# The physics behind the "starburst" effect

Discussion in 'Medium Format' started by graham_wootton, Jan 24, 2002.

1. ### graham_wootton

Does anyone know the precise physical explanation for the "starburst"
effect you get when shooting into the sun (or other very bright light
source) at small apertures? The effect is definitely related to the
diaphragm blades as the points of the star are always equal to or a
multiple of the number of blades. I believe it is due to diffraction
but cannot envisage how the spread of light becomes so wide. Some
writers have attributed the effect to reflection off the edges of the
diaphragm blades but I cannot envisage how that would work either.
Any ideas?

2. ### scott bacon

I believe 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 has been a while since my college physics curriculum, so maybe someone better or more recently educated can provide a more precise answer.

3. ### alex_tsow

I believe this effect is a result of light passing through the film and reflecting off of the back plate of the camera. The diaphragm blades could credibly determine the number of spokes. I think the "anti-halation" layer of film is intended to minimize this effect.

4. ### vijay_nebhrajani

It does not only have to be diaphragm blades - consider: when you clean your lens in the much advertised "circular" motion, you may leave very fine (for lack of a better word) streaks. These "streaks" would act like a "starburst" filter. Eyeglass wearers whose lenses are not absolutely clean will be familiar with the effect.

I think that anything that causes diffraction will cause the starburst effect - this includes dust particles, diaphragm blades, shutter blades (leaf shutters), cleaning streaks, glass imperfections etc.

But yes, it is caused because of interference.

5. ### jay_piper

Folks describing interference at edge of aperture are right, Light spreads out (diffracts) perpendicular to the edges of the blades, with star spikes at right angles to flat/straight edges of the aperture opening.

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...And this effect may be less pronounced/more diffuse with rounded iris touted on some newer lenses

Yes yes... very interesting indeed! However i must say you all
forgot to mention the appenditure recoliation affect...

Very minute in its ways as a simple foliation synthesis but still it
is the more present affect of the statbust. I suggest to fix this you
simply attach an EGM-Spec9 focus scope to your camera and it
will reprecate all the problems for you ...

So, for those of us trying to avoid this phenomenon; it's caused by the leaves of the shutter and not the leaves of the aperture, correct? Meaning that a focal plane shutter camera will not have starbursts and a 5 bladed leaf shutter, for example will, yes?

8. ### dave_s

Not correct. It's a result of the small aperture. Images taken with a focal plane shutter commonly show this effect.
A question this thread made me think of-- what happens when you have a completely circular stop instead of a bladed iris diaphragm? I guess it would be what Jay Piper said-- a diffuse glow around the highlights, because the circular aperture is like an infinite number of (infinitely small) aperture blades. This would only come up with really old lenses with Waterhouse stops, or certain process lenses, but it's an interesting 'thought experiment'.

9. ### q.g._de_bakker

This would only come up with really old lenses with Waterhouse stops, or certain process lenses, but it's an interesting 'thought experiment'.​
Or with just about any lens used wide open.
;-)

10. ### dave_s

Or with just about any lens used wide open.

But the 'starbust' effect this thread is only visible at small apertures, due to diffraction. Am I missing something?

Not correct. It's a result of the small aperture. Images taken with a focal plane shutter commonly show this effect.​
This is true with focal plane shutter cameras at small apertures, but not large ones. However, it happens to me at all apertures, not just the small ones, with my Hasselblad leaf shutter lens.
http://www.photo.net/medium-format-photography-forum/00XEIt

Therefore my assumption is, I need a focal plane shutter camera with a large aperture to avoid this effect, correct?

12. ### q.g._de_bakker

But the 'starbust' effect this thread is only visible at small apertures, due to diffraction. Am I missing something?​
A combination of the following:
a diffuse glow around the highlights, because the circular aperture is like an infinite number of (infinitely small) aperture blades. This would only come up with really old lenses with Waterhouse stops,
Or with just about any lens used wide open.
it happens to me at all apertures, not just the small ones,​