Enhanced focussing screens
Compiled by Q.-Tuan Luong for
If you find it difficult to focus with your ground glass, you might
want to consider a replacement. Two things can make a ground glass
difficult to focus on:
In addition, the hot spot makes it difficult to assess the composition.
A finer glass will tend to have a worse light fall-off. Two solutions
are to add a Fresnel lens to even up the illumination, or to use a
Although they do their job, the problem with Fresnel lenses is that
they work optimally only for one focal length.
the other hand, ALL the users of the Boss screen seem to like
it. The only drawback seem to be that this screen might be damaged
by extreme temperatures (hot and cold). I was relatively satisfied
with the screen on the 5x7 back of my tachi, which was a plastic
fresnel/ground combination protected by a sheet of glass. I found the
stock screen of my Canham difficult to focus on. I tried using a
fresnel lens, but I got weird problems with it. Then I switched to a
Toyo focussing screen, which has a considerably finer grain. This made
focussing easier but the hot spot was terrible. Eventually I switched
to a Boss screen, and I found it worked fine.
- Darkness off axis, especially off-axis
Some general remarks about focusing screens. Most of these are
"field lenses" usually of the Fresnel type, so they can be flat, which
direct the light going through the ground glass toward the expected
location of the viewer's eyes. For this scheme to work right the
focal length of the field lens must be such as to focus the light
from the lens at a convenient distance for viewing. This will be
affected by the location of the lens so will vary with focal length of
the taking lens and to a lesser extent with distance focused on. It
is the angle of the cone of light leaving the lens which is important.
It also depends on the whole optical path being centered. For this
reason field lenses do not work well when the camera movements are
used. The parts of the screen away from the optical center will be
darker than with a conventional GG. The same for using WA lenses.
The angle of the light striking the edges of GG are acute and a
different FL of field lens is needed to make the image uniform.
I tried a fresnel lens made by Arca-Swiss. The illumination is a bit
more even than before, but I don't find the image to be much
brighter. The thing which bugs me is that when I use wide angles,
there is a very weird effect, with concentric bright rings which look
like double images. They are extremely disturbing with a 90 lens, and
noticeable with a 120 lens, to the point of preventing me from
assessing the composition of the image in the corners. Rod Klukas
from Photomark says that with any Fresnel this is going to happen for
some focal lenghts (he observes the same with this 45 lens on 4x5),
and as a consequence there is a folk he knows who has different backs
with different fresnels that he switches as he switches lenses. Tuan
The Beattie Intenscreen
The Beattie is, basically, a plastic "ground-glass" with a
built-in fresnel lens on one side of the "ground-glass".
get a glass cover plate to protect the plastic screen. I decided
that I didn't like the Beattie and took it off and went back to using
a plain ground glass and fresnel lens. Why?
It really is brighter but:
even said that the plastic screens aren't always perfectly flat so
prefect focus in the center of the screen may give out of focus
The probably do make pretty good backup ground glass for carrying
I would imagine they are much less fragle than real glass, and backups
The grain is much coarser than other ground glasses that I"ve used so
that while the scene is easier to see because it's brighter, it's
harder to focus because of the coarse grain of the GG being more
visible. For me the net effect was no change in ease of focusing.
The GG becomes strongly directional. Moving your eye away from the
axis of the lens causs the image to go very dark. So as soon as you
any significant amount of rise/fall or shift, the image "blacks
Not a problem for those who don't use much movement, and less a
with long lenses, but I found it to be nearly impossible to focus
using a 90mm or shorter lens along with any amount of
The Linhof Super Screen
The Linhof Super Screen also incorporates a fresnel, but it didn't
interfere with focusing in the same way as a Beattie Intenscreen.
I think the Linhof was brighter than
the Boss, but I did not care for the way it had bowed in the Technika
it was installed in. To make matters worse, I looked at it one day,
noted the bowing, then went back about a week later, and it was bowed
differently. I didn't particularly care for the idea that the damn
thing was not stable (it's a plastic screen). Paul Butzi
The Boss screen
The "Boss" screen, made in the Netherlands, is a thin layer of a light
scattering wax/paraffin mixture between two glass layers. In contrast
a "ground" glass there is no visible grain, it is brighter and quite
contrasty. In addition you dont need an extra fresnel lens for most
length. I use one on my 4x5 with lenses from 75mm to 600mm and I am
pleased with it. With the 75mm there is a hot spot in the center, but
corners of the format are still clearly visible without fresnel. They
have screens fitting most view cameras and you can also choose between
a model with clear corners (similar to cut-away corners) and the
rectangular shape. If I remember it right, the manufacturer advises
temperatures should not exceed 55 degrees C, but I never had any
in summer. I noticed, however, that at temperatures below
-10 degrees C, some Newtons rings appeared after some hours in the
at the edge of the ground glass, indicating a slight separation
the glass and the wax layer. This effect was reversible, though.
