[Leica] Leica IR filter

[henningw at archiphoto.com (Henning Wulff)]

>Focus compensation would vary according to the film?   Why?
>
>Bob
>
>----- Original Message ----- From: "Marshall Hunt" <huntmc2@gmail.com>
>To: "Leica Users Group" <lug@leica-users.org>
>Sent: Tuesday, January 17, 2006 1:07 PM
>Subject: Re: [Leica] Leica IR filter
>
>
>Compensation varies according to the film, as well as the lens. The focusing
>distance should be adjusted by 1/200 to 1/400 of the focal length of the
>lens, at infinity. Which fraction to use depends upon trial and error.

Compensation should depend on the spectral response of the film. 
Those such as HIE that are sensitive further into the IR region need 
more compensation than films such as Konica 750 or Maco 820 that 
don't have as wide a spectral response. The further the incoming 
'light' is into the IR region, the further behind the film plane the 
main bundle will be focussed.

Compensation also depends on the design of the lens; many modern 
lenses have a much flatter spectral response and don't need as much 
compensation or none at all into the near infrared, whereas the 1/200 
compensation is needed for older (60's or older) lenses using HIE. In 
any case, stopping down to middle or small apertures is needed with 
HIE in any case as hardly any lenses display severe aberrations at 
850-900nm that can be brought into line by stopping down.

-- 
    *            Henning J. Wulff
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