[rollei_list] Re: Bright Screens
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 2 Jan 2012 13:24:29 -0800
----- Original Message -----
From: "Bob Marvin" <marvbej@xxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, January 02, 2012 9:15 AM
Subject: [rollei_list] Re: Bright Screens
I've never had a satisfactory answer to this question:
Back in 1965 I had the original.screen of my 2.8E replaced
with a Rolleiclear screen, which was a considerble
improvement, especially in terms of corner brightness. I
still do more tha 3/4 of my photography with that camera
and have no trouble focusing. Would I be likely to see
much difference with a Maxwell or other bright screen?
Bob Marvin
Sent from Samsung tableto
Perhaps this being sent from some sort of pocket
machine explains the lack of editing. In general one should
quote only the segment of a digest one is specifically
replying to rather than the entire digest.
There are some basic things about all of these screen
brigheners: One, the total light energy can not be larger
than what ever comes out of the lens. The "brightening" can
come from either having the scattered light from the screen
directed toward the eye, or, in some cases, a more efficient
transmission. Generally, these brigheners are some form of
field lens. When one interposes a textured surface like a
ground glass in the optical path of a lens the light
striking it is is scattered. The angle over which the
scattering takes place depends on the nature of the
texturing. In general, the coarser it is the less scattering
there is. Since the light which is not scattered tends to
continue in whatever direction it started the corner
brightness of all ground screes appears to fall off when
viewed from the center line of the screen. If one moves
one's eye toward one or another corner the corner will
appear brighter but all other parts of the screen will
appear darker. A finer grained screen is a more efficient
scatterer of light so that it will appear _more even_ but
the brigher parts will be darker than a coarse screen. By
interposing a field lens the light from the margins and
corners is deviated toward the center, actually focussed on
the eye, so that the overall brightness will appear to be
rather uniform. However it will fall off quickly when the
eye is moved away from the point of focus.
Screens that have some depth become hard to focus on
because there is no longer any well-defined optical plane.
Such increase in effective depth can come from the ridges of
a Fresnell type field lens or from thickened screens such as
one made of two thin sheets of glass with fine wax inbetween
them.
there is no perfect answer to this. The Rolleigrid,
which works pretty well, is a plastic Fresnell screen but it
has a plane circle in the center allowing sharp focusing to
be done there.
I should also note that away from the center of a ground
glass the fact that the light impinges on the surface at an
angle may also affect the effective point of focus.
I hasten to state that I have no idea of what Maxwell
makes his screens.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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