Dumb Question About Replacing a Ground Glass

[Frank Petronio]

I guess this has always inspired unconfidence.... But if I replace a ground glass with another of a slightly different thickness, it doesn't matter because we are focusing on the ground side, on the inside of the camera. As long as the glass is flat and held firmly in place so it can't shift, no worries, right?

[Shen45]

Right :)

As long as there isn't a Fresnel.

The standard with just a ground glass is .190 inches [Give or take a small amount]

[Frank Petronio]

But it shouldn't matter how thick the glass (within reason) is on something like a Sinar, since the surface the glass hits or touches is machined into the back very acurately (I assume)?

[Shen45]

That's correct Frank. The "toothed" surface is where the image forms and it will sit in the same register as the film plane when it is moved out the way to insert a film holder.

[Frank Petronio]

Then, why doesn't everyone just put their fresnels on the outside of the ground glass (to the photographer, not the lens) and stop worrying about focus shift and calibration?

[Nathan Potter]

The fresnel should go between the ground glass and the photographer. It also works between the ground glass and the lens but since the in focus image plane appears on the frosted side of the ground glass you would need to add a shim of thickness equal to the thickness of the fresnel to the image side of the holder to capture the focus point on film. I've never seen much difference in fresnel brightness between in front of or behind the ground glass. Ease of use dictates a fresnel behind the ground glass. Of course with any arrangement the fresnel must be removed from the image path when taking the picture.

Nate Potter, Austin TX.

[Ken Lee]

The Sinar has ledges of sorts, where the rough end of the ground glass needs to rest, and where the small plates fasten it to the camera back. As long as the rough side is sitting there, with the rough side facing the lens, the rest of the glass can be as thick or thin as you like.

I just got a 5x7 plain ground glass for my Sinar, from PhotoFinder (http://shop.ebay.com/merchant/photofinder_W0QQ_nkwZQQ_armrsZ1QQ_fromZQQ_mdoZQQ_sopZ1) on eBay, for a very reasonable "Buy It Now" price. It works just fine.

I am glad to be rid of the lines, but I am keeping my old GG in case I need to shoot something architectural.

[Kirk Fry]

"The fresnel should go between the ground glass and the photographer. It also works between the ground glass and the lens but since the in focus image plane appears on the frosted side of the ground glass you would need to add a shim of thickness equal to the thickness of the fresnel to the image side of the holder to capture the focus point on film."

Graphics are desided to have the fresnel to go between the gg and the lens. The bummpy part next to the GG. The backs are machined for the thickness of the fresnel. So if you take the frensel out you need to shim the ground glass for the thinkness of the fresnel.

K

[Myron]

So who knows how to grind your own glass???

[C. D. Keth]

So who knows how to grind your own glass???

I've done it. It's very simple. The glasses I ground myself have been the ones I was happiest shooting with, too.

[Rafael Garcia]

So who knows how to grind your own glass???

Not difficult: Try grinding a Dobsonian mirror!

[C. D. Keth]

Not difficult: Try grinding a Dobsonian mirror!

I'll pass. I'm patient but not that kind of patient.

[Nathan Potter]

Myron, I've ground a lot of glass for various purposes. You can start with a piece of regular window glass which is slightly greenish soda lime, so called. Better yet use borosilicate glass which is crystal clear to the eye (but a bit hard to come by).

Preferably start with a piece slightly larger than what you need to end up with and after frosting cut to exactly the right size. Learn to cut glass by practicing on junk pieces.

Lay the unground glass on a soft cloth on a flat surface. Add several drops of water then add grit and mix to the consistency of a rather liquid flowing slurry. The grit you need to buy from a lapidary supply house - try on line suppliers. Grit comes in numbered sizes and made of different materials. Silicon carbide is one type, alumina another and etc. The hardness of each is slightly different but that's no big deal. I like silicon carbide. I'd say you will be looking for a grit size between 150 to say 800. The lower the number the coarser the grit and the rougher the screen you end up with. Generally the higher the number the finer the screen and the less bright the image until you get to grit so small, > than say 800, where you'll start to polish clear areas.
I like around 360 grit for general purposes.

Once you've got your slurry on the glass use a very flat holder of hard material, I use another piece of glass about 2 X 2 inches with a small handle epoxied on the upper side for grabbing. With circular motion and always keeping some of the slurry between the holder and screen grind away. A 6 by 7 inch piece might take 15 minutes of grinding. Grind uniformly out beyond the eventual size that you will use so as to avoid edge uneveness effects. If the grinding handle seem to start sticking you'll need to add more grit and water. The amount of pressure you apply while grinding will have some bearing on the size and depth of the microfractures you cause in the surface of the screen. It is these microfractures which scatter the light from the lens and cause a real image to be formed. The sidewall angle of the fractures determine in part the brightness of the viewed image. I'd say I usually grind at about 1 to 2 lbs/sq. inch pressure. You'll have to experiment a bit but it's really pretty simple stuff.

Nate Potter, Austin TX.

[C. D. Keth]

Preferably start with a piece slightly larger than what you need to end up with and after frosting cut to exactly the right size. Learn to cut glass by practicing on junk pieces.

Nathan, I guess now is a good time to ask something I've been wondering about. Is there a trick to cutting glass once it's been ground? A couple years I ground a new glass for my 5x7 thinking I'd cut it down after I ground it and I screwed up the cut and had to make another. I just cut that one first but I tried cutting the piece of glass I had been grinding with and I couldn't get a nice straight cut on that either. I've cut a lot of glass for framing without problems but that frosted glass gave me all kinds of problems.

[SaveBears]

Christopher,

Make sure you use a straight edge and score on the non-ground side of the glass, then a quick snap, and you should be good to go..