[LargeFormat] Bad Kodak Lens

[Jim Hemenway]

Thanks Richard... I'll try your experiment.

Jim - http://www.hemenway.com



Richard Knoppow wrote:

> ----- 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