What creates stars around lights??? Advice please.

[masshuu]

as Lou ann pointed out in teh comments to the blog post twilight tips comments you get star bursts from lights. why and how does this happen?

here is an example of a photo i took a few weeks ago:



Both the car's headlamps and the street lamp above have star effect.

Why is this???

[Jamesc359]

It's caused by diffraction. Light is entering your camera and striking the edges of your diaphragm (aperture) blades. Some of that light gets diffracted off the blade's edge and continues on to the sensor. Imagine how a ball will bounce off the edge of something. Depending upon where it strikes along the edge, it'll be deflected at a different angle.

You can actually tell how many blades a lenses diaphragm has by counting the number of light streams produced. An odd number of blades produce 2x the number of light streams, while ones with an even number of blades will produce the same amount of streams. This is because with an even number of blades the light from the opposite blade is overlapping, where there is no blade on the opposite side of an odd numbered system. So your lens either has 7 or 14 blades. I'm leaning towards 7 for some funny reason.

If you don't want this effect to be as pronounced, opening your aperture up to it's largest possible size will decrease the effect. Selecting a smaller aperture will of course do the opposite and increase the effect.

[wulf]

Further to that, the effect is more pronounced as you stop down to smaller apertures. You are unlikely to see much diffraction at f/5.6 but go down to f/22 (and compensate with a slower shutter speed) and you should reliably create the effect, certainly at night when there are a few bright light sources.

Wulf

[Nicole]

Though the 50mm f/1.8 really does produce fairly pronounced starbursts even at f/4.5



The starbursts are even more noticeable on the print I have of this.

[masshuu]

excellent guys, thanks for that. this shot was taken at f5.0 so now i know and can play some more.

very helpful

[wulf]

That is interesting. I don't remember getting a lot of starbursts with wider apertures so you have prompted me to go back to a shot that I took with what I thought I knew in mind:



Unfortunately, I didn't record the exact aperture for that shot (manual lens) but it was small (at least f/16) because I thought that was the key factor. I wonder if, instead, the rule is that bright lights in reasonable focus form starbursts? You can see a prominent one where I was focusing and hints of a starburst on another further back and near the right of the picture. The relatively dim lights of the Christmas tree and the lights that are well outside the sharp zone are not affected.

If that is the case, then perhaps the small aperture theory appears to work because there is more chance of catching a strong light in focus rather than at a point where "bokeh" steps in and rubs out the star lines?

Wulf

[Jamesc359]

I think it has more to do with a lights intensity than it's being in focus. If you look closing at your image, there's a light to the left of the Christmas tree that's out of focus, but still has fairly strong points on it's start burst.

The rest of the lights in the background aren't really all that bright because they're either very dim (because of distance and amount of light emitted) or being directed away from the camera and towards the building. They may still be blown out, but the intensity just isn't the same.

Also, if you crank up the gamma, you'll see that the Christmas tree lights do produce star bursts as well. They're just so subtle that your eye can't see them without enhancement.

[silentnight_913]

You can ensure points of light have this effect by using a cross screen/star filter.

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