[pure-silver] Re: 4x5 ground glass on Cambo
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 7 Sep 2012 16:35:58 -0700
----- Original Message -----
From: "Bogdan Karasek" <BKarasek@xxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, September 07, 2012 2:57 PM
Subject: [pure-silver] 4x5 ground glass on Cambo
Hi, and my follow up question.
I have several backs for my 4x5 Cambos and they come with
the Cambo labelled gg with the grid and a 6x9 outline.
Using an 8x loupe, I find that the corners are a tad, a
lot?, darker.
Can I do better (brighter) than the Cambo gg? I have
heard praise for the Beattie Intenscreen; is the hype
justified? I believe that the Beattie uses a Fresnel
lens in front of the gg like the gg/fresnel on my Crown
Graphic. But it's hard to judge when I have 26" of
bellows extension and light coming through on a 210/5.6.
And the Beattie's are expensive :(
I'll listen to all recommendations. Thanks!
Cheers.
Bogdan
The caveat when using a field lens for the GG is that
it works provided the focal length of field lens is right
for the camera lens and that no tilts or swings are used.
The field lens is designed to gather the divergent cone of
light from the camera lens and make it somewhat convergent.
Although the ground surface of the GG should scatter the
light striking in all directions equally it does not so that
light which strikes it in a direction toward the viewer's
eye continues that way resulting in a hot spot. Usually,
the coarser the grind the hotter the spot. The field lens
will focus the light from the GG (or striking the GG) so
that its approximately parallel. If the light from the lens
strikes the field lens at an angle the lens will focus it at
an angle so that the ground glass may appear very dark
unless the eye is moved so that its on the optical axis of
the lens. This effect is considerably less for a plain
ground glass than when the field lens is used. The focal
length of the field lens is ideally about equal to the
distance from the camera lens to the focal plane. That
results in the light emitted from the field lens + ground
glass being approximately collimated. If any camera
movements are contemplated an easily removable field lens
will be appreciated.
Some "bright" screens are not field lenses but simply
very highly diffusive material, such as a layer of fine wax
between two thin sheets of glass. The image from such a
panel may seem bright but will not be very sharp. Most
field lenses are Fresnel lenses molded in plastic sheets.
These can be used on either side of the ground glass but the
ideal location is on the inside with the ground and ridged
surfaces facing each other in order to minimize reflections.
This is usually not a practical location unless the focal
distance can be adjusted to compensate. The results with
the field lens on the viewing side is usually not much
different than the ideal location. The Ektalite field lens
found in the Graflok back for Speed Graphics is mounted
inside and the back made to compensate its thickness.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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