Re: [Leica] Retrofocus. was sharpness discussion

["Bryan Willman" <bryanwi@seanet.com>]

I should use more words.

On an *SLR* you can't have any part of
the lens closer than the end of the sweep 
of the mirror, unless you want to lock up
the mirror and give up viewing!
(Nikon made a 7.5mm fisheye that worked
 exactly that way.)

On an RF camera, you could have the lens
sitting inside the camera body, and make an
image.  However, the lens design has to fit 
in there (which could limit speed) and if it
blocks the metering cell that reads the pattern
on the shutter curtain, the meter won't work.
Finally, if you are going to focus, you need
enough of the lens in front of the body for
the user to reach a focusing tab and the
tab to turn the rangefinder cam/follower.
Also need an aperture ring.
Finally, the focus level, cam, and aperature
ring aren't normally desireable as parts of
the picture, so a lens with a very wide angle
of view (like a 10mm or 14mm) will need to
be retrofocus to avoid having parts of itself
in the image.

(The 15/16mm Hologon designs for Contax/Leica
  sit way down inside the camera body, have fixed
  aperature, are focus by guess (at least on G2).
  I don't know about metering.)

As an aside, one extreme case is the 35mm Grandagon
(view camera lens) I have mounted on a tecknikardan
board.  The rear element clears the ground glass by
about 1 mm.  Attention to detail is required to keep
the camera out of the picture.

bmw




- -----Original Message-----
From: Chris Bitmead <chrisb@ans.com.au>
To: leica-users@mejac.palo-alto.ca.us <leica-users@mejac.palo-alto.ca.us>
Date: Tuesday, May 26, 1998 9:12 AM
Subject: Re: [Leica] Retrofocus. was sharpness discussion


>Bryan Willman wrote:
>> 
>> The real point here is that Retrofocus lens is
>> physically longer than its focal length.  So,
>> a 21mm for an M camera must be retrofocus,
>> since you can't fit the nodal point within 21mm
>> of the film plane.
>
>What do you mean "can't" ? Do you mean you can't without
>interfering with the metering?
>