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Re: Coatings & Refurbishment Project



At 10:57 AM 12/19/96 PST, you wrote:
>My understanding of coatings is that they affect both contrast and
>resolution and are an integral part of the lens design, not an afterthought.
>With that understanding in mind, however, I am still curious . . . has
>anyone ever coated or multi-coated a Triotar, and, if so, what was the
>impact upon performance?  Anybody know where this can be done?
>
>Also, I'm refurbishing the Rolleicord II, type 5 I have.  I need a source
>for either re-silvering the original mirror or to purchase a replacement
>mirror.  I would hate to have to send it off for such a small repair.
>
>One last item.  If I were to replace the (dim, dim, dim) orginal screen with
>a later Rollei-clear screen of the sort used in the "T", would I then be
>required to recollimate (sp?) the focusing system because of the difference
>in the thickness of the two screens?  And is that a home project, or would
>it require a trip to Oceanside or N.J.?
>
>Thanks. 
>
>
  Coating affects contrast because it reduces flare which is light reflected
from the glass-air surfaces of the lens.  It does not affect lens
performance in any other way and certainly does not affect resolution.  The
degree to which coating will improve contrast depends on how much flare the
lens has.  This is related to the number of glass air-surfaces.  A lens with
minimum surfaces, such as a Dagor (4 surfaces) is only improved a little,
complex lenses, like zoom lenses may not be practical at all without coating.  
  Single coating has its full effect at only one wavelength or color.
Single coatings work for photographic lenses because the band of light of
interest is only about an octave wide so the coating has some effect over
the range.  Multi-coatings have effect over an extended band of colors.  The
band can be extended by using more layers.  At some point one reaches a
diminishing return with complex coatings.  
  A Triotar or Tessar has six air-glass surfaces and will show a noticeable
improvement with coating.  Single coatings will produce acceptable results.
  I doubt that aftermarket coating of these lenses is ecomical.  I would say
that definitly trying to multi-coat a single coated lens is not a good idea.
  Also note that some lens aberrations can look like flare.  Particularly
uncorrected sperical aberration as observed in Dagors and similar lenses and
in many other lenses when used wide open. Probably never of concern with
Rolleiflex or Rolleicord lenses.
- ----
Richard Knoppow
Los Angeles,Ca.
dickburk  

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