Re: Lens question please_Gundlach, Turner-Reich series II No. 1

<jjs> wrote in message 
news:13r8viujmvit6e0@[EMAIL PROTECTED]
> "AAvK" <notforspam@[EMAIL PROTECTED]
> wrote in message 
> news:L1djj.926$Ca7.438@[EMAIL PROTECTED]
>
>> What about for mono films?  I think because of this 
>> problem I
>> would never use this lens for color films!
>
> I took it that Richard was addressing B&W. If aberation 
> occurs in a particular color for which the film is 
> sensitive, a correcting filter will help.
>
> Black & White film is sensitive to color (obviously) and 
> not equally sensitive to all colors. Filters are 
> im****tant.
>
>
    Lack of color correction affects both B&W and color 
although it manifests itself in different ways. Lenses have 
two kinds of color aberration: one is longitudinal chromatic 
aberration, the other is lateral chromatic aberration. The 
first is the one we usually think of where the lens focuses 
different colors at different distances: in effect the focal 
length changes with color. This is inherent due to the 
characteristics of glass and is reduced by balancing the 
dispersion of the positive and negative elements by choosing 
the kinds of glass used for them. The newer low-dispersion 
gl***** make the job of correcting color while maintaning 
other corrections easier.
    Lateral color is a difference in the _size_ of images at 
different colors. The images are all in focus but different 
in size so that there is still fringing. In fact, because 
the images are all sharp the fringing may be more noticable. 
Lateral color can be reduced by symmetry or by choice of 
glass types. In non-symmetrical lenses it is sometimes 
difficult to correct.
    In a convertible lens, where the cells are similar but 
can be used alone the correction for lateral color due to 
symmetry is lost when only one cell is used. Some of these 
lenses have cells which are corrected fairly well and others 
show considerable color fringing when single cells are used 
but all are inferior to the complete lens (two cells).
    Color problems may actually be more appearant in B&W 
because the film records everything at once and the eye can 
not longer ignore some of the information at it can in color 
pictures.
    Now, if the colors recorded throught an aberrated lens 
are limited the aberration will have less effect. Some very 
old lenses are not very corrected for the red end of the 
spectrum because before about the mid 1920's most film was 
orthochromatic (not sensitive to red) or even "ordinary" 
(not sensitize to any color but blue). A filter which 
reduces red light may make such lenses sharper. Green 
filters cut off some blue light and some red light so are 
good choices where a lens has serious chromatic aberration. 
A cyan filter will somewhat duplicate orthochromatic film 
and will usually also improve sharpness when the lens has 
some chromatic aberration.
    My experience with the T-R lens is that the individual 
cells have very noticable color fringing. The Zeiss 
Convertible Protar is better although still having some 
fringing. Even the older convertible version of the 
Schneider Symmar has some fringing, actually a little worse 
than the Protar.
    A general purpose lens which was very well corrected for 
both kinds of color aberration is the Kodak Commercial 
Ektar. These are Tessar types and don't have the advantage 
of symmetry but were very carefully designed to eliminate 
color problems because they were intended to promote the use 
of color film for commercial photography. Most symmetrical 
or nearly symmetrical lenses have good correction for 
lateral color, the Goerz Dagor, Dogmar and of course Artar 
being examples.


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Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]