Re: Focus Shift with SFX200

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

At 19:51 07/07/97 -0600, you wrote:
>At 08:23 PM 07/07/97 -0700, you wrote:
>>Hi George. I haven't used the Ilford SFX stuff, but I have used Kodak's 
>>infrared in my Leica Ms. The Ilford captures a smaller amount of the 
>>infrared spectrum than does the Kodak, so I would guess the focus 
>>adjustment would be less, or certainly no more. Anyhow, just focus on 
>>your subject, then move the focus a tad closer -- that is, move the lens 
>>outward. On the 50mm Summicron, moving the focus point to about the f/4 
>>or f/5.6 mark on your depth of field scale should do it. I presume the 
>>Summilux is similarly marked.
<snip>
>Thanks for the info.  I suppose that with TTL metering it will be pretty
>much hit-or-miss as far as correct exposure is concerned.  Bracketing may
>not be practical with the subject matter I am looking for.
>   I'm used to those red dots on non-Leica lenses, which show how much to
>shift the lens at infinity.  It should be interesting to see what happens.
>I'll try to see how much a slight shift of the lens throws out the
>rangefinder image, so I can guestimate the focus on the fly and use a
>small>aperture to allow the extra Depth of Field to bail me out as needed.
>-GH

FWIW, I have here a 1950's collapsible 50mm Summicron, which actually has
the infrared focus mark on it! (Leica stopped using them around that time as
they felt they were misleading, being only accurate for infinity). This IR
correction mark is on top of of the right dof marking for f2.0. As luck
would have it I also have the 50mm Summilux-M to compare, and the dof
markings appear to be spaced similarly, but very slightly further apart,
which would seem appropriate for a physically slightly wider lens of the
same focal length. Both lenses focus from infinity down to 3 ft 5" (as near
as dammit) with 180 degrees rotation of the focusing helix. My tentative
conclusion from this would be that the IR correction point for infinity for
the Summilux would also be on the right dof mark for f2.0. 

Joe Berenbaum