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Question for you optical gurus...

 
 
MarkČ
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      05-07-2007
I know people sometime use "star filters" to create similar effects...
....but what exactly is going on when these bright-light star effects are
created with no such filter?
What causes it?
http://www.pbase.com/image/78346396/original
That is an un-edited image (save for a slight levels adjustment and size
reduction).
I'm assuming the number of points is dictated by the number of aperture
blades(?)...but I don't know.
I don't spend a lot of time number crunching, but I know there are folks
here who know this stuff inside and out (Littlewood? Bill? Roger?).

Just curious... And thanks in advance to anyone who can shed light on this
one (pun intended).

MarkČ

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www.pbase.com/markuson


 
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Roger N. Clark (change username to rnclark)
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      05-07-2007
MarkČ wrote:
> I know people sometime use "star filters" to create similar effects...
> ...but what exactly is going on when these bright-light star effects are
> created with no such filter?
> What causes it?
> http://www.pbase.com/image/78346396/original
> That is an un-edited image (save for a slight levels adjustment and size
> reduction).
> I'm assuming the number of points is dictated by the number of aperture
> blades(?)...but I don't know.
> I don't spend a lot of time number crunching, but I know there are folks
> here who know this stuff inside and out (Littlewood? Bill? Roger?).
>
> Just curious... And thanks in advance to anyone who can shed light on this
> one (pun intended).
>
> MarkČ
>

It is diffraction from the blades of the iris.

Roger
 
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MarkČ
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      05-07-2007
Roger N. Clark (change username to rnclark) wrote:
> MarkČ wrote:
>> I know people sometime use "star filters" to create similar
>> effects... ...but what exactly is going on when these bright-light
>> star effects are created with no such filter?
>> What causes it?
>> http://www.pbase.com/image/78346396/original
>> That is an un-edited image (save for a slight levels adjustment and
>> size reduction).
>> I'm assuming the number of points is dictated by the number of
>> aperture blades(?)...but I don't know.
>> I don't spend a lot of time number crunching, but I know there are
>> folks here who know this stuff inside and out (Littlewood? Bill?
>> Roger?). Just curious... And thanks in advance to anyone who can shed
>> light
>> on this one (pun intended).
>>
>> MarkČ
>>

> It is diffraction from the blades of the iris.
>
> Roger


Thanks. That makes perfect sense, of course.
I don't often shoot night scenes like this...so rarely see it/worry about
it.

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Images (Plus Snaps & Grabs) by MarkČ at:
www.pbase.com/markuson


 
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Paul Furman
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      05-07-2007
MarkČ wrote:
> I know people sometime use "star filters" to create similar effects...
> ...what exactly is going on when these bright-light star effects are
> created with no such filter?
> What causes it?
> http://www.pbase.com/image/78346396/original
> I'm assuming the number of points is dictated by the number of aperture
> blades(?)


Yep, you are stopped down to f/11. It won't do that wide open where the
opening is perfectly round (or with rounded aperture blades?... maybe
not perfectly round?). I'd guess it's an incremental thing, not positive.

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achilleaslazarides@yahoo.co.uk
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      05-07-2007
On May 7, 7:58 am, "MarkČ" <mjmorgan(lowest even number
here)@cox..net> wrote:
> I know people sometime use "star filters" to create similar effects...
> ...but what exactly is going on when these bright-light star effects are
> created with no such filter?
> What causes it?http://www.pbase.com/image/78346396/original
> That is an un-edited image (save for a slight levels adjustment and size
> reduction).


Diffraction:
http://en.wikipedia.org/wiki/Diffraction

 
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TheDave©
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      05-07-2007
> Mark2 wrote:
> I know people sometime use "star filters" to create similar effects...
> ...but what exactly is going on when these bright-light star effects
> are created with no such filter? What causes it?
> http://www.pbase.com/image/78346396/original
> That is an un-edited image (save for a slight levels adjustment and
> size reduction). I'm assuming the number of points is dictated by
> the number of aperture blades(?)...but I don't know. I don't spend a
> lot of time number crunching, but I know there are folks here who
> know this stuff inside and out (Littlewood? Bill? Roger?).
>
> Just curious... And thanks in advance to anyone who can shed light
> on this one (pun intended).
> Mark2


I lost a photo contest once because the judge thought I "cheated" by
using a star filter when I hadn't. Virtually identical type picture as
your example, just different location.
 
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Jeff R.
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      05-07-2007

"Paul Furman" <(E-Mail Removed)> wrote in message
newsHz%h.939$(E-Mail Removed)...
> MarkČ wrote:
>> I know people sometime use "star filters" to create similar effects...
>> ...what exactly is going on when these bright-light star effects are
>> created with no such filter?
>> What causes it?
>> http://www.pbase.com/image/78346396/original
>> I'm assuming the number of points is dictated by the number of aperture
>> blades(?)

>
> Yep, you are stopped down to f/11. It won't do that wide open where the
> opening is perfectly round (or with rounded aperture blades?... maybe not
> perfectly round?). I'd guess it's an incremental thing, not positive.
>



At f/11: http://www.mendosus.com/armidale/uralla.jpg

At f/3.5, wide open: http://www.mendosus.com/armidale/uralla-2.jpg

--
Jeff R.


 
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MarkČ
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      05-07-2007
Jeff R. wrote:

> At f/11: http://www.mendosus.com/armidale/uralla.jpg
>
> At f/3.5, wide open: http://www.mendosus.com/armidale/uralla-2.jpg


I get only "Server Not Found"

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www.pbase.com/markuson


 
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Bill Funk
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      05-07-2007
On Sun, 6 May 2007 20:58:52 -0700, "MarkČ" <mjmorgan(lowest even
number here)@cox..net> wrote:

>I know people sometime use "star filters" to create similar effects...
>...but what exactly is going on when these bright-light star effects are
>created with no such filter?
>What causes it?
>http://www.pbase.com/image/78346396/original
>That is an un-edited image (save for a slight levels adjustment and size
>reduction).
>I'm assuming the number of points is dictated by the number of aperture
>blades(?)...but I don't know.
>I don't spend a lot of time number crunching, but I know there are folks
>here who know this stuff inside and out (Littlewood? Bill? Roger?).
>
>Just curious... And thanks in advance to anyone who can shed light on this
>one (pun intended).
>
>MarkČ


I've noticed the same thing, and since it changes with the aperture
used (the points are stronger with a smaller aperture), I'll have to
go with the idea that it's caused by the aperture blades and the
difraction they cause.
I never checked to see if rounded blades (for a more round aperture at
smaller apertures) lessens the effect, but I would imagine so.

--
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private jet provided for her in South Carolina
and demanded the more luxurious Gulfstream III.
She was on her way to California. She can't be
seen in something the valets are embarrassed
to park.
 
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acl
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      05-07-2007
On May 7, 5:31 pm, Bill Funk <(E-Mail Removed)> wrote:

> >MarkČ

>
> I've noticed the same thing, and since it changes with the aperture
> used (the points are stronger with a smaller aperture), I'll have to
> go with the idea that it's caused by the aperture blades and the
> difraction they cause.
> I never checked to see if rounded blades (for a more round aperture at
> smaller apertures) lessens the effect, but I would imagine so.
>


It's diffraction. Diaphragms with an even number of blades result in a
pattern with the same number of rays, while those with an odd number
result in patterns with twice that number. For diaphragms with more
blades, there are more rays but they get dimmer. It is not hard to
imagine having so many that the end result is a circular aperture, and
what the effect would be...

 
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