[rollei_list] Re: Bright Screens
----- 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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