The Online Darkroom Store

Thursday, September 11

Making a ground glass

By David M.


Regular commenter and correspondent, the mysterious David M., has sent me a piece he's written on the subject of making a ground glass for those people, like me, who struggle to see much on their existing one in dim light or have a broken one to contend with.

There aren't any pics to go along with the text but it's all self-explanatory thanks to David's way with words. It was my travails with the Speed Graphic in poor light that prompted David's intervention. I might yet have a go at making a new ground glass to see if it's any better than the Speed Graphic's original.

For those of you to whom the idea also appeals, here's how it's done. Take it away, David.

Sometimes the ground glass in a large format camera is rather dim. Or it might be cracked.

First, check that it’s clean. You can wash it in the way you’d wash ordinary drinking glasses in warm water and a mild detergent. There are only two things to remember. Firstly, there may be markings on the glass, for roll-film holders, or a grid to assist in composition. Make sure that it’s waterproof before you commit. Secondly, it’s best to rinse the glass in the cleanest warm water you can find and let it drain dry. Drying it with a cloth may deposit little fibres on the rough side, which you don’t want.

If this doesn’t work, or if the screen is broken, you can buy a new one. You’ll find that eBay is good for this.

My kitchen table. You'll see that I'm using valve grinding paste and 3-in-One oil.
Later I switched to WD40 and some anonymous aluminium oxide powder, which
gave a finer screen.

Or you can make your own. Not as difficult as it might seem. Surely, a precision machine like a large format camera needs something rather complicated? There are various hi-tech and hi-cost screens available, but they all have one thing in common – a surface that intercepts and scatters light.

You can make a temporary screen from ordinary glass cut to size with “invisible” tape stuck to the side facing the lens. It works, but it’s not entirely satisfactory. Even tracing paper will suffice in an emergency.

You can grind a new screen with almost any sort of abrasive powder. Valve grinding used to be common before motor-cars went all digital and you can still find it. I discovered some aluminium oxide on a website that has since vanished.

You need two sheets of glass, about 2mm thick. Clip frames are commonly sold in pairs and will serve very well. Get the next size up.

Now, using your skill and judgement, cut one of them to the exact size you need. It’s unlikely that you’re as bad at cutting glass as I am, but you need to get it right. Check the size again. If you want clipped corners, cut them off now. Using some coarse emery paper, grind down the edges until they won’t cut you. More importantly, they won’t wear grooves in your camera, either.

Check that it will fit the camera by putting it in place. Don’t omit this step. It’s quite hard to trim a millimetre off the edge of a piece of glass. How do I know this? Ha!

Cover a flat surface with layers of newspaper and put the uncut sheet on it.

Halfway. You can see that the glass from an IKEA clip-frame is not quite flat.
The actual difference is very small indeed – unmeasurable with a micrometer.
This shows how it looks halfway through, with various stages of grinding
convenientlyarranged in stripes. If you used a small block of thicker glass as
a grinding tool, this will probably not happen. It needs to be another piece
of glass. Anything else, either softer or harder, will give you problems.

Put a little pile of your abrasive on the glass with a spoonful of water, then put the other glass on top and shuffle it round and round. Change direction every few minutes. The aim is to get the grinding evenly distributed. I put a rubber sucker on the top glass to help with holding and pushing it. Some people use a smaller piece of thicker glass, which is rather easier to hold, as a grinding tool. You might prefer to do this and the end result will be the same.

Eventually, the grinding powder will grind itself down and stop cutting. Slide the top glass off (it will be hard to pull it upwards because of surface tension) and add another little heap of the powder and another spoonful of water. And back to the shuffling.

I have been deceiving you: I don’t use water. It evaporates and makes a sort of glue that makes the work harder. I use WD40 as a lubricant – smellier and muckier but it works well. It needs more newspaper to keep the table top clean.

Keep going until you think it’s done. It won’t be done. Wash and rinse off the sheet and let it drain. You can dry it with kitchen paper or a tea towel this time.

Hold it up to the light and try to look through it at some high contrast scene like a light source. Here and there, you’ll glimpse something visible through the milkiness. Note these places and put the sheet back on the lower glass and continue the shuffling. If you put your fingers on the thin spots as you push the glass back and forth, you can imagine that it’s helping. Eventually, it will really be finished and you can wash it carefully (carefully because you don’t want residual bits of grit near your lenses) and install it in your camera.

Some people suggest using a coarse grind first and then a finer one. There seems to be little point in this, other than to give you more work. If your glass is so bumpy that it needs coarse grinding, get some flatter glass.

Now we come to the bit where I can give you no advice and you will have to make up your own mind.

The general rule is that a coarse-ground screen will be brighter, but a finely ground one will be easier to focus. This is reasonable, as a very coarse grind will obscure the fine detail that we depend on for accurate focusing.

On the other hand, a fine screen will not seem as bright. Again, this is logical as an ultimately fine screen would have no texture at all and would therefore be transparent.

If you like having a grid or markings for smaller formats on your screen, you can draw them on the matt side with a fine clutch pencil or a fine pen. Make a template on paper first and trace onto the glass.

Now, fit your new screen and give it a go. Remember to place the ground side of the glass towards the lens and away from the eye. This is the standard way to do it. If you find it’s still too dim you can easily (Ha!) make another one, using your new skills, but with a coarser grind. If it’s hard to see fine enough detail, you might try a finer grind.

The screen on a camera. This is the view under the dark cloth. Admittedly,
it's a brightly-lit scene. I think this has the fresnel that I cut down from
something on eBay, probably part of an overhead projector.  Remember
overhead projectors? All that keystoning and boredom. At least Powerpoint
has dealt with the keystoning.

If all combinations seem dim, remember that your eyes will accommodate and the screen will get brighter if you stay under the dark cloth for a while, without popping your head out. You do use some sort of dark cloth, don’t you? If that fails, creep up on your piggy bank and… (the details are too horrible to write) …a newer and faster lens. A wide-angle lens inevitably seems darker in the corners and you might have to move your head around to see it all. There is a partial answer to this.

Fresnel screens: when the light from the lens strikes the screen, it is scattered in all directions, and not all of it is directed towards your eye. A Fresnel screen is simply a lens, working rather like the condenser lens in an enlarger that re-directs this light towards your eye. There are all sorts of expensive (and no doubt excellent) versions available, but you might find that one of those flat book magnifiers works quite well.

Some people find the pattern of lines distracting and there are two things you can do to help. Take care to adjust your loupe to focus exactly on the focal plane – the matt side of the glass – or you might construct your Fresnel so that it can be removed after you’ve composed the scene (when you need to see the whole screen) and precise focusing (when you don’t).

Don’t forget to load your holders and good luck.

1 comment :

Kevin Vinke said...

Excellent site 👍😎
I tend to take things apart and not throw thing away. I took apart a laptop screen and found two fresnel sheets( horizontal and vertical ) and a ground glass type sheet. This on its own or all together bonded on an anti reflection Perspex base make a good screen.