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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