[LargeFormat] Bad Kodak Lens

[Richard Knoppow]

----- Original Message ----- 
From: "Jim Hemenway" <Jim@Hemenway.com>
To: <largeformat@f32.net>
Sent: Tuesday, March 16, 2004 7:14 PM
Subject: Re: [LargeFormat] Bad Kodak Lens


> I expected to find focus shift on the 13 1/4 - 23 1/4
Protar that I use
> on my 11x14.  But I lucked out.  So far I haven't seen,
(or I don't know
> if I've seen) any focus shit on any of my lenses.
>
> Jim - http://www.hemenway.com
>
>
  I would expect there to be some. However, spherical adds
rather than cancels when cells are combined so the spherical
of a single Protar or Dagor cell is no worse than the
combined lens.
  I have been using a good quality 10X triplet magnifier to
examine the ground glass image. This is pretty critical. Try
the following experiment. First, focus as sharply as
possible with the lens wide open. Then stop it down two or
three stops. Then see if changing focus will sharpen the
image any. If it does, then open it again and look at the
image with the lens wide open. If there is focus shift you
will find a definite difference in the focus positions for
the two stops. The larger error as far as defocusing is in
the wide open image after focusing critically when stopped
down. Generally, the lens will move away from the ground
glass as you stop down.
  Since last posting I've compared two Speed Graphic Zeiss
Tessars of late 1930's construction and a Bausch & Lomb
Tessar of considerably older vintage, the latter on a view
camera. Neither Tessar has much focus shift although both
have a little. One is slightly better than the other and is
a lens I've always considered extra sharp. The B&L Tessar is
quite sharp but shows some shift. Its hard to compare
because its a longer FL lens, but, from the amount of
softness of the wide open image, its probably not quite as
good as the Zeiss lenses. I think focus shift is mainly of
concern when some method of focusing other than looking
directly at the image is done. Even the Kodak Ektar that
started this all off is actually a very sharp lens when
stopped down, its just that the critical focus using the RF
is too far from the actual critical focus. I had not
realized before just how much focus shift many lenses have.


---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com