Re: [Leica] Noctilux and filters and IF

["Joe B." <joe-b@dircon.co.uk>]

At 23:18 22/01/00 -0700, you wrote:
>The filter will have an effect on focus but it is not a factor on lenses as
>short as 50mm. Dick Gilcreast in his excellent article (LHSA Viewfinder) on
>the 135mm Hektor says:
>
>"The one caveat I should mention in using the Hektor is in the use of
>filters with the lens in the rangefinder mount. In common with most other
>long lenses, Leica's as well as everybody else's, the glass in the filter
>will change the infinity setting very slightly, making the lens focus just a
>little beyond infinity at the largest aperatures. It is well to either stop
>down to moderate aperatures or use reflex......"

Aha. I think maybe in the case of this 50mm lens, dof is so shallow that it 
might have an effect similar to the above.

>Now as I am sure you know, inferred light focuses differently than the
>visible spectrum. Leica does not put the generic IF dot on their lenses as
>the actual amount of focus shift varies with type of film and filtering
>used. They say to test first. The focus would not have to shift much at f1
>for things to go awry, so I suspect this is where your particular problem
>came from.
>John Collier

This is why I specified that it was Infrared Ektachrome- this film is 
focused normally, unlike some b&w IR films. Using Kodak High Speed Infrared 
film with a Noctilux would be madness. Utter madness! Unless stopped down 
so far as to make the maximum aperture of the Noctilux irrelevant.

Joe B.