P3D Re: TDC Vivid Viewer

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Jon, regarding this, you can forward to this person my comments.
   
>The TDC vivid viewer has APOCHROMATIC lenses , the highest category 
>of color corrected lenses.
>The rest of our viewers (red button etc etc) have ACHROMAT
>lenses, and are not color corrected for the 3 primary colors,
>usually only corrected to a reasonable tolerance.  Achromats
>obviously work great and we can not see any color aberrations
>through them.  Apochromats are a  step higher, so it's a better
>lens that commands a premium price.  This is especially needed
>for enlarger lenses, but makes the TDC that much better.

I think that's nonsense!

- Where is it written that the TDC Vivid has APOCHROMATIC lenses?
  I have in front of me the TDC Stereo Vivid Deluxe Viewer
  instructions and under "lenses" it writes: "The cemented 
  achromatic lenses have an effective focal length of 42.7 mm"
  So even the TDC literature does not claim any special type
  of lenses, other than the common "achromatic".

- According to Rudolf Kingslake in "Optics in Photography" (great
  reference! - still have a few copies left for sale), apochromatic
  lenses are made of crystalline fluorite or other unusual types
  of glasses, have stronger elements (higher curvature) and are
  limited to smaller apertures.  I doubt that this kind of lens 
  would be put in a viewer.  I have taken the lens apart and does 
  not look any different than the regular 20 mm diameter lenses found 
  in red button and Revere/Wollensak viewers.

- In my opinion, and to my well-trained eye :-), the lenses in the 
  TDC viewer are, optically, in the same group with the red button, 
  Revere/Wollensak, and other fine achromatic lenses from that period 
  of time.  They do not appear to be better in any special way.  

- TDC was not asking a premium price in the '50s for this viewer (as 
  they should have if they were using any type of premium/expensive 
  lenses).  The price asked for this viewer was $17.50 vs. $19.75 for 
  the Realist red button, $18.50 for the Revere and $23.95 for the 
  Wollensak.

I wonder where this "apochromatic" idea originated from... If someone 
wants to pay a premium price for this fine viewer because it is 
relatively scarce, that's fine.  But not for any special type of 
lenses that the viewer does not have.

George Themelis 


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