RE: [Leica] Focusing the M6

["Ken Lai" <ken@compose.com.hk>]

Assuming a 28 mm lens at an aperture of 2, the closest eye is at the edge of
the frame (worse case) and is at a distance of 0.7 meter from film plane
when you rotate the camera so the eye is in the center of the frame when you
focus,

Now if you rotate the camera back to the original position so the subject's
eye is now at the edge of the frame, the actual distance to the eye from the
film plane is now 0.888 meter which means a 0.888-0.7=0.188 meter focus
discrepancy.

However the 28mm lens at f2 and 0.7 meter has a DOF of around 0.026 meter in
front and 0.028 meter behind the point of focus which means in this case the
DOF does not cover the focus discrepancy and the focus is off by
0.888-0.7-0.026=0.162 meter or 16.2 cm.

So technically, the focus can be off if your subject is not in the center of
the frame. But how to solve this, I have no idea.

Ken Lai



> -----Original Message-----
> From: owner-leica-users@mejac.palo-alto.ca.us
> [mailto:owner-leica-users@mejac.palo-alto.ca.us]On Behalf Of John
> Collier
> Sent: Wednesday, October 25, 2000 9:53 AM
> To: leica-users@mejac.palo-alto.ca.us
> Subject: Re: [Leica] Focusing the M6
>
>
> My solution! I do not worry about it. I focus, recompose and
> shoot. Works
> fine everytime. Mind you there is one trick....use film in
> the camera and
> you too can see that it works! ;-)
>
> John Collier
>
> > From: Jesse Hellman <hellman@home.com>
> >
> > What is the best way to solve the following: you are
> shooting close-up,
> > say at two feet, at F/2 or wider. The closer eye of your subject is
> > off-center. There is nothing dead-center on which to focus.
> So you focus
> > on the eye and then swing the camera back to frame. Now,
> obviously, the
> > plane that passes at a right angle through the eye
> intersects the lens
> > axis at a distance closer to the film plane than the
> distance from the
> > film plane to the eye. The farther the eye is towards the
> edge of the
> > film the greater this becomes. With a reflex you can avoid
> this problem
> > by focusing on the groundglass.
> >
> > At wide f-stops the depth of field is very small at close distances.
> > What is the best way to handle this? A lens with a curved
> field might do
> > better in this regard than one with a flat field. So as the
> lenses get
> > better, this effect has become greater. My own solution is, if it is
> > possible, to try to set up something (like the subject's
> hand) where I
> > think the center would be. But with quick action and more
> candid shots
> > that simply is not possible. Or, you could focus a bit closer than
> > indicated, but that seems like guessing.
> >
> > What do you do? I have never seen this issue discussed.
> >
>