[pure-silver] Re: Kodak Enlarging Lens


----- Original Message ----- From: <genej2@xxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Tuesday, September 27, 2005 3:25 PM
Subject: [pure-silver] Re: Kodak Enlarging Lens


I'm not at home or I'd check out the "number", but my Anastigmat is a Tessar, f4.5. Very Old. 20's, teens or earlier would be my guess. My new favorite portrait lens. BTW, I'm going to disagree with the assertion of narrow coverage on the Kodak dialytes. just ain't so. i have a 135 that will cover 4x5 easily with considerable movements, and pretty well stopped down a bit.

This is unusual. The constrast is between lenses like the Dagor, which has a very large circle of illumination but one that requires stopping way down to get sharp results in. A Dagor will cover 87 degrees at f/45, at larger stops it will cover a smaller angle. A Dialyte, like an Apochromatic Artar or Dogmar (similar to the 70 series K.A.) has a relatively smaller circle of illimination but the circle of good definition is nearly as large wide open and does not get much larger as the lens is stopped down. The limit of coverage of many lenses is the stigmatic node. This is the point at the margin of the lens where the two stimatic fields (radial and tengential) cross (they start out together at the center). Beyond this point the two fields tend to deviate from each other quite rapidly in the Dialyte so the image quality becomes poor very quickly with increased image angle beyond this point. It is possible that Kodak came up with some way of reducing the deviation of the two fields. Tessars are midway between the Dagor and Dogmar types. That is, it has a circle of illumination which is fairly large and the part of it that has good definition increases significantly as the lens is stopped down.
Kodak seems to have discontinued most of the Dyalite lenses after about 1946 or 47. One which survived is the famous 203mm, f/7.7 lens. This was sold originally as the No.70 K.A. and later sold as a K.A. without the number (I think because it was the only one left). When coated this lens was sold as a Kodak Ektar. The 100mm Enlarging Ektar is also a Dialyte.

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Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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