I experienced a
partial separation (1-2square cm) between the wax and (one of the)
layers when the camera was subjected to temperatures below -15degrees
(5 deg. F) for several hours. It shows up as an area with Newton's
its not a crack pattern. You can easily test if its really separated,
by applying a little bit of pressure with your fingers on the glass:
when the Newton rings move, you have a separation ( and not some ice
from breathing on your glass :-) ). Most of the time this was
and the rings disapperared when the camera was warmed up again, but
time (really cold) it was permanent. As you have mentioned, it is
possible to "repair" this area by _careful_ heating with a hair dryer
_just_ until the wax melts and becomes clear, then let it cool again.
Dont forget to tape the edges of the inner glas, or else some wax will
creep out (I forgot). The repaired area looks a little more
when the GG is held against the light, but in the camera it is
only noticeable at certain angles. I assume this difference can be
reduced by changing the cool-down process of the wax, but I did not
to make a test series with my Bosscreen.
Be aware that according to the manufacturer separation can also happen
the screen is bent by tightening the mounting (screws or whatever
is used) of the ground glass too much.
Despite the cold temperature problems I would still recommend the
Bosscreen; its a real improvement over my former setup (Linhof ground
glass and Fresnel).
Bosscreen is made by a Dutch company called Stabitech (formerly
"Stabilix", which went bankrupt towards the end of 2003).
The company sells direct as well.
The new website of the company is http://www.stabitech.nl
You can place an order by email with them and wire your payment
to their bank account. They can cut a screen at any size to
fit your camera, but this took more than a month in my case.
It's imported in the US and probably sold retail as well by
Bromwell Marketing in Pittsburgh (412-321-4118). It was extremely
difficult to get a screen through the specialty LF retailers.
I have heard that the temperature limit is around 140F, which
is easily reached in summer in a car trunk. Chris Newman writes
"I am on my third one, one damaged by extreme cold the other by
moderate heat (90 degrees)". Myself, I didn't have any problems
until a summer trip to the Rocky Mountains, (where it doesn't get
that hot). Since then, I have the dreaded bubbles. They do not
interfere so much that the screen would not be usable, however are
enough of a nuisance that I will replace it.
One frequent concern about the Boss screen is that the position of
the plane of focus would be wrong, but this has been addressed
Basically, the sheets of glass of the Bosscreen are two different
sizes. The one which goes closest to the lens is small enough to fit
INSIDE the area on which the ground glass usually rests. It is the
inner surface of the piece of glass on the viewing side which actually
seats on the routed out area that the ground glass normally sits.
This then puts the inner surface of this outermost piece of glass in
the correct focal plane, which means, of course, that the wax is
actually too close to the lens at this point. To compensate for that,
there is a piece of plastic (they call it "tape") along the edges on
the inner side of the glass which sits in the rout. This tape is
sufficient to correctly space the glass back far enough to put the wax
in the correct focal plane. David Fokos.
The layer of glass between the lens and image forming plane probably
affects the focus position. Probably why the real Boss screen comes
with some shims. Otherwise the smaller front glass should put the wax
layer in the same position as the ground side of the ground glass
(assuming you are using a camera designed to have no fresnel between the
ground glass and the lens, some cameras like most speed graphics were
designed to always have a fresnel mounted in front of the GG). I've
heard that the layer of glass moves the focus point back by about 1/3
the thickness of the glass if I'm remembering right. I'm sure it
depends on the index of refraction of the glass, but this probably
doesn't vary much unless you are using some exotic glass type.
Viewing Screens for Large Format Cameras by
Ron Wisner. Explains how a ground glass work.
List compiled by Craig Blurton (originally posted in
a LF Forum thread):
FOCUSING SCREENS MADE BY THIRD PARTY MANUFACTURERS
Maxwell Precision Optics UNOFFICIAL website
Maxwell Precision Optics
Contact Bill Maxwell <firstname.lastname@example.org>
Velveteen View screen
Contact Greg Garret <email@example.com>
EXAMPLES OF FOCUSING SCREENS MADE/SOLD BY MANUFACTURERS FOR THEIR CAMERA
Ebony Fresnel Lenses
Canham Fresnel Lenses
Linhof Super Screen
Sinar Fresnel Lenses
Wisner Fresnel Lenses
Wista Fresnel Lenses
EXAMPLES OF GENERIC FRESNEL LENS THAT CAN BE ADAPTED FOR USE AS A FOCUSING SCREEN WITH A CAMERA
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