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Mysterious ring inside the lens
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PostPosted: Sat Mar 28, 2015 9:37 am    Post subject: Mysterious ring inside the lens Reply with quote
I am wondering if anybody can help.

Seller on eBay selling very nice Schneider 50mm/0.95 lens (c-mount), which is slightly different than usual versions.
The seller describes the black ring seen when looked inside the lens (see pic).
Says it is unlikely to affect the images, but sold "as is"
Any ideas what it might be? Just curious...
Rado


[/img]


PostPosted: Sat Mar 28, 2015 9:40 am    Post subject: Reply with quote
There is a thin plastic sheet inside the lens with a black dot in the center. You should be able remove it if you open the lens.


PostPosted: Sat Mar 28, 2015 11:10 am    Post subject: Reply with quote
It is there to get an even lighting without vignetting.
It is on a glass surface.

Last edited by exaklaus on Sat Mar 28, 2015 11:25 am; edited 1 time in total


PostPosted: Sat Mar 28, 2015 11:21 am    Post subject: Reply with quote
http://overgaard.dk/leica-leitz-90mm-Thambar-f-22.html


PostPosted: Sat Mar 28, 2015 12:32 pm    Post subject: Reply with quote
I have this on my ernitec tv c mount lens only with 3 rings. Gives ring like bokeh on mine.



Last edited by imagedit on Tue May 05, 2015 5:19 pm; edited 1 time in total


PostPosted: Sat Mar 28, 2015 1:21 pm    Post subject: Reply with quote
calvin83 wrote:
There is a thin plastic sheet inside the lens with a black dot in the center. You should be able remove it if you open the lens.


Mine was on plastic too. Image quality improved after removal.


PostPosted: Sun Mar 29, 2015 12:50 am    Post subject: Reply with quote
exaklaus wrote:
It is there to get an even lighting without vignetting.
It is on a glass surface.


Wow! I would like to know the logic in that design choice -- to block the bright central rays to acheive more even lighting -- that little piece of plastic also makes the lens a little slower (at f/0/95!!!) than it could be!

However, I think the fellow in the article http://overgaard.dk/leica-leitz-90mm-Thambar-f-22.html has got it wrong. He writes:

Quote:
the lens comes with a front filter with a black dot in the middle so as to ensure that the center of the image isn't sharp.


A dot in the center has nothing to do with sharpness! For example, mirror lens has such central blockage...

Without dot, focus may change more as aperture is closed down...


PostPosted: Sun Mar 29, 2015 3:28 am    Post subject: Reply with quote
I believe he means that the image is softer but still has great amount of detail visible, and the OOF areas are greatly affected.


PostPosted: Sun Mar 29, 2015 6:44 am    Post subject: Reply with quote
From what I know it was used by a sensor inside the system where it was attached to. I also have such lens(es) which have such inside, basically a ND filter.


PostPosted: Sun Mar 29, 2015 9:44 am    Post subject: thanks Reply with quote
Thanks all for quick answers! you guys are living on this site ,are you? very knowledgeable responses...


PostPosted: Sun Mar 29, 2015 12:23 pm    Post subject: Re: thanks Reply with quote
radissimo77 wrote:
...you guys are living on this site...

I think you figured us all out Smile.


PostPosted: Sun Mar 29, 2015 8:14 pm    Post subject: Reply with quote
No, this is to stretch the transmission range, without getting to much diffraction due to small iris settings.

When you close the iris much, the neutral density filter fills the complete aperture. And with a high density (low transmission) it allows T settings probably up to f/1000, but with much better sharpness than such a iris f-stop setting would have. Furthermore more reliable than very small iris opening.

This was used for CCTV, where you have outside very bright light at a sunny day, and low light at night.

The Thambar filter was for completely different use.


PostPosted: Mon Mar 30, 2015 12:42 am    Post subject: Reply with quote
Thanks for explaining ZoneV. Very Happy


PostPosted: Mon Mar 30, 2015 8:01 pm    Post subject: Reply with quote
so the question is:

does it blend?


I mean:

is that frickin' ring removable? Rolling Eyes


PostPosted: Mon Mar 30, 2015 8:05 pm    Post subject: Reply with quote
radissimo77 wrote:
so the question is:

does it blend?


I mean:

is that frickin' ring removable? Rolling Eyes


Mostly yes. But getting there isn't easy with zooms having min. 12 elements Smile.

I suspect when those rings are put on plastic material it degrades over time. So the image quality is worse then initially intended.


PostPosted: Tue Mar 31, 2015 4:43 am    Post subject: Reply with quote
On a Schneider Kreuznach lens I tested to remove this. As I remember it was on a glass plate, and with this removed the optical setup was wrong - as expected. Glass or plastic inside the optical path at least changes the distances.

I want to test it more, but have to find the lens again.
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