because each blade's midpoint makes a point, and the corners between two blades makes a point.

notsnot, close, but no cigar. The blades are the cause, but the doubling effect is not due to "corners + midpoints". Rather, the stars are diffraction effects from the symmetry of the blades, and diffraction causes the doubling.

There's a +1 diffraction order, and a -1 diffraction order, for every blade. The two are indistinguishable (the polarity is arbitrary, essentially), so the apparent effect is a doubling of the diffraction source "number of features".

Sorry if my techno-speak is kinda stilted today...
posted by IAmBroom at 12:05 PM on June 9