[LargeFormat] 480 Apo Nikkor Cell Spacing Question

[Jim Hemenway]

Richard:

Thanks for the explanation.

Jim Hemenway


Richard Knoppow wrote:
> ----- Original Message ----- 
> From: "Jim Hemenway" <Jim@hemenway.com>
> To: <largeformat@f32.net>
> Sent: Sunday, January 25, 2004 4:38 PM
> Subject: Re: [LargeFormat] 480 Apo Nikkor Cell Spacing
> Question
> 
> 
> 
>>I have two process lenses, while they give good coverage
> 
> for 11x14 at
> 
>>infinity, there is so much of the outer areas which are
> 
> just a little
> 
>>out of focus, that they're really only good for 8x10.
>>
>>A retired optics guy from Polaroid, told me that the
> 
> process lenses have
> 
>>just one element less than their non-process lens twins.
> 
> This is
> 
>>because they were not meant for infinity use but instead
> 
> for 1:1
> 
>>copying... that it was cheaper to not include the missing
> 
> element since
> 
>>it wasn't needed.
>>
>>I don't know how true this is, but I can tell you that
> 
> I've given up on
> 
>>them for 11x14 use EXCEPT for macro work, when they are
> 
> sharp all over
> 
>>the 11x14 piece of film.
>>
>>Jim - http://www.hemenway.com
> 
> 
>    I think the culprit is coma. Coma is automatically
> corrected in a symmetrical optical system. By system I mean
> that the lens must be symmetrical _and_ the distance to
> image and object must be the same, i.e., unity
> magnification. At other distances some coma appears, the
> amount depending on the type of lens. Four element process
> lenses of the type sometimes called a Dialyte, such as the
> well known Apochromatic Artar, have rather stable
> corrections with distance. These lenses have goo performance
> at infinity focus but do have some coma there. Coma varies
> with the square of the stop so stopping down gets rid of it
> pretty quickly. At f/22, an f/9 Artar will be free of coma
> at infinity. For lenses which are to be used mostly at
> infinity the corrections are often optimized by shifting
> some power from one cell to the other. This improves coma
> correction (and some other aberrations) at infinity.
>   Since this can be done by adjusting element powers or even
> spacings I don't know what the extra element might have
> done.
>   Process lenses have rather narrow coverage. Dialytes
> around 45degrees at infinity and Tessars maybe 48. The
> Artar, although its rated at only 45 degrees at infinity
> focus _will_ cover an image circle of a diameter equal to
> the focal length when stopped down, but image quality toward
> the margins is somewhat degraded. Wide angle process lenses
> are usually of the Plasmat type. Again, they are more
> limited in coverage than standard lenses of this type,
> perhaps 60degrees rather than the 80 degrees typical of
> general purpose Plasmats. I don't know which type Nikon
> process lenses are. Their general purpose LF lenses are slow
> Tessars.
> 
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@ix.netcom.com
>