Explain like I'm 5.......... GG thicknesses

**BennoLF** · 2-Mar-2020, 12:15

Hey y'all,

I'm travelling to NM this summer for 3 months and bringing my 4x5. I'll be doing lots of backpacking so I'm looking to save weight and make my kit as rugged as possible.
I have a question about groundglasses. Does the thickness matter for accurate focus? I'm thinking of making a spare out of some styrene in case I break my GG in travel or a fall. They're not quite the same thickness. It seems to make sense to me that if the lens-side of the GG stays the same the thickness shouldn't matter, i.e. how far back the user-side of the GG extends. There are obviously other considerations such as sharpess, brightness, etc. All I'm wondering about at the moment is whether or not the focus would be affected by a thinner GG. I'm fairly new to LF so explain like I'm stupid haha.

Benno

**RichardRitter** · 2-Mar-2020, 12:23

No thickness does not matter. The image is formed on the ground/ frosted side.

**Bob Salomon** · 2-Mar-2020, 12:27

If you have a Technika or TK 23 it is very critical and the back has to be adjusted if it is not the proper thickness.
If it is most other cameras it doesn’t matter how thick other then it might be too loose if too thin for the hold downs or it might be too thick for the hold down screws. If the first you might have to add shim washers under the screws. This might make any weight saving negligible. Or use shorter screws, if you can find the. If too thick you might need longer screws.

OTOH one of the acrylic bright combination gg/fresnel screens might be lighter then what you have now.

**Mark Sawyer** · 2-Mar-2020, 12:28

You're right, the thickness doesn't matter. What matters is that the ground plane is in the right place, so make sure your spare gg seats correctly. Also, if it's much thicker, be sure the clips and screws that hold it in place are adequate. And for a backpacking camera, gg protectors are cheap and easy to make and provide pretty good protection for your original gg...

**Mark Sawyer** · 2-Mar-2020, 12:32

Originally Posted by Bob Salomon

If you have a Technika 23 it is very critical and the back has to be adjusted if it is not the proper thickness.

Why is that, Bob? The focal plane remains in the same place, and the inner surface of the thinner gg rests on the same plane as the thicker gg...

**BennoLF** · 2-Mar-2020, 12:45

Ok sick, thanks y'all! Now just to figure out how to lug a 4x5 and film around on my back..........

**Bob Salomon** · 2-Mar-2020, 13:17

Originally Posted by Mark Sawyer

Why is that, Bob? The focal plane remains in the same place, and the inner surface of the thinner gg rests on the same plane as the thicker gg...

There are adjusting screws on top and beneath the gg on the 23. If you install a gg that is thicker then the original one and adjust the wrong ones everything will be out of focus.
George Tice learned that the hard way while shooting with his TK23 in Maine. He broke the gg one day and went to a local repairman who improperly installed the wrong gg. Everything he shot on the rest of the shoot was out of focus!
When he found out he immediately had Marflex fix it and he bought a new, spare, back in case he did this again.

**Mark Sawyer** · 2