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Jeffk
Tinkerer
Username: Jeffk
Post Number: 9
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2011 - 06:12 pm: | 
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The photos below show my fully-functioning unbelievably nice Nixe from the front, and how I set the focus of the camera using a glass plate stolen from a halogen driveway lamp. There is no way to focus the camera other than hope the ruled scale is correct, or check it with a ground glass with the shutter on T before loading it. This makes the whole photography process time-consuming and basically restricts it to landscapes at infinity, but that's fine and if the focus is perfect the images can be printed up to poster size and remain nice with an antique look.
The one time I used the camera, I put scotch tape on the front of the glass plate and checked the focus with a magnifying lens, but the granular structure of the tape made it difficult to get it right. I have the bug to shoot another roll through it, but I want to do better on the focus - I tried sanding one surface of the plate with everything from 240-2000-grit sandpaper, but the result is just a not-very-dense network of scratches.
Any thoughts on how to do better? If there ever was a ground glass that came with this camera, it is long lost so I'm forced to make my own.
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Michael_linn
Tinkerer
Username: Michael_linn
Post Number: 34
Registered: 04-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2011 - 06:18 pm: |
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I have not tried this yet on glass, but craft shops such as Michael's sell solutions which will etch a frosted surface on glass. Something like Armour Etch should do the job. |
Br1078lum
Tinkerer
Username: Br1078lum
Post Number: 164
Registered: 11-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2011 - 10:12 pm: |
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Lay the glass on a thin bed of fine sand and water, and move it around in a varying pattern. That will give you the proper surface for focusing.
PF |
Mr_flibble
Tinkerer
Username: Mr_flibble
Post Number: 10
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 19, 2011 - 11:50 pm: |
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sello and scotch tape works well enough on a smaller scale (like viewfinders for box cameras). I would imagine that anything bigger than a 35mm frame would be too much.
The armour etch sounds like an interesting idea to try some time. |
Hanskerensky
Tinkerer
Username: Hanskerensky
Post Number: 122
Registered: 05-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 20, 2011 - 04:35 am: |
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Another way is try to get a vintage glass plate film, develop till you have a light uniform image or sand it as mentioned before. |
Mareklew
Tinkerer
Username: Mareklew
Post Number: 250
Registered: 03-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 20, 2011 - 11:26 am: |
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Go to the dentist of your choice, must be a friendly one, mind it, and ask, where do they get their mixing glass from. These plates are used for mixing paste that the tooth fillings are made with. They come cheap, I paid for a small one something about $0.5.
I don't know what the thing is called exactly, it's very good quality glass with a rough etched surface that's perfect for focusing and is almost undercut-free, i.e. it has no cracks going below the surface (you don't want grains of glass getting loose from it when you mix a filling on it).
YMMV, I don't know if you can get it in the size you need.
Marek |
Logandiana
Tinkerer
Username: Logandiana
Post Number: 19
Registered: 04-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 20, 2011 - 06:24 pm: |
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I don't personally own one, but I have seen some up close before and they are nice.
http://www.hopfglass.com/ |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 11
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 20, 2011 - 11:09 pm: |
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Thanks for the tips! I think I'll start with the etch and see how it works, since that probably requires the least effort - I will post results when I have them. Another thought is sandblasting, but I'd have to find a shop willing to do a quick odd job for close to nothing as a favor - simpler to try the etch first. |
Tamas
Tinkerer
Username: Tamas
Post Number: 4
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 20, 2011 - 11:16 pm: |
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Jeffk,
Execution frosted glass.
Link:
1 - http://www.astronomy.ro/forum/files/sticla_mata_596.png
2 -http://www.youtube.com/watch?v=fkPlUM37j60
3 - http://www.apug.org/forums/forum216/78392-making-ground-glass-focus-screen.html |
Tamas
Tinkerer
Username: Tamas
Post Number: 5
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 20, 2011 - 11:49 pm: |
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With little change can be polished scratched lens surfaces.
