[rollei_list] Re: Tripod versus Handheld
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Thu, 18 Aug 2011 14:57:48 -0700
----- Original Message -----
From: "Frank Deutschmann" <frank.deutschmann@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, August 18, 2011 2:02 PM
Subject: [rollei_list] Re: Tripod versus Handheld
Richard brings up something I never understood: why do
sheetfilm cameras reference the back of the film? Seems
to me making the film substrate an exact thickness with
near zero variation batch to batch is asking a lot from
film manufacture! Always seemed to make more sense to me
to reference the emulsion side against the rails, which is
what I assumed all rollfilm (35mm and mf) cameras do; I
assumed the plate was just there for moderate support. But
then on more reflection, that can't be right either, as no
plate would really be needed, just backing pressure at the
rails. Curious.
-frank (mobile: +1 203 962 3834)
I think the reason is that its a lot easier to do it
this way. The film is really not pushed against the back of
the holder, its just held in place at the edges. On most
holders the flap pushes against the film but that is only at
one end. I don't know how one would locate the film at the
emulsion side without a glass plate. The support thicknes is
controlled very accurately, at least by Kodak. There is an
ISO standard on the thickness. However, some films came on
thinner support, I think Technical Pan did. Some recommended
putting thin black paper under the film to adjust for the
difference. Most sheet film is 0.007 inches in thickness,
the emulsion thickness doesn't really matter and is much
thinner.
Some special purpose cameras like aerial mapping cameras
did use glass plates to locate the film but these all have
all sorts of problems with dirt and scratching the film when
its transported. Other cameras used vacuum backs but those
also locate the film from the back. In an aerial mapping
camera or in some scientific applications, where one wants
the ultimate in resolution a glass plate or a vacuume back
is justified but not in general photography.
Using a pressure plate is somewhat misleading since it
can not put pressure on the center of the film, only the
edges, unless used with a glass plate. The film can still
bulge.
Note also that a plane glass plate is not without
optical effects unless the light passing through it is
absolutely planar. If the light goes through at an angle a
plane glass will introduce both spherical aberration and
chromatic aberration. Note that in a camera the light from
the lens to the film is _always_ coming from an angle except
at the exact center. Probably the effect of a very thin
glass is negligible but for some very precise uses it has to
be calculated.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
---
Rollei List
- Post to rollei_list@xxxxxxxxxxxxx
- Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe'
in the subject field OR by logging into www.freelists.org
- Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with
'unsubscribe' in the subject field OR by logging into www.freelists.org
- Online, searchable archives are available at
//www.freelists.org/archives/rollei_list
Other related posts:
