The spaces refer to the distance from the edge of the film holder
that rests on the camera to the surface of the film. The ground glass
should be in the same position. Since the frame of the back in which
the ground glass is held rests on the same reference surface as the film
holder the ground glass should be the same distance from the edge of the
back as the film is to be. Getting to the edge may require removing the
ground glass frame from the back. However, if you are measuring the
difference between the ground glass and a holder with film it can be
measured from any convenient surface since you just want the two to be
the same. I may be beating this to death but I want to be clear about it.
The flat used to locate the micrometer must be rigid enough so
that it does not deflect when making the measurement. In other words
don't use something thin like a wooden ruler. Also be careful that the
micrometer is exactly perpendicular to the GG or the film. If you
wanted to be really, rilly, rilly fancy you could use a laser
interferometer (just kidding although it would tell you if things were
flat and level). I have gotten remarkably sharp negatives from a
wooden view camera with wooden holders so it can work very well. Some
years ago I photographed a nearby electric distribution station with my
ancient Agfa Universal View and a 12" Goerz Apo Artar. I found to my
amazement I could read the labels on the transformers. This was at
perhaps a 200 foot distance and the labels are maybe five or so inches
across.
FWIW Apo Artars are exceptionally sharp lenses, even old ones and
even at infinity focus.
On 7/13/2015 12:59 PM, mark@xxxxxxxxxxxxxxxxxx wrote:
IF I understand this correctly, the frosted or front face of the ground glass must be .197 from the wood on the back +/- .007 of an inch? Think I do have a depth micrometer around here somewhere.
I also greatly appreciate the offer of a ground glass, and I very well may take you up on it after I experiment a bit here.
I don't look at this as misery or a time waste, even if it fails. Though at times it is frustrating, I am also learning a great deal and the education is worth the time.
-------- Original Message --------
Subject: [pure-silver] Re: making ground glass
From: `Richard Knoppow <dickburk@xxxxxxxxxxxxx
<mailto:dickburk@xxxxxxxxxxxxx>>
Date: Mon, July 13, 2015 12:51 pm
To: pure-silver@xxxxxxxxxxxxx <mailto:pure-silver@xxxxxxxxxxxxx>
But, make sure the thickness is right. Most window glass
is pretty thick, say 1/4 inch where camera glass is about half
that. The thickness is critical only in that it must fit into
the back.
In most camera backs the location of the ground surface, and
hence the focal plane, is determined by the back, not the glass.
The reference surface is the one the ground surface of the glass
is held against. The construction of the back is such that the
ground surface is automatically in the same location as the
film. Here is a list of the ANSI film plane positions:
ANSI Standards for film plane placement in sheet film cameras.
Size Location Tolerance + or -
4x5 0.197 0.007
5x7 0.228 0.010
8x10 0.260 0.016
Dimensions in inches.
Film thickness is 0.007 The above does not include
film thickness.
Smaller formats than 4x5 are the same as for 4x5.
If the back is removable from the camera its easy to
confirm the ground glass plane is the same as the film plane. You
need a holder that you know is good. Load a sheet of film in the
holder. Use a depth micrometer to measure the ground glass
position in reference to any convenient surface. I lay a piece of
thick sheet metal across the back, a piece of thick glass will do
as well. Now put the holder in the back with the dark slide
removed and measure again, the two should be the same. Note that
in many holders the film is not actually pushed against the
plenum. You should check your film holders the same way only lay
the reference surface against the sides of the holder. I check
near the corners and at the center. I've found that many older
wooden holders are warped and some brands of plastic holders are
even more warped. Generally the best are the Riteway holders up
to 4x5 (they may have also made 5x7) and for larger holders the
ones made by Graflex or Kodak (same holders). Check both sides
since the plenums may have moved from the center.
On 7/13/2015 9:45 AM, Myron Gochnauer wrote:
By far the simplest thing is to go to a place that sells glass
and ask them to cut you a few pieces to the exact size you need.
There need be nothing special about the glass. Ordinary window
glass will work fine with a large format camera. You’re looking
through the glass to see the side that is ground (and illuminated
by the image from the lens). Almost any piece of commercially
available glass will be clear enough and flat enough for this
purpose. The slight greenish tinge of inexpensive glass is
unlikely to matter at all since you use the GG only for composing
and focussing.
My local glass supplier usually smooths off the edges and corners
for me, too, which makes handling very nice. (They use a large
wet-belt sander.)
Myron
On Jul 13, 2015, at 1:23 PM, mark@xxxxxxxxxxxxxxxxxx
<mailto:mark@xxxxxxxxxxxxxxxxxx> wrote:
What I have seems to be working, but it is a mess. Locally I
have no idea where I could get the silicon carbide. It would
have to be ordered, but that is not total road block.
My biggest potential problem I think is going to be cutting the
glass to fit. The glass I am using is left over from another
project, and truthfully I don't remember what kind of glass I
used. I have never been able to get any kind of straight cut
with the scoring method. It never breaks right for me. It
looks easy and I am sure its something to do with the way I
apply pressure to break it. Really doesn't matter. What ever
the reason I stink at that.
