[Leica] Flat vs curved field, was Focusing the M6

[Christer Almqvist <christer@almqvist.net>]

>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.
>
>Jesse


  Jesse, this is an interesting issue to contemplate on a rainy night. 
I hope Henning or Erwin or somebody really knowledgeable can put this 
straight for us.

It seems to me that you think a lens with a flat field, when properly 
focused on a straight wall plastered with newspaper will render 
everything equally sharp on the negative provided the film plane is 
parallel to the wall and you focused on the middle of the wall. 
Thisin spite of the corners being further away from the camera than 
the middle of the wall on which you focused with your M. 
Consequently a curved field lens will render all parts of a curved 
wall equally sharp provided the camera is properly focused and placed 
in the middle of an imaginary circle of which the wall is a part.  I 
do not know if I have interpreted you correctly, and if the above is 
true or not, but I would be interested to know.

If the above is true with regard to flat lenses, and if Leicas have 
flat l field lenses, then you will be off focus if you focus on a 
person and then turn the camera to bring the focused area to the side 
of the picture. Your distance will be set too far for you to get the 
person on which you focused sharp when he is then pictured off 
center. I think there is a solution to this problem, perhaps not a 
perfect solution, but something that helps.

For snapshots I use the blind focusing method. I always reset my lens 
to infinity, and when I see something I want to picture but do not 
have the time to focus properly, I just turn the lens - without 
looking at it - to what I think is the correct setting. I am 
surprised how often this works extremely well.
To be fair, I have to admit that I train a lot. You can train in 
front of the TV if the program is bad, you focus on the screen, then 
on the window, then on your feet, then on your beer bottle, or while 
waiting for the bus, anywhere, anytime... You will soon get the feel 
for what is right.

Now, in your case, you would use the rangefinder to focus on the 
person you want to be placed near the side of the picture, then turn 
the camera to get the person off center and at the same time turn the 
lens a bit further away from infinity than what the rangefinder told 
you. How much is 'a bit'? Make a few test, even without film, but 
then it is easier with two persons, one in the middle of the picture 
and the other on the side.
- -- 
Christer Almqvist
D-20255 Hamburg, Germany and/or
F-50590 Regnéville-sur-Mer, France