[pure-silver] Re: Lens quality, filter degradation

----- Original Message ----- 
From: "Scott Hamming" <Scott.Hamming@xxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Wednesday, March 23, 2005 1:24 PM
Subject: [pure-silver] Lens quality, filter degradation


> Hi all,
>
> Recently on a photo website, likely APUG or photo.net - I 
> don't recall =
> which, a poster made the declaration that placing filters 
> on a 'high =
> quality' lens (in this case the reference was Rolleiflex 
> Planars and =
> Xenotars) degraded the optics to the level of the filter, 
> and thereby =
> nullifying the need for a 'quality' lens.  The last point 
> is clearly an =
> overstatement, esp. considering the source, but I was 
> curious as to what =
> the actual facts might be.  It seems there may be several 
> factors to =
> consider, such as coatings and glass quality, but are 
> there other =
> considerations, and what degree of impact might they have? 
> As for =
> myself, I always use a lens hood when possible and more 
> often than not a =
> yellow Hoya multi-coated filter when shooting.  Thanks,
>
> Scott
>

   I think this may be due to a misunderstanding about an 
optical principal. A plane parallel block of glass can 
introduce some aberrations (spherical and chromatic) in an 
image forming beam of light _providing_ the light rays are 
passing through at an angle. Where the light is also 
parallel the glass has no effect. Now, this assumes the 
glass, or other substance, is homogenic and clear. When used 
on the front of a lens focused on a distant object the light 
from the object is essentially parallel so the filter will 
have little effect. If the filter is made of good quality 
optical glass, which is homogenious and with highly parallel 
sufaces the glass itself will not have much effect on the 
light. Some flare can be created by the glass-air sufaces if 
not coated.
   When a filter is used behind a lens the light is 
convergent so the filter can degrade the lens performance. 
For this reason behind the lens filters, when used, are 
generally of gelatin. Gelatin has the advantage over glass 
and plastics of having a very low index of refraction and 
being very uniform and thin. The effects of the plane 
parallel surface depend on its ratio of index to air and to 
its thickness so the closer the index is to air and the 
thinner it is the better.
   To make it concise, the effect of a high quality, coated, 
filter on the front of a lens, when used for normal 
photography (not super close ups), is almost nil.

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

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