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alex from holland
16-Nov-2010, 06:09
Hi all,

what would you suggest as a max thickness for a ground glass of a ultra large camera.
The ground glass is going to be about 20 x 20 inch.
I would prefer 2mm but that is going to be too thin ??


alex

Walter Calahan
16-Nov-2010, 07:38
That sound too thin for such a large camera. Not that I have any experience larger than 8x10.

BrianShaw
16-Nov-2010, 07:42
I have no experience with format that large, but even with 4x5 would be concerned that 2mm may be too fragile. Also, may bow and throw your focus off when you put a loupe to it. Since the ground side is toward the lens the thickness shouldn't matter too much. I'd err toward increasing the odds of survivability and go thicker.

GPS
16-Nov-2010, 08:30
Hi all,

what would you suggest as a max thickness for a ground glass of a ultra large camera.
The ground glass is going to be about 20 x 20 inch.
I would prefer 2mm but that is going to be too thin ??


alex

After some checking up on it - the "normal" classical home windows in Europe have 3 mm thickness. They are washed/touched with no special precautions, hence...etc.

bobwysiwyg
16-Nov-2010, 10:17
As far as I know, here it the U.S., a typical window pane is 1/8th inch thick which is a hair over 3mm also. And, as noted, take a fair amount of "use" over a pane area much larger than a GG.

Jay DeFehr
16-Nov-2010, 11:28
Alex,

The thinner, the better, as far as brightness goes. For what it's worth, a view screen doesn't have to be glass. A thin sheet of plastic stretched over a frame, like the head of a drum, for instance, would do just as well, and be much brighter, lighter, and much less fragile. This is a feature of all my lightweight camera designs.

GPS
16-Nov-2010, 11:39
Jay, just plainly curious - what kind of plastic you have in mind that can be stretched so tightly that it doesn't bow under a loupe pressuring on it?

Vaughn
16-Nov-2010, 11:40
The Ritter 7x17 I have on loan uses plexiglass. There are trade-offs for the lightness and the lack of worry about breaking glass, but one could use glass in the studio and plex out in the field.

alex from holland
16-Nov-2010, 11:47
wow, interesting thoughts.
I have tried grinding a plastic groundglass, but without succes.
How do you guys solve that problem ?

Jay, i would love to see some pictures of your solution. Sounds just great.

alex

Jay DeFehr
16-Nov-2010, 12:44
GPS,

Mylar is the most commonly used material in this type of application; it's optically clear, durable and can withstand high tension membrane stresses, and it's heat-shrinkable. Push on the head of a drum sometime, and see how much force is required to flex it. A loupe is not a problem.

Alex,

Mylar, and many other plastics can be chemically etched to produce the required tooth to form an image. Very fine etchings are brighter than coarser ones, but create hot spots. A fresnel is typically used to even out the light, but costs a little brightness. An exotic solution is two films, one etched, one fresnel, but a perfectly serviceable screen can be made with just drafting mylar, which comes with a tooth for taking pencil drawing (good for making grids, etc). I'm at work in Alaska right now, so I don't have photos, but once you see a sheet of drafting mylar, you'll understand how it works. I'll make some photos when I get home.

Jay DeFehr
16-Nov-2010, 12:51
BTW, most sheet film is coated on mylar, and I've made screens from fixed out sheets. APHS Ortho works well.

GPS
16-Nov-2010, 16:37
No doubt you can make a screen from a stretched mylar or even a sheet film. They will never have the rigidity of a glass gg though... (the guy is talking a 20x20in gg!)

Jay DeFehr
16-Nov-2010, 18:43
GPS,

I think the questions to ask are: how rigid is rigid enough, and what is the difference in deflection between a tension membrane and a pane of glass? Until these questions are answered, one can't intelligently weigh the compromises.

The weight of a 20x20 GG is significant, and a bright screen is even more important for larger formats than for smaller ones. Besides, I think users of ULF learn not to lean on the GG with their loupes.

Michael Kadillak
16-Nov-2010, 19:12
Hi all,

what would you suggest as a max thickness for a ground glass of a ultra large camera.
The ground glass is going to be about 20 x 20 inch.
I would prefer 2mm but that is going to be too thin ??


alex

You are in waters that have only been charted by a few selected people making cameras. I personally would not touch mylar with such an application on a 20x24 camera. I like medium thick glass because it is what history and time have consistently used. Weight is a non event between glass and an alternative material because the camera is the beast all by itself even without ANY viewing screen.

