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How do lens flares really work.


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#1 Albion Hockney

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Posted 15 February 2015 - 10:47 AM

I understand the basic way lens flares are created.... bright source shinning toward the lens causes internal reflections of elements or can cause a "flashing look" where everything is washed out. I have a couple questions about what the specifics though I have never been able to really nail down.

 

 

Difference between the "washed out" look and the "internal reflection colored flares" ....What causes each of these. Is a sharper point source create the reflections vs a softer light for the hazy washed out effect? does it have to with the angle of the light source?

 

 

Opening up a lens..... how do flare charectoristics change in general with aperature changes. does a wide open lens flare easier?

 

 

Playing around on set I generally get to a point I'm happy with when I need to flare up a lens in someway (usually just looking for a soft hazy look) but it always takes sometime and I have never been able to really nail down the causes of each effect.

 

thoughts?


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#2 David Mullen ASC

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Posted 15 February 2015 - 11:56 AM

Focal length also has an effect on the flares. Internal coatings will color the flare even while reducing some of the flare.
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#3 Dennis Couzin

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Posted 16 February 2015 - 11:31 PM

Veiling glare and ghosts.  Lens surfaces reflect specularly.  The edges of lenses, and the interior of the lens housing, and scratches and dirt on the lens surfaces reflect diffusely.  Those diffuse reflections can't make ghosts, only veiling glare.  But the specular reflections can produce both veiling glare and ghosts.  What makes one rather than the other is a matter of focus. 

 

Light enters the lens from a small light source.  It could be within the picture area or not.  The main effects are the results of two specular reflections.  The first reflection surface can be any except the front surface.  The second reflection surface must be scenewards of the first reflection surface.  If the lens has N air-to-glass surfaces, there are then N(N-1)/2 different reflection scenarios.  They all occur.  The surfaces are curved so for each scenario there's a different virtual image.  The lens elements filmwards of the second reflection surface focus do or don't focus it onto the film.  The very much unfocused scenarios contribute veiling glare, while the well-focused ones make ghosts. 

 

So the total light in the veiling glare plus the ghosts is pretty much determined by the number N and the quality anti-reflection coating.  The proportion of the total light in veiling glare vs. ghosts depends on the particulars of the design.  I don't know if there are simple indicators.  There's a 1980 paper by T. Kojima et al., "Computer simulation of ghost images in photographic objectives", Proc. SPIE, 237, 504, which should answer that.


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#4 Albion Hockney

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Posted 17 February 2015 - 10:18 AM

Thanks Dennis! this is the kinda information I was looking for ....and David yes tottaly forgot about focal lengths obviously has a big effect.

 

I really want to get a better understanding of how this works as to obtain better control on set and maybe do some tricks!

 

So basically the amount of elements a lens has is going to add potentional flare scenarios + how good that coating is ....but I wonder if there are some generalities that can be made about the angles that most lenses flare at and the best way to create the flashing "veiling" effect.

 

Dennis can you explain what you mean about ghosts? are you talking about the direct colored circular flares you get? or are you talking about something else.

 

 

I will look up that paper!


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#5 Dennis Couzin

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Posted 17 February 2015 - 03:44 PM

Albert, yes, ghosts is the standard optics term for what you're describing as "direct colored circular flares". 

 

As David Mullen explained, the colors are due to the coatings.  A coating can be very neutral transmission-wise, e.g., 99.9% red, 99.8% green, 99.9% blue, but this means the reflection is 1 unit red, 2 units green, 1 unit blue, quite colored.  And since there must be two reflections to produce a ghost it is, in the example, 1 unit red, 4 units green, 1 unit blue, strongly colored.  Since ghosts were a much larger problem before coatings, color does not figure in the definition of ghost, nor in their analysis.

 

I don't know if focal length has an especial effect.  A cemented doublet makes a fair telephoto lens.  N=2.  So just one ghost-producing scenario which you can calculate from basic geometric optics.


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