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

[Jesse Hellman <hellman@home.com>]

The problem is greatest at the widest lens openings. I suspect that those
who have not found it, like Tina, generally use smaller lens openings, and
not at the closest distances. I wondered if the effort to correct for flat
field for a RF lens is going astray, and that some field curvature can
actually be helpful for those of us who do not shoot walls.

Particularly, it seems to me that this effect is another one that diminishes
the value of the widest apertures. It 's another reason to use faster film
and so be able to use F2.8 instead of F1.4. How often do we put the subject
right in the center? I discovered this early with my Summilux 50, and
noticed that I did have to correct to get portraits sharp. It is curious to
me that this issue has not been widely discussed.

Jesse

Christer Almqvist wrote:  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