My recommendation is to have a conversation with people that know what they are talking about for such a unique application. I would contact Ebony and Keith Canham and see what they have to offer on the subject. Information is power and you need some for this question.

Good Luck!

John Bowen
16-Nov-2010, 19:29
Richard Ritter shared with me that one REAL problem with glass is that when it breaks, it tends to put some nasty cuts in bellows.

Drew Wiley
16-Nov-2010, 19:53
How on earth do you keep any kind of plastic flat? You already have a problem keeping the big sheet film from bowing (though I've heard of people using the same
kind of double-sided Post-It tape that I use in my 8x10 holders). Polycarbonate isn't
very bright or terribly flat. Optically coated acrylic is baked out prior to coating, and
might be more resitant to bowing toward temperature, but won't etch well. And heck, real glass shattering isn't good for smaller view camera bellows either - that's
why I use a cover when not in use! Real glass has distinct advantages when you
need something which actually maintains its plane of focus.

IanMazursky
16-Nov-2010, 20:10
I would be interested to know what the best way to chemically etch plexi or mylar.
What chemicals (acids) and or compounds would be best? and what is the procedure?
I want to replace my 12x20 GG glass with something less prone to breakage.

EdWorkman
16-Nov-2010, 20:25
in response to questions about grinding plastic:
Take a look at the offerings of Surplus Shed, which has "plastic ground glass', IIRC made by Kodak. I cut a piece for 8x10 and found it satisfactory. I concluded to use it for my will-it-ever-be-finished 7x17, albeit there will be a vertical joint, probly right down the center, as I learned from my Cirkut.
Last i looked, this stuff wasn't offered in anything approaching 20x20 however

Michael Kadillak
16-Nov-2010, 20:32
My suggestion is driven by what Drew mentioned relative to maintaining a perfectly rigid focus plane. In case you have not aware of this fact - 20x24 sheet film can be $15-$25 a SHEET when you can find it. Even a modest deflection would drive you friggin nuts.

Since any sensible LF/ULF photographer carries their prized possession in a protective case breaking a ground glass would be a terribly unusual event all things considered. You would have to have the camera tip over or drop it handing it. If you wanted some "breakage shard" protection all you need to do is add a clear adhesive backing to the back of the ground glass. This would essentially keep any broken glass from embarking on a suicide mission targeting the bellows.

If you have a ULF camera don't worry about breaking the ground glass. Think of taking care of the camera when it is out of its case as that is a more reasonable risk assessment and the ground glass tags along for free. Sometimes we create more problems when we concern ourselves with trivial things.

Drew Wiley
16-Nov-2010, 20:38
There are some pretty strange things out there if one looks. I once bought a box of
thin Zeiss glass which was coated in such a matter that it is not only optically superb
and very flat but nearly breakproof with ordinary handling. It is obviously tempered and unlike anything else I've ever seen, and I have absolutely no idea what it was
originally made for. I bought it surplus and use it in one of my 8x10 enlargers. It
was hell to cut; but I have special equipment for tempered glass. Over the years I've collected samples of all kinds of special glass and plastic types, some no longer
available. If I were a ULF designer I'd be looking into one of these optically coated
acrylics for high-end instrument panels. Somewhere around here I've got a bunch
of tech specs. It costs like hell, but might be a lot more resistant to deformation and
scratching than ordinary acrylic. But how you'd etch it is another subject and would
require some experimenting. Maybe someone has some scratched scraps that they
can't otherwise sell.

Jay DeFehr
16-Nov-2010, 20:56
Maybe you gentlemen are right. I defer to your experience.

Drew Wiley
17-Nov-2010, 10:55
It it were a coated acrylic, you obviously couldn't mechanically abrade it, or you'd be
sacrificing the very quality which keeps the sheet dimensionally stable. You'd have to
experiment with something sprayed or laminated onto the surface. Regular acrylic would be easy to abrade, but then again, wouldn't remain flat. In fact, the degree of
bowing could be substantial unless you were working with something so thick that it
would be way heavier than glass in the first place.

ic-racer
17-Nov-2010, 12:22
Well, lets see...a 4mm focus spread wll be covered by f32, so if you press in to the plastic 2mm with the loupe you will be ok if you scene has a small focal spread to begin with.

