Etching Ordinary Glass into Ground Glass?

[F4U]

What I want to do ischekt the infinity focus of a 120 folding camera to make sure it jives with the infinity stop and rangefinder. What I propose is to use the glass out of a cheap dollar store picture frame and etch one side, just well enough to use it as a temporary ground glass, and put a magnifying loop on it with a black cloth over my head and camera. Obviously a collimator would be perfect. But many a Speed Graphic has been home-done by a tripod and something far away. After that I have no further use for the glass. Any ideas on how to get the glass etched well enough or that simple check? Sandpaper, a sandblaster, or a jackhammer?

 

[BrianShaw]

Grinding with silicon carbide powder probably gives the best result. For quick and dirty, use some wet/dry sandpaper. Unless you happen to have some valve grinding compound in the garage.

Google Search

 

[reddesert]

You'll get better results if you search for DIY grinding ground glass - when people say "etching" they are often referring to chemical etching. There are a number of threads, videos, etc, on grinding your own ground glass. It's not hard to make a nice surface for focusing. You'll need two pieces of glass and some grit - I used 600 grit silicon carbide. That can be obtained at some stores that cater to rock hobbyists, tool shops, online, and so on. I think my box was $8/pound a while ago, a pound is enough for several lifetimes.

 

[koraks]

Unless you happen to have some valve grinding compound in the garage.

That's what I've always used.

And I third the suggestion of grinding, not etching. To etch glass, you either have to be very, very patient, or you have to use relatively nasty acids. Grinding is overall a safer and reasonably quick process. You'll notice the process accelerates as you 'break through' the shiny/smooth surface of the glass and get some purchase on it.

 

[Dan Daniel]

If all you want is a center splotch of grinding, you can probably get there with a sheet of 400 or 600 grit wet/dry sandpaper, some water, and patience.

 

[Donald Qualls]

I always used to take a flat piece of plastic from a CD jewel case and cover it with frosted adhesive tape ("Invisible" tape). Five minutes, if you have some CD blanks left over from when burning a CD made sense. A piece of clear glass will work too -- the tape forms the frosted surface you're after. Tape side toward the lens, please.

 

[radialMelt]

I recently tried Donald's suggested approach to calibrate a rangefinder and it was totally sufficient (not to mention cheap and easy to put together).

 

[ic-racer]

Yes, semi-transparent tape as suggested above.

 

[beemermark]

Scotch tape is your buddy here

 

[btaylor]

I was going to suggest Donald’s suggestion as well- I have used the Magic Tape solution as well. I have also used a processed piece of film which worked well for me in calibrating a view camera. I drilled holes through the divider of an old broken sheet film holder then inserted a piece of processed sheet film- the film acted like a ground glass and I could see precisely if the film plane was congruent with the ground glass.

 

[ags2mikon]

I have used 2 glass plates and fine valve lapping compound to do just that. If you get the water based type you can add a little water to it to keep it grinding smoothly.

 

[Romanko]

Does it need to be glass for a single use GG? A piece of clear plastic would do the job. You can grind/sand it lightly or use some translucent tape as others suggested.

 

[mshchem]

Back in the crazy days when you could get deadly poisons at your local store. An old camera magazine, 1930's early 40's recommended mixing Mallinckrodt roach powder (pure sodium fluoride) with glacial acetic acid to produce hydrofluoric acid.

Probably some 1200 grit carbide paper was pretty hard to find in 1939.

I like the Scotch tape method. No one needs to suffer for infinity focus.

 

[F4U]

Thanks. I had forgotten the tape method. Ill get a dollar store 2 1/2 x 3 1/2 inh picture frame and steal the glass out of it. I don't have any CD's to harvest the case. Although Ive got lots of vinyl records and reel to reel tape. CD's are just a fad that'll never catch on. Not here anyway.

 

[koraks]

Any other piece of transparent plastic will do fine just as well. For setting infinity focus you don't really need to use actual glass. I never do.

 

[Donald Qualls]

you don't really need to use actual glass.

The only advantage glass has in this use case is rigidity, and it's easy to avoid flexing a piece of stiff plastic if you pay attention.

