Re: [Leica] Noctilux and filters and IF

[John Collier <jbcollier@home.com>]

on 22/1/00 8:25 pm, Joe B. at joe-b@dircon.co.uk wrote:

> I have used filters with the Noctilux in situations where
> it was the only thing to do, like for example with Infrared Ektachrome. I
> didn't get many correctly focused shots that time, and I wonder if possibly
> the filter affects the focus accuracy with such a wide aperture lens.

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......"

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

I like the idea of being able to focus beyond infinity, sort of Buzz
Lightyearish. (yes I know you view camera people can go "to infinity and
beyond" anytime you like)