Archived posting to the Leica Users Group, 2004/07/07

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Subject: [Leica] "focus then reframe" (aka fungus in camera)
From: jean.louchet at (Jean Louchet)
Date: Wed Jul 7 22:42:29 2004

> >
> >  with a 35mm, at aperture 2 "focus then reframe" is OK from 2 metres
> >               at aperture 1.4 "FTR" is OK from 3 metres.
> >
> >At shorter distances, one has to reduce the focussing by, say, 10 - 12%.
> >
> >I did not consider focussing on an object which will finally be right in a
> >corner (no practical interest).
> >
> >OF COURSE we luggers will take much, much better pictures now :-)
> >
> >If anyone is interested in having the equivalent results with a 50mm, let
> >me know (private e-mail), I may well open another page on my web site.
> >
> >Jean
> While these calculations and formulae are correct, the point is still 
> moot due to the almost universal field curvature, especially of fast 
> lenses, at shorter distances.

Thanks for having checked - it is easy to get messed up in this area. It 
is true that I did not take field curvature into account.  However, I
think that even in high aperture lenses the field curvature while non
negligible is generally small compared to this "diagonal effect" unless
one is shooting very close (say, under 50cm). I have just tried on my old
design (1973)  Nikkor-S 1.4/50 and between 1 and 2 metres I still have to
apply a correction of ca. 8 - 10% on the distance to compensate for the
"focus from reframe"  error. This looks about the same (or even a bit
more) than given by theory.

> > After these calculations, you won't be closer to the truth or focus, 
> _and_ your subject will be gone to sleep or just plain gone.

Right! this is why it is better to make the calculations at home on a
rainy day (or have them done by another lugger) rather than when shooting 


Replies: Reply from bdcolen at (B. D. Colen) ([Leica] "focus then reframe" (aka fungus in camera))
In reply to: Message from henningw at (Henning Wulff) ([Leica] "focus then reframe" (aka fungus in camera))