If you scene has a large focal spread, say 20mm, (requiring f128) then an additional 2 mm is only a 10% errror and probably won't make much differnece either.

GPS
17-Nov-2010, 12:41
Well, lets see...a 4mm focus spread wll be covered by f32, so if you press in to the plastic 2mm with the loupe you will be ok if you scene has a small focal spread to begin with.

...

That would be true in the best case - if you first arrived at the correct focus middle value (+-0) and then flexed the screen. You wouldn't do that - with the loupe you first flex the screen and then try to get to the sharp zone.
Besides, it is awfully unpleasant to focus with a loupe on a spring which itself is moving... :)

Drew Wiley
17-Nov-2010, 12:53
Let's see ... a twenty inch wide piece of 1/8" acrylic can easily bow a quarter inch;
then if the film sags a quarter inch itself with the camera pointed slightly down; then
since focus doesn't mean a hill of beans, you can ignore a loupe all together; and then
the tripod leg slips a little ... but heck, it's a contact print, and will look perfectly sharp
to anyone who lost his reading glasses, provided the lens was stopped down to f/756
and nothing moved during the 37-hour exposure!

jb7
17-Nov-2010, 13:11
GPS,

Mylar is the most commonly used material in this type of application; it's optically clear, durable and can withstand high tension membrane stresses, and it's heat-shrinkable. Push on the head of a drum sometime, and see how much force is required to flex it. A loupe is not a problem.

Alex,

Mylar, and many other plastics can be chemically etched to produce the required tooth to form an image. Very fine etchings are brighter than coarser ones, but create hot spots. A fresnel is typically used to even out the light, but costs a little brightness. An exotic solution is two films, one etched, one fresnel, but a perfectly serviceable screen can be made with just drafting mylar, which comes with a tooth for taking pencil drawing (good for making grids, etc). I'm at work in Alaska right now, so I don't have photos, but once you see a sheet of drafting mylar, you'll understand how it works. I'll make some photos when I get home.

Sounds like a very elegant solution Jay-

For a big image, I suppose there's not much need to use a magnifier more than about 2x or 3x.
Reading glasses might work, or those binocular magnifiers...
no need to deform anything- by leaning on it-

How would you go about etching a fresnel into a sheet of Mylar?

Drew Wiley
17-Nov-2010, 13:22
Different brands of mylar have a slightly different degree of frosting. If you tried mylar,
you'd want the sheet around 5-mil and frosted on one side only. How you'd tension it
would be an interesting problem. If your tried heat shrinking there is a risk of hot spots with uneven clarity, or damaging the camera itself. Would take some finesse. One good
quality about mylar is that it is quite dimensionally stable, and it is fairly inexpensive to
replace. Just don't confuse it with frosted acetate, which is also sold for drafting.

Jay DeFehr
17-Nov-2010, 13:35
Joe,

These guys know how to make thin film fresnels:

http://www.microsharp.co.uk/technologies/fresnel-lens-films/

It might even be possible to have a fresnel on one side of a film, and an etched surface on the other, for a very bright, evenly illuminated screen.

jb7
17-Nov-2010, 13:47
Thanks for the link- interesting stuff-
doesn't look inexpensive- at £100 for a sample kit of 2 A4 sheets of linear, I shudder to think what a special order of 20x20 circular might cost-

Though probably competitive in price with a Maxwell, I suppose-

Drew Wiley
17-Nov-2010, 16:14
Joseph - it's a pretty long shot that any of that fresnel film stuff would work very well
if at all for focusing an image. Very little in common with camera fresnels. I've worked
with linear fresnels etc in optical design for light distribution problems, and consider
most of this kind of thing unsuable for image formation per se. Be better just to test
out that frosted mylar idea if you need an alternative to glass. And a piece of custom
fresnel ULF size would probably cost more than a luxury car - they have to make the
precision moulds.

