Indeed. Based on the known performance of mirror lenses, some of which
have a greater obstruction than my piece of tape, I knew that it would
be hard to tell the difference between shots with and without the tape.
The only effect of the tape is to block some light and introduce some
more diffracting edges to add to the existing diffracting edges which
are the edges of the lens cells and aperture blades. The end result is
a slight loss of contrast due to increase diffraction... common in very
fast mirror lenses since they have very large obstructions.
I've lost track of why I did this originally but I think it was to show
that a lens with big scratches or a big chip out of the front element
could still perform very well if the damaged area is filled in with
India ink to prevent light transmission through the damaged area.
Chuck Norcutt
C.H.Ling wrote:
> A black tape is totally different from dust, if you have hundreds of tiny
> pieces of dust (easily happen in real life if you left a filter on air for a
> few days), the contrast of the image will be reduced, it is due to the
> semi-transparency and color (usually very light color) of the dust which
> reflected the light and building of bright spots when sun light is shinning
> on the glass surface. Of course if there are only a few tiny pieces of dust
> will not affect image quality.
>
> I don't need black tape for test, my 500mm mirror lens has a big one on the
> front element but it does not simulate the real life situation.
>
> C.H.Ling
>
> ----- Original Message -----
> From: "Chuck Norcutt" <chucknorcutt@xxxxxxxxxxxxxxxx>
> To: <olympus@xxxxxxxxxx>
> Sent: Wednesday, January 09, 2008 9:45 AM
> Subject: [OM] Re: focus and ? about lens filters
>
>
>> 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?
>>
>> Yes, I did perform the test but at the request of Dr. Diffraction. Dr.
>> Focus said he didn't expect any impacts in his domain.
>>
>> Chuck Norcutt
>
>
> ==============================================
> List usage info: http://www.zuikoholic.com
> List nannies: olympusadmin@xxxxxxxxxx
> ==============================================
>
>
>
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
|