[rollei_list] AW: Re: Rollei 35 mm SLR Flange focal distance
- From: <Dirk-Roger.Schmitt@xxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Tue, 9 Sep 2014 20:39:32 +0000
Emanuel,
thanks a lot, but this information confuses me totally!
I do not understand what a lens vertex is.
In the light from Carlos' information I do not understand the 44,67 mm for the
focal point.
Second: From that, what Carlos was writing, the 44,67 mm are the distance from
bayonet seat and pressure plate plane.
The 44,50 mm are the distance from the bayonet seat to the focusing screen
plane, i.e. from the bayonet seat to the focal plane of the lens.
Considering this, you can imagine that the 44,50 mm are the distance from the
bayonet to the inner film gliding tracks, i.e. to the surface of the film,
where the 44,67 mm are the distance to the outer tracks against which the
pressure plate is pressed. In between should be the film channel with a width
of 0.17 mm. This makes sense.
In principle, it is now to question, which is the best focal point for the
lens. Shall it be on the surface of the film (44,50 mm), at the back of the
film (44,67), or in the middle (i.e. 44,59 mm).
Also to consider that in the center the films moves/bends/ a bit forward in
direction to the lens after it has been transported. Considering the latter
information, I can believe that the 44,50 was the distance on purpose for the
inner tracks for the film AND the plane of focus, allowing to bend the film a
bit, hence with 44,50 mm focal plane you meet the middle of the film thickness.
Similiar discussion you find here:
http://www.fredmiranda.com/forum/topic/665776
For example, look to Canon EF: The flange focus distance normally is reported
to 44.00 mm. But there you find 44.14 mm. The difference is the thickness of
the film channel, here 0.14 mm. (Lower tolerance than at Rollei which take 0,17
mm by the way).
Or here:
http://photo.net/nikon-camera-forum/00acWi
( Nikon tolerance +/- 0.015 mm, even much lower than Rollei gear...)
Or, more specialized, here:
http://www.dantestella.com/technical/flange.html
Dirk
flange focal distance = distance between the lens bayonet seat and the focal
point = 44,67 mm (an official Braunswcheiger spec _should_ have a comma as a
decimal separator ;-) )
back focal distance = distance between the last lens vertex and the focal point
= 44,50 mm
Hence we get the clue for the "variable" figures quoted on the 'net
1/ confusion between flange focal distance (mechanical - bayonet seat) and back
focal distance (optical - lens vertex)
2/ flange focal distance = 44,67 mm explains why some documents mention
44.6 and some other 44.7
3/ those Rollei figures are not really related to the M42x1 flange focal
distance = "register" in English, quoted as 45.46 mm (not really close to
44.67) in this other document:
http://www.graphics.cornell.edu/~westin/misc/mounts-by-register.html
I home, Dirk-Roger that you can now have a quiet sleep.
--
Emmanuel
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