Jack Dahlgren
17-Nov-2010, 17:24
Let's see ... a twenty inch wide piece of 1/8" acrylic can easily bow a quarter inch;
then if the film sags a quarter inch itself with the camera pointed slightly down;

The ideal focus screen would sag exactly the same amount the film does.

GPS
17-Nov-2010, 17:41
The Ritter 7x17 I have on loan uses plexiglass. There are trade-offs for the lightness and the lack of worry about breaking glass, but one could use glass in the studio and plex out in the field.

If Alex used let's say 5-6mm thick plexiglass he would not gain much to speak about in weight, he would loose something in the luminosity of the gg picture and he would gain quite a lot in the shatter resiliency.
If he used glass it would teach him to take care of his camera...;)

Jay DeFehr
17-Nov-2010, 18:12
Drew,

Microsharp has the technology and expertise to make any kind of thin film fresnel for which one has an application. I don't think their fresnels are molded, I think they're laser cut.

civich
17-Nov-2010, 18:21
Alex,
If you decide to try a plastic gg for your ULF project here is some info which may be of use.
Transluscent film as described by Jay is available from any art/engineering supply in sizes larger than your 20x20 and in various degrees of opacity. Some comes with an adhesive side but I would avoid these due to the difficulty in eliminating the inevitable air bubbles.
To apply use a 'window film application solution' available at any big box home improvement store and follow the instructions. A hard rubber roller (from an art or hobby center) will help work out the air bubbles.
The resulting plastic ( I used polycarbonate, Lexan brand ) "ground glass" is easily marked with grid lines. Is plastic suitable for your ULF? - dunno - it works fine for my 8x10 and is cheap and in the unlikely event of breakage easily replaced. Is it a quality gg? - for a lot more money you could get something brighter and more fragile but, then again, you will probably not be slinging that bad boy around in a backpack!
-Chris

ic-racer
17-Nov-2010, 18:34
That would be true in the best case - if you first arrived at the correct focus middle value (+-0) and then flexed the screen. You wouldn't do that - with the loupe you first flex the screen and then try to get to the sharp zone.
Besides, it is awfully unpleasant to focus with a loupe on a spring which itself is moving... :)

Actually if image focal spread is small (ie flat object or infinity), then a 2mm flex while focusing will give a focal point that will encompass the subject rendering it sharp at f32 (4mm total = 2mm on either side).

Drew Wiley
17-Nov-2010, 18:40
Jay - thanks for that clarification. There's a domestic outfit that also does this kind
of fresnel work, but it's been awhile since I dealt with them. I don't think a linear
lens could be done well laser-cut, due to thickness, but thin-film circular patterns could be. Doesn't mean they'd visually focus well. And how much money and testing would it take to get things right for a single camera? And how dimensionally stable would something like this be? I've wasted enough money myself on analogous projects. If it were me, and I couldn't tolerate glass, I'd just use polycarbonate - you'd lose about 8% transmission, but it would stay flatter and be tougher than acrylic, and can be sanded or whatever. If I was younger I think it would be a fun
project to rethink a ULF camera and build one; but I'm more eager these days just
to find time to use the gear I've already built - so off I go to the dkrm as soon as
the news is over! The mylar drafting film idea would be interesting is one could
design a drop in sub-frame, with the mylar properly tensioned, then clipped into the camera back. I use a lot of frosted mylar for masking and color separation work,
and it varies quite a bit in minor characteristics, depending on the source.

Jay DeFehr
17-Nov-2010, 22:48
Drew,

I understand your position, and it's a reasonable one. I have a personality disorder that compels me to evaluate every design I confront for improvements. Some have suggested it might be a form of autism, while my Julia says I "suffer an unquiet mind", referring to bipolar syndrome, I think. One of many unfortunate consequences of whatever defect with which I'm afflicted, is a growing catalog of simultaneous projects my son affectionately refers to as "The great unfinished works". Among them is an ultralight, self casing ULF camera. A big part of the design involves integration, and the way a tensioned membrane screen fits into the design makes for a big weight savings. There has never been a camera like the one I'm designing, and maybe there are good reasons for that. On the other hand, what's not to like about a 14 x 17 camera that weighs as much as some 4x5s?

jb7
18-Nov-2010, 02:35
Yes, I've seen some of those 4x5's ...