Still another method that doesn't require glass or plastic at all is to pull the thread taught between the film rollers on either end of the frame gate, and watch the aerial image -- when the thread is in focus at the same time the aerial image is, you're calibrated.

 

[F4U]

The only advantage glass has in this use case is rigidity, and it's easy to avoid flexing a piece of stiff plastic if you pay attention.

Still another method that doesn't require glass or plastic at all is to pull the thread taught between the film rollers on either end of the frame gate, and watch the aerial image -- when the thread is in focus at the same time the aerial image is, you're calibrated.

Thread? Aerial image? Can you explain or link me to such an experiment? I've never heard of it. Thank you.

 

[Donald Qualls]

Every positive lens produces an aerial image -- it's in the same location where a ground glass would intercept a sharp real image, but it's MUCH brighter. The image you'd see in a binocular or telescope eyepiece is an aerial image from the objective lens/mirror, magnified by the eyepiece. We don't usually use aerial images for focusing because our eyes will unconsciously focus on the image, even if it's not in the correct plane.

The thread (which is in the intended film plane) gives you something to focus on so you can tell whether the aerial image is in focus at the correct position. In a rifle scope, the crosshair does this job, but we usually adjust the eyepiece to focus on the crosshair and only look at subjects that are far enough away (minimum around 25 meters) to be within depth of field (rifle scopes tend to be fairly slow lenses), but some scopes have a focus adjustment on the objective to bring the aerial image to focus at the crosshair.

If you have a medium format SLR, you can take off the prism or hood and dismount the focusing screen and stretch a thread across where the screen goes and see the same thing. Focus your eye on the thread, and adjust focus until the subject is also sharp. Doing this at the focal plane in a folding camera or similar works the same way -- focus on the thread and adjust focus until your infinity target is also sharp.

This is also how grain focusers work for enlarging -- they intercept the projected image and put a crosshair or other reticle at the focal plane equivalent to the plane the base of the unit stands on. The positive lens in the eyepiece puts the crosshair at optical infinity, so your eye relaxes and you adjust the fine focus on the enlarger until the projected image (which, due to the focuser's magnification, will usually include visible film grain) is critically sharp.

 

[BHuij]

If your only goal is to use occasionally for collimating a lens, you might be able to get away with just using some slightly textured Scotch tape on the glass.

When I have made ground glass in the past for various DIY camera projects, I have tried valve grinding compound (not sure what the abrasive actually was in the matrix), wet-dry silicone carbide sandpaper, and powdered aluminum oxide abrasive with water between two sheets of glass. I've always just used inexpensive float glass from the hardware store.

I read an article once upon a time about making your own ground glass... and now I can't locate it. In the text though, the author said that mass produced commercial ground glass is usually done with silicone carbide grit, because it frosts the glass surface quickly and gives a decent image. But the really high-end expensive ones are done with the softer aluminum oxide. If hand grinding, it's a difference of several hours; the aluminum oxide is much slower. But silicone carbide chips flecks of glass out of the surface, while aluminum oxide does not. So if you put the time and elbow grease in, you will end up with a screen that can resolve much finer detail without sacrificing brightness.

I don't know if a single claim from that article is true. What I can say is that when I have made ground glass using silicone carbide, it came out perfectly usable. I'm pretty sure my commercial ground glass that came with my Intrepid 4x5 was made with silicone carbide, and I generally don't struggle to nail focus with it. When I recently made a 6x6 ground glass for a 3D printed technical camera that takes Mamiya/Graphic roll film backs, I decided to try the aluminum oxide powder instead. It did indeed take a lot of hours of grinding, but I think the screen might actually be a little nicer.

One of these days when I'm feeling bored, I'll grind a new 4x5 glass for my Intrepid with aluminum oxide. Maybe while I listen to a football game on the radio. Then I can do a true side-by-side comparison and see if one is significantly better than the other.

But I digress. Virtually any sandpaper in the range of 320-600 grit and a few minutes of scuffing, and I think you'd be able to make a suitable tool for collimating lenses at infinity. And indeed, try the Scotch tape first. If it's good enough, then you will have saved yourself 30 minutes of sanding