Re: [Leica] Image plane

[Walter S Delesandri <walt@jove.acs.unt.edu>]

Hi, Pascal.....it probably doesn't concern photographers, or shouldn't....
it does, however, concern repair folks.....ALL lenses project a slightly 
curved field, even if the curvature is masked by the "out of focus points" 
being within the human eye to discern....they're still curved.....

The greatest example I can think of is the old Super Ikonta B.....other 
are similar....when a repairman is focusing this camera/RFDR, he MUST 
adjust the focus BEHIND the film rails....for two reasons....the curvature 
is such that if he focuses in the center, at the film rails, then the 
edges will be very out of focus wide open....IF the RF and Lens are focused 
so the center is sharp on the film rails....also, the film flatness is somewhat 
iffy, and the center is farther from the rails than the edges!!!...this adds 
to the problem (in machining, I believe it's called "stacked" tolerances...
those which add up to negatively impact accuracy, rather than correct each other)

So one must focus the camera so that the "sharp" central image, at close distances, 
is a fraction of a MM behind the rails  (we used to use a coupla thicknesses of 
masking tape!!!)....this will cause the center of the image to be BEHIND the 
film, the edges slightly IN FRONT....thereby minimizing the errors.....

Great camera, I used to carry one, but you have to be careful with close/wide 
aperture shots, for these reasons!! (the mechanical "iffiness" of folders 
makes this a problem with all large folders, but I still LOVE them!!!)

So no, the end user ("Photographer") doesn't need to know much, but like 
any art/craft, the more he knows the better he'll be.......I'll bet that 
90% of the RFs used by pros are somewhat out of adjustment (infinity)....
most are out slightly vertically, as well.....many users don't regularly 
check the infinity, and thereby probably suffer some loss of sharpness at 
closer distances/wide apertures.....one has to find a reasonable balance 
between a devil-may-care "journalist" attitude, and being so obsessed with 
"ultimate quality" that he is unable to "work".....I know of at least 
a couple of those (not on this list-hehe-I'm talking about in my neck of the 
woods...)
I wish that I was less "technique" obsessed, although I've been chastised for 
being the opposite on this list...just goes to show you that all things are 
really relative...!!!...

Walt


On Wed, 25 Oct 2000 13:11:41 +0200 Pascal <cyberdog@attglobal.net> 
wrote:
> >Choosing the location of the film plane is done by the designer and 
> >influences 
> >the image quality. When we photographers do focus, we try to position the 
> >projected image exactly on that film plane.
> 
> At the risk of sounding odd, why does that concern us, photographers?
> 
> I would think that the lens is designed to project a sharp image on the 
> film plane in the camera (wherever it is), and that the whole camera 
> construction should ensure that we can visually see through the 
> viewfinder when the image will be well focussed.
> 
> Am I missing something here?
> 
> Pascal
> NO ARCHIVE
> 
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