[rollei_list] Re: 'Old' Zeiss glass question...uncoated lenses


----- Original Message ----- From: "Carlos Manuel Freaza" <cmfreaza@xxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, April 28, 2006 7:25 PM
Subject: [rollei_list] Re: 'Old' Zeiss glass question...uncoated lenses


--- Richard Knoppow <dickburk@xxxxxxxxxxxxx>
escribis:


   Why would an uncoated lens be desirable for
surveillance
work?


Richard, it seems uncoated lenses are better than
coated lenses for low light and night vision
applications (acording some surveillance cameras
propaganda), perhaps a coated lens cuts or modifies
some type of light rays dimishing even more a poor
lighting, but really I'm speculating about this
point.-

All the best
Carlos


I thought this might be the answer. While flare can result in ghost images or other concentrated light it mostly results in an even fog over the image. This reduces the shadow contrast while having little effect on the highlights. It is something like a film with a long toe. I think those who prefer uncoated lenses are assuming that either the added light will push the film over the line where it will record an image at low light levels or that the flare light acts as a sort of pre-flashing. I am not so sure it does either. The light loss due to flare can be significant. In a lens with eight glass-air surfaces, like a Biotar, the loss is around 33% for an efficiency of about 66% with the flare light increasing this by about 6%, but the flare is not image light. The transmission efficiency of a multiple coated lens is 90% or more. This is only a fraction of a stop difference but the shadow contrast is much greater for the coated lens.
I wonder if anyone has actually experimented to see if the flare light from such a lens increases shadow speed as much as the loss of transmission lowers it. Further, if the image contrast is low enough it may become useless. Perhaps there is a basis in actual experience for this but, until I see it, I remain a bit skeptical.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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