I've got an old Tachihara focusing screen for my 8x10 which is made of Acrylic with a glass grid on top (it's a tiny bit undersized and slightly scratched up). I'd like to make my own acrylic ground glass screen (so I can go out in the field and not worry about breaking a glass screen) but am unsure of how to get a good ground finish, can I just use a fine grade of silicon carbide or should I think about another grinding medium?
-
Thank You For Visiting... Please Click Here and sign up to contribute and reply to posts and download the resources.
Acrylic Ground Glass
- Thread starter jhotopf
- Start date
Hadn't thought of that. Maybe a thicker piece of acrylic then.
I built an acrylic ground glass for a 5x4 DIY camera, in 3mm clear acrylic, using a frosting spray to "grind" the "glass". From memory, and it was quite a few years ago, the stuff was used for dulling reflections when photographing glass objects. It works quite well under a dark cloth, and the thickness of the acrylic prevents any movement. A friend 3-D printed the surround from PLA+, using a film holder as a model.
- Joined
- Aug 14, 2016
- Messages
- 84
3mm acrylic should not flex at 4x5, but it might at 8x10. I doubt it, but there is only one way to be sure.
I have used frosting spray, but there are two caveats - make sure the spray is even and from far enough back to get an even coverage, and you can't get it off easily (if at all).
Hand lapping 8x10 means a really flat surface (probably glass), and using a large, firm, weighted backing on the acrylic to get an even pressure. I used to hand-lap geological specimens, and getting a flat smooth surface was as much an art as science.
I once (successfully) ground a 4x5 acrylic sheet using a large oscillating sander and fine carborundum sheet. The unit almost covered 4x5, so getting an even grind was not too difficult. Much more of a challenge at 8x10. Not my first choice, by any means.
Do you really expect to break the existing glass?
I have used frosting spray, but there are two caveats - make sure the spray is even and from far enough back to get an even coverage, and you can't get it off easily (if at all).
Hand lapping 8x10 means a really flat surface (probably glass), and using a large, firm, weighted backing on the acrylic to get an even pressure. I used to hand-lap geological specimens, and getting a flat smooth surface was as much an art as science.
I once (successfully) ground a 4x5 acrylic sheet using a large oscillating sander and fine carborundum sheet. The unit almost covered 4x5, so getting an even grind was not too difficult. Much more of a challenge at 8x10. Not my first choice, by any means.
Do you really expect to break the existing glass?
Graham, your last question brought an anecdote to mind.
John Blakemore was preparing to spend some time camping and photographing in a very remote area. He had similar worries and took several spare screens with him. All his spares were smashed, but the one in his MPP survived.
In any case, acrylics are quite soft, (hence the difficulty in grinding) and need protection from the abrasive effects of the loupe. All the plastic screens I have encountered are supplied with a thin sheet of glass to protect them from this.
John Blakemore was preparing to spend some time camping and photographing in a very remote area. He had similar worries and took several spare screens with him. All his spares were smashed, but the one in his MPP survived.
In any case, acrylics are quite soft, (hence the difficulty in grinding) and need protection from the abrasive effects of the loupe. All the plastic screens I have encountered are supplied with a thin sheet of glass to protect them from this.
Yes, broken 8x10 screens twice so far. Never broken a 5x4 though!