A ground glass focusing screen is easy to make. It is one of those jobs where you can’t really go “too far”. So with some time, patience, and a moderate investment in supplies you can make your own screen in around 30 mins.
Prefer to buy one? I aim to keep a few ground glass screens available for purchase.
Equipment
400 Grit Silicon Carbide (or Aluminium Oxide) abrasive powder
Glass cut to desired ground glass size. 2mm thickness suits many cameras. You can do this with a glass cutter or ask your local glazier.
Glass cut to approximately half the size of the main ground glass. This will be your “rubber”. HINT: Make this a useful size so that you can have a smaller ground glass as a bonus. Grinding a whole plate GG? Make the “rubber” a half plate size.
Block of wood. Palm sized.
Double sided tape. Medium tack.
Water. Ideally held in a squirty bottle.
Kitchen towel (tissue).
A flat board. Optional Heavy granite cutting board.
Setting up
Cover your worktop for protection against water.
Place a sheet of kitchen towel onto a flat board, ideally a granite cutting board.
Add a few drops of water to stick the paper to the board.
Place the large glass sheet on top. It will “stick” to the board.
Attach block of wood to the smaller glass sheet. This is your rubber.
Sprinkle a small amount of abrasive powder onto the large glass sheet.
Squirt some water on.
Process
Apply circular motions and figures of eight until completion.
You will need to reapply abrasive powder and water as necessary.
Turn the glass around (not over!) every so often to even out your left or right handed bias.
Check your progress by rinsing and drying the screen.
For the final wash use a soft sponge and dish detergent. Stand on edge with kitchen towel underneath to wick away water.
Sounds easy doesn’t it?
A Few Key Points
Assess your progress only when the glass is dry. The ground glass screen will appear transparent when wet. Give it a rinse every 15 minutes or so and wipe dry.
Every ground glass is a compromise between brightness/apparent illumination and fine grain size. Many old ground glasses have a very milky appearance and whilst the image shown on the ground glass is apparently bright corner-to-corner, it is almost certain that the coarse grit used to achieve this has caused pitting and chipping that makes a less than ideal focusing surface. On the other hand if 1200 grit is used the grind will be too fine and you will see the rear element of the lens as a hotspot through the ground glass. For this reason I recommend a 400 grit abrasive powder.
Not all abrasive powders are the same. Lower quality powders may be contaminated with coarser grits leading to scratches. I have always been happy with Veritas Silicon Carbide Lapping Grit.
It might seem like a good idea but don’t be tempted to use an oil-based valve lapping compound. Oil is messy. This would make it harder to check progress.
The smaller glass sheet used as a rubber should be around half the size of the large glass you are grinding. Much larger and the sheets will stick together very easily and your grit-water slurry will be lost off the edges more quickly. On the other hand a much smaller rubber will take longer and perhaps have a tendency to “dish” the main glass, making it a tough job to easily grind the edges. As mentioned above make this smaller glass a useful size so that you can have a smaller ground glass as a bonus. Grinding a whole plate ground glass? Make the “rubber” a half plate size.
Can I tape the glass sheet down with double sided tape? You could, but this would make checking progress more difficult.
I hope there are some useful pointers in there. Feel free to get in touch if you have any questions. Alternatively you can purchase a ready-made ground glass.