I mean that the lens surface not treated with anti-reflection layer.
 |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 13
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, October 22, 2011 - 09:44 am: |
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The Armour Etch worked pretty well, thanks for the tips! Go outside next to a garden hose, slop the stuff on one side of the glass with an expendable butter knife, wait 5-10 minutes, hose it off, then clean it with Windex. Much better than scotch tape, though the frosting is still motled on fine scales that show up with a good magnifying glass. I'll experiment a bit more with sanding, but within the wide-open depth of focus I can probably already get it as good as it can be. |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 14
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, October 22, 2011 - 10:49 pm: |
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Follow-up, a light sanding with 2000-grit sandpaper makes it even better! Works very well. But, checking the focus with three other cameras (Contax IIIa, Super Ikonta 531, Exa Ia) against the lens settings and the internal ground glass on the Exa SLR, I see that systematically when the cameras say they should be in focus, the ground glass at the back is a bit off. For example, in all three cases, a cabinet 25 feet away is out of focus unless I set the lenses to infinity. Any ideas why this would be? One camera, I could blame the camera, but three of them by the same amount in the same direction? |
Hanskerensky
Tinkerer
Username: Hanskerensky
Post Number: 125
Registered: 05-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 23, 2011 - 01:39 am: |
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Except for the obvious (matt surface on wrong side) i can only think of wrong place to rest your groundglass on the filmgate, warped groundglass, etc.. So just everything you can think of what influences the lens-groudglass distance.
I had something similar with my autocollimator were i had to press a mirror against the cameras filmgate. In the end i glued mirror-tape on a film and loaded that in the camera so the back could be closed and the film pressure-plate could do its game. The overall results were just better. |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 15
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 23, 2011 - 10:40 am: |
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Hmm, perhaps the glass is not in exactly the same plane as the film would be - to make sense, the glass would need to be about 100 microns back away from the lens compared with the film, which is small (and much, much smaller than the glass thickness, I can definitely make things blurry by flipping the glass plate ;-)). Not sure why it's systematically off in three cameras, though the plate is rather large compared with the 35 mm and 120 film format (it's about right for the 9x12 Nixe). Have to dig deeper here... My first thought was that all three cameras are accomodating the film coating layer thickness (in front of the emulsion), but that's gotta be much less than 100 microns, and more importantly the correction would go the other way. |
Hanskerensky
Tinkerer
Username: Hanskerensky
Post Number: 126
Registered: 05-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 23, 2011 - 03:13 pm: |
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The emulsion side of the film is allways the reference side so the guidings of the filmgate are on that same reference plane. The pressure plate ensures that the film (no matter what thickness it has) is pressed against those guidings .
So the matt (frosted) side of the glass should be hard pressed against those 2 guidings. On a 35mm camera these guidings are the 2 shiny rails upwards and downwards of the filmgate.
If your glass is that large probably another feature of the camera (i.e. a roller or a sprocketwheel) doesn't allow it to lie flat against the guidings. |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 16
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 23, 2011 - 04:49 pm: |
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Fixed, I simply hadn't been careful enough at the 100 micron level. The only camera of the three that lets me use this plate without *any* obstructions at all is the Contax IIIa, and if I'm careful with that camera and avoid the sprocket with the plate then the focus makes sense.
Note I was referring to the thin plastic layer on the film that's in front of the emulsion - the emulsion is only right at the surface on uncoated films that probably aren't sold any more, and were used for soft x-ray/vacuum ultraviolet light detection (for example Kodak 101-07). In all other films, the actual detection layer is behind a protection coating. |
Tobias_f
Tinkerer
Username: Tobias_f
Post Number: 4
Registered: 06-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 30, 2011 - 07:58 am: |
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Sorry to pipe in rather late. I remember Emmet Gowin telling me, years ago, that he used to grind his GG with 1000 grit carborundum (silicon carbide), the method would be to get two identical sheets of glass, to size, and possibly protect one side with contact paper, or plastic sheeting and waterproof tape, put a pinch of the carborundum with water between the two sheets, and slide them in circular movements until the surface is even.
Subsequently, I have purchased a tin of "valve honing paste", which is easier to get than small amounts of carborundum, and usually a tin has two grades, one for honing, and one for polishing the seat of a valve in an engine. Experimenting with these should give you good results also.
Emmet also had the habit of cutting a small hole, say 6mm, in the centre of the GG so that he could focus more accurately with a lupe, simply on air.
t |
Tamas
Tinkerer
Username: Tamas
Post Number: 12
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 30, 2011 - 09:40 am: |
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"Emmet also had the habit of cutting a small hole, say 6mm, in the centre of the GG so that he could focus more accurately with a lupe, simply on air."