If it was some form of laminated glass cutting with the right
blade and a jigsaw should be no problem if I take my time. Frame glass and I think it might work. Might crack, but maybe
not. Depends on how much temper was in the glass. IF its
tempered glass its going to be a mess. Gloves and eye protection
for sure and maybe the shop apron too. Will find out fairly
quickly. IF its going to shatter, it will shatter fast, and
come to think of it I may have some glass from an old 4x5 frame
laying around.
Appreciate all the help
-------- Original Message --------
Subject: [pure-silver] making ground glass
From: Myron Gochnauer <goch@xxxxxx <mailto:goch@xxxxxx>>
Date: Mon, July 13, 2015 7:46 am
To: Pure Silver group <pure-silver@xxxxxxxxxxxxx
<mailto:pure-silver@xxxxxxxxxxxxx>>
. . . I went to auto parts store and bought a small tube of
valve grinding compound. That's slow, but it is working.
It is vastly easier if you buy some straight silicon
carbide. I don’t know of any local source for most people,
but if you are going to make ground glass in the future, you
should order some from a telescope-making supply. Here’s
one that might do:
http://www.willbell.com/ATMSupplies/ATM_Supplies.htm It’s
located in Richmond, Va.
As you can see at that website, silicon carbide is pretty
cheap. And it never goes bad! I’m still using left-overs
from 1962.
I can’t recall what grade I use for ground-glass. I *think*
220 is okay. I’m pretty sure 80 was too coarse for my
tastes, and 400 or 500 was definitely too fine. You don’t
need much, either. 4 oz. will make a lot of ground glass.
The other thing you need is a “grinding tool”. For 4x5 to
8x10 ground glass, I use a piece of glass that is roughly 4
inches square and a half inch thick. The exact dimensions
don’t matter. It just has to be big enough to grip, and
rigid enough to transmit even pressure. You can probably
make something out of a piece of plate glass glued to a
block of wood.
To grind the glass:
1) Place several thicknesses of newspaper on a sturdy work
table. (Telescope makers often use an upside down barrel so
they can walk around it.)
2) Place the glass-to-be-ground on top of the newspapers.
The newspapers will cushion and “grip” it slightly.
3) Sprinkle a little carbide onto the glass. Keep it in an
area a bit smaller than your grinding tool. You’ll quickly
learn how much to use.
4) Sprinkle a little water onto the carbide and place the
tool on top. Move the tool around gently to wet all of the
carbide (you’ll feel it), and then grind back and forth at
various angles using moderate pressure. There’s nothing
magic about these motions. Just keep them fairly random and
short enough that you don’t constantly scrape off the
grinding compound with the edge of the tool. With a 4 or 5
inch tool I think I use a back-and-forth stroke of something
like 3 inches total.
5) You may need to add a bit of water from time to time,
and in a short space of time you will feel and hear the grit
disappearing (it breaks down). When it seems like you’re
not really *grinding* any longer, rinse off the glass and
tool under running water and start over again with a fresh
charge of silicon carbide and water.
6) Within two or three cycles you should start seeing the
glass beginning to look frosted. Check as you go along to
see where it needs more work, and continue until it is
ground and frosted uniformly.
7) If you later decide the surface is too fine or too
coarse, you can regrind it with a different grade, although
it will be a little more difficult seeing how your work is
progressing.
The main difference between this procedure and one using
automobile value grinding compound is that this procedure is
completely water-based, so it is much easier to assess how
you are doing, make adjustments, and clean up. The basic
idea is the same, though.
Myron Gochnauer
goch@xxxxxx <mailto:goch@xxxxxx>
Got about 15 minutes into it and its clear the glass is
grinding down as it should. I am guessing its going to
take at least another half hour or so, maybe longer to get
it like I want, but it is working. Will it be factory, but
from the looks of it I think it will be quite serviceable. There is something to be said for having done it yourself.
Got to pick up a blade tomorrow that will fit in the jigsaw
that will cut glass. No way I could ever get a clean cut
with a typical glass cutter. Tried a few times and never
mastered the scoring, or just never could get a clean break.
This project is giving me a great appreciation for the work
of the early pioneers. When you see the images they
created with equipment that would be considered primitive,
after doing this I can appreciate the work even more. I
have one huge advantage they didn't. I have their
experience and all those that came after them.
A while back someone posted they had a method to shim the
ground glass to make sure that what you saw as in focus on
the ground glass was in focus on the film. Would love to
hear the procedure and if they prefer they are welcome to
email me off list.
Slow but sure, and this past couple of weeks have been a
pain to find any time at all to do much of anything. One
day when I push the shutter for the first time, it will all
be worth it, at least I hope.
Mark
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-- Richard Knoppow
dickburk@xxxxxxxxxxxxx
WB6KBL
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