[Leica] "focus then reframe" (aka fungus in camera)

[bdcolen at earthlink.net (B. D. Colen)]

Jean - I'd love to see the photos these calculations have produced? Do
you have a website, or photos posted in one of the galleries?

 B. D.

-----Original Message-----
From: lug-bounces+bdcolen=earthlink.net@leica-users.org
[mailto:lug-bounces+bdcolen=earthlink.net@leica-users.org] On Behalf Of
Jean Louchet
Sent: Thursday, July 08, 2004 1:42 AM
To: Henning Wulff
Cc: eric clerjeaud; Pierre Collet; Leica Users Group
Subject: Re: [Leica] "focus then reframe" (aka fungus in camera)


> >
> >TO SUMMARISE:
> >  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 
:-)

Jean


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