It is an old idea.
And I was thirty years ago but was not boring.
I used hydrofluoric acid to make glass mat.
In the middle was covered with some wax.
So I got a frosted glass with transparent center.
As magnifier i used a lens of f = 100 mm acromat.
The camera was a Voigtlander - Bergheil 9x12 lens Heliar f = 150 mm. |
Scott
Tinkerer
Username: Scott
Post Number: 167
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 30, 2011 - 11:49 am: |
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How do you focus "on air"? Where is the image in this case?
A rigid piece of plexiglass is what I use, sanded matte on on side. About 2-3mm thick. Easy to cut to size with a hacksaw. Near the edges, I glue a couple of bits of plexiglass to provide a raised area with which to anchor the glass in place with rubber bands. |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 23
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 30, 2011 - 05:41 pm: |
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The image is in a plane in space behind the lens, so you can focus on that plane with a magnifying glass or eyeloupe if you know where the plane is supposed to be (where the film will be when you load it) and then change the lens focus until the two planes match. Trouble is you have no idea where the plane is supposed to be without putting something else in that plane and scattering light far outside the numerical aperture of the lens, hence the ground glass.
You could place a clear hole in the middle of the ground glass, but it's actually not clear to me that this helps, and it only could help if you use the ground glass to focus an eyeloupe on the rear surface of the frosted glass and then keep it focused in the same plane. You're also limited by the numerical aperture of the camera lens, probably much smaller than that of the eyeloupe or magnifying glass, and you have to look directly through the lens, so you are forced to put your eye in a particular place and look in a particular direction in order to focus the lens.
I dunno. |
Scott
Tinkerer
Username: Scott
Post Number: 169
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 30, 2011 - 07:59 pm: |
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I have never seen or heard of an image in "the air." Sounds creepy to me!
The glass provides the plane upon which the image is diffused into a 2-dimensional image.
As for the aperture, open it as far as it will go-- if it is focus accuracy that you want. You want as small a DOF as pssible when you are calibrating the focus, because if you put it at f16, then almost everything will appear more-or-less in focus, and you will have a very hard time deciding where to set the lens foucs. |
Jeffk
Tinkerer
Username: Jeffk
Post Number: 24
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 30, 2011 - 08:19 pm: |
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Without the ground glass, you have a Keplerian telescope system - the camera lens is the objective, the magnifying glass or eyeloupe is the eyepiece. Just as with a telescope, you have to adjust the eyepiece position to get a sharp image. When testing a camera lens, you have the added constraint that the image between the objective and eyepiece has to be in a particular place, which is where the film is supposed to go. |
Michael_linn
Tinkerer
Username: Michael_linn
Post Number: 40
Registered: 04-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, October 30, 2011 - 08:35 pm: |
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"Aerial Image" focusing has been around a while in photography, particularly in motion picture cameras. As it has already been described in this thread, I'll just sum things up by saying that the focus does not involve an accurate approximation of focus at the film plane as the human eye and it's ability (particularly in younger eyes) to force a focus by itself makes for inaccuracy. |
Connealy
Tinkerer
Username: Connealy
Post Number: 57
Registered: 02-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, October 31, 2011 - 07:41 am: |
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Accurately collimating a lens with even the best ground glass is pretty close to impossible in my opinion. Save yourself a lot of trouble and time by using an slr to focus on a target through the front of the lens with an slr. Mike Elek and Rick Oleson have good articles illustrating the technique. Using a moderate-length telephoto on the slr makes the process a little easier. |
Steve_s
Tinkerer
Username: Steve_s
Post Number: 190
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, October 31, 2011 - 08:27 am: |
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I agree that focusing a lens on a ground glass can be very difficult, particularly if the lens aperture is relatively small, but it has always seemed to me that using a second camera as a collimator simply introduces a whole bunch of extra potential errors.
I'd use a good ground-glass and a well-lit subject for medium format, or for 35mm, a viewfinder screen from a scrap SLR (a microprism screen or a split image rangefinder, though with the latter you have to be very careful that it is positioned and viewed exactly on axis). |
Tamas
Tinkerer
Username: Tamas
Post Number: 13
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, October 31, 2011 - 10:54 am: |
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Bringing infinity for cameras:
https://kyp.hauslendale.com/classics/forum/messages/2/22762.html?1319276033
Last post. |