I think I suffer from a similar affliction- we won't argue about severity.

Perhaps we could collaborate? That way, there's twice as much stuff we could fail to accomplish-

I like the Mylar idea, I might be borrowing a sheet from you at some point.
I think the fresnel idea is wonderful too, though probably overkill for the larger formats- but there's nothing to stop you having a range of focal lengths-

Look forward to seeing some of these projects happen-

GPS
18-Nov-2010, 04:51
Actually if image focal spread is small (ie flat object or infinity), then a 2mm flex while focusing will give a focal point that will encompass the subject rendering it sharp at f32 (4mm total = 2mm on either side).


That would be true in the best case - if you first arrived at the correct focus middle value (+-0) and then flexed the screen. You wouldn't do that - with the loupe you first flex the screen and then try to get to the sharp zone.
Besides, it is awfully unpleasant to focus with a loupe on a spring which itself is moving... :)

:rolleyes:

BrianShaw
18-Nov-2010, 07:53
... If he used glass it would teach him to take care of his camera...;)

Is GPS the screen name moniker for "Confusious"? This saying should be in a fortune cookie! :D

Jack Dahlgren
18-Nov-2010, 08:04
:rolleyes:

I don't put what I use as a loupe on the surface of the ground glass.
You can certainly focus without touching the glass.

Bob McCarthy
18-Nov-2010, 08:24
Have you concidered the high strength glass Steve Hopt offers as an option to his regular ground glass. Thats a big piece of glass.

http://www.hopfglass.com/borosilicate.htm

bob

alex from holland
21-Nov-2010, 12:40
Thanks all for the feedback !
I think we will go for the 3 mm glass.

Jay, i am still curious how your solution looks like. Can you post some pictures of it ?
thanks

alex

John Jarosz
21-Nov-2010, 16:51
I recently saved a process camera from the trash. The ground glass is 20x24 inches. I measured the ground glass and it's approx. 6.5 mm thick.

John

alex from holland
23-Nov-2010, 01:13
Thanks John,

A process camera is a fixed construction.
if i would take that thickness the glass only would have a weight of aprox 4,3kilo.

alex

Jim Cooksey
24-Jan-2011, 14:26
Richard Ritter uses plastic for his ULF screens, though he'll make a glass one for you if you insist. Also Steve Hopf can make one out of 1mm(!!) thick borosilicate glass, which ought to be pretty light, even in the big sizes.
Jim Cooksey

Scott --
24-Jan-2011, 14:36
I don't put what I use as a loupe on the surface of the ground glass.
You can certainly focus without touching the glass.

+1

jb7
24-Jan-2011, 14:53
Different brands of mylar have a slightly different degree of frosting. If you tried mylar,
you'd want the sheet around 5-mil and frosted on one side only. How you'd tension it
would be an interesting problem. If your tried heat shrinking there is a risk of hot spots with uneven clarity, or damaging the camera itself. Would take some finesse. One good
quality about mylar is that it is quite dimensionally stable, and it is fairly inexpensive to
replace. Just don't confuse it with frosted acetate, which is also sold for drafting.


Drew, might you have some specific brand in mind?
I did have a look at the time of this post, not altogether successfully-
Sourcing anything other than a sheet of paper is difficult here...

Nathan Potter
24-Jan-2011, 15:34
I have used mylar film that is frosted on one side, clear on the other side (drafting mylar) as a focusing screen. Some mylar is better than others but all I've used tend to yield varying degrees of hot spots that make them inconvenient rather than unusable.
For very large format I think I would explore attaching the mylar to a glass plate using a clear adhesive; possibly UV curable. I might pick a glass thickness of a couple of mm. minimum but certainly not over 4 mm for weight considerations. The difference between soda lime and borosilicate glass in density and modulus is small, although my experience is that the borosilicate seems more durable to shock even tho that doesn't make sense. Youngs modulus for borosilicate is about 64 GPa where soda lime is 72 GPa.

But hell, why not just grind a piece of glass and be done with it. The lapped glass will give a much better off axis rendition of the image than mylar simply due to its better scattering properties.

One could attach a mylar film to lexan or the like, using adhesive, but then you're back to a flexing clear base material with a quite unattractive modulus.

Nate Potter, Austin TX.