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[OM] Re: focus and ? about lens filters

Subject: [OM] Re: focus and ? about lens filters
From: Roger Wesson <roger@xxxxxxxxxxxxxxxxxxx>
Date: Wed, 09 Jan 2008 17:41:17 +0000
I was looking so hard for minor differences in the contrast that I 
didn't see the dark shadow at all until you pointed it out, Amin.  I'm 
reminded of the fantastic Ig Nobel prize-winning experiment which showed 
that a gorilla walking slowly across the field of vision is normally not 
seen by people asked to look for something different:

http://www-psychology.concordia.ca/fac/deAlmeida/PSYC352/Pages/Simons_Chabris_1999.pdf

But anyway, I'd be surprised if the shadow was the tape on the lens.  It 
wouldn't normally show up like that - depth of field would blur it out 
so much you wouldn't see it.  It's similar to the secondary mirrors of 
reflecting telescopes (or mirror lenses), which can obstruct 10-25% of 
the aperture and yet don't show up as a dark patch in the middle of the 
field.  It's normally reckoned that if less than about 20% of the 
aperture of a lens or telescope is obscured, then the effect on image 
quality is negligible.  This page has a fairly technical analysis in 
case anyone's interested:

http://www.telescope-optics.net/obstruction.htm

Roger

Chuck Norcutt wrote:
> Interesting.  I don't know how I could have missed that before other 
> than my general tendency to sometimes be wholly unobservant of the most 
> obvious things (my wife will tell you).  But I'm not yet convinced 
> because I know of no optical phenomenon of the "crippled" lens which 
> should cause what looks like a shadow cast on the flower.  But that 
> doesn't mean there isn't one.  I just don't know about it.
> 
> These were taken by natural light with about 10 minutes in between so 
> it's possible something is casting a shadow that wasn't before.
> 
> Chuck Norcutt
> 
> Amin Sabet wrote:
>> Even in the thumbnail versions, it seems clear to me that the first
>> one (the one on the left) is the one with the black tape.  The 100%
>> crop avoids the most affected area, which is a a sizeable dark area a
>> bit more than 2/3 up and just left of center.  -Amin Sabet
>>
>> On Jan 8, 2008 8:45 PM, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
>>> Ah, yes, the giant dust spec test.
>>> <http://www.chucknorcutt.com/dust%20spec%20test/index.htm>
>>> Both of these shots were taken with an A1 which has a front element
>>> approximately the size of a Zuiko 50/1.8.  One shot was taken normally.
>>>    The other is the same except that there is a piece of black
>>> electrical tape approximately 3/4" square (19mm) stuck on the front
>>> element.  Since the front element is about 40mm in diameter the tape was
>>> blocking about 29% of the glass area.
>>>
>>> The third shot is a comparison of the 1:1 pixel level detail from both
>>> images.  Can you tell which one is which?\

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