Archived posting to the Leica Users Group, 2001/02/01
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At 11:41 AM -0500 2/1/01, Austin Franklin wrote:
>Why do you get double the number of points when you have an odd number of
>blades?
>
>> -----Original Message-----
>> From: owner-leica-users@mejac.palo-alto.ca.us
>> [mailto:owner-leica-users@mejac.palo-alto.ca.us]On Behalf Of Vozeh,
>> Colin
>> Sent: Thursday, February 01, 2001 11:19 AM
>> To: 'leica-users@mejac.palo-alto.ca.us'
>> Subject: RE: [Leica] star pattern in lights when stopped down...
>>
>>
>> I'll take a guess and say diffraction - light bending around
>> edges. Each
>> straight edge produces another "spike" of light, so the star
>> has as many
>> points as the iris has blades. It's always more pronounced
>> the more you
>> stop down. It happens to all the light rays coming into the
>> lens, but of
>> course the bright sources are what you notice. I guess this
>> is why the
>> greatest sharpness for all lenses typically doesn't occur at small
>> apertures. For what it's worth, to my eye this is one
>> instance where fewer
>> iris blades actually make a nicer image.
>>
>> C.
>>
>>
>> -----Original Message-----
>> From: Austin Franklin [mailto:austin@darkroom.com]
>> Sent: Thursday, February 01, 2001 9:36 AM
>> To: leica-users@mejac.palo-alto.ca.us
>> Subject: [Leica] star pattern in lights when stopped down...
>>
>> What is the optical phenomenon that causes the star patter
>> that is equal to
>> the number of aperture blades, seen when stopped down, in some lenses?
>>
>>
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>>
>>
The effect is diffraction based. It occurs at all times, even with
wider apertures, only the diffraction effect is fairly constant, so
only becomes visible when the rest of the opening providing the image
is small.
Each edge produces a diffraction 'star point' in each direction. When
there is an even number of blades, the opposite blades produce
diffraction 'star points' in the same directions, so you have: two
blades, two 'star points'. When there are and odd number of blades,
each blade will produce two 'star points'; therefore double the
number of 'star points' as blades.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
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