[rollei_list] Re: Rolleinar Parallax Compensation
- From: Emmanuel Bigler <Emmanuel.Bigler@xxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Sat, 23 Jul 2011 11:53:16 +0200
Le 23/07/2011 03:32, Don Williams a écrit :
Does anyone know what distance the prism is supposed to correct the
framing precisely?
Hi Don and Rollei friends !
I do not know the precise answer to this question, since, as you say,
it would require some precise optical and mechanical measurements to
be sure.
However this is my guess. Call it "a conjecture" to be verified or
contradicted by the real thing.
As you know, under the Rolleiflex TLR ground glass (GG) you have a
built-in mechanical parallax compensator. A moving frame, or a set of
blades, connected to the main focusing system. Actually the ground
glass is bigger than the 56x56 mm = nominal "6x6 cm" image frame, the
height of the GG is larger than 56. When the camera is focused to
infinity, the frame is set in a certain position. From memory, the
frame is at its "lowest" position in the lower part of the GG image
when focused at infinity.
When focusing to closer distance ranges, the frame moves "up". Now,
keep the Rollei focused to infinity and add a #N Rolleinar: N = 1, 2
or 3. Infinity will then be "converted" to 1/N metres (** see note 1),
and the shortest focusing distance, instead of being 1 metre on a
standard Rolleiflex with a 75 or a 80 mm lens, will become 1/(N+1)
metre. In fact the shortest focusing distance is .9 metre or so, about
3 feet, measured from the object to the film plane, an old convention
for distance engravings. A minimum focusing distance of .9 metre
instead of one metre ensures a minimum of overlapping focusing ranges
between different Rolleinars.
Now my guess is the following: when fitted with a #N Rolleinar, the
camera being set to infinity (minimum of extension of the lens TLR
panel), the parallax compensator is by construction set to infinity.
Hence the #N Rolleinar prism HAS to properly frame for the distance of
1/N metre.
What I do not know is: does the moving frame continues to properly and
precisely correct parallax effects when focusing to the closest
distance with a Rolleinar #N in place? The only thing that we can say
without calculation, is that the displacement of the frame is is the
proper direction, and that actually some compensation takes place ...
And as Carlos says: if you are not satisfied with your Rollei-TLR for
close-up work, take your SL-66 ! ;-);-)
--
Emmanuel from France, who fell "in love" with Rollei TLRs 34 years
ago but still resists the SL-66 .. for how long ? ;-);-)
(** note 1) "metre" since my spell-checker uses the British dictionary ;-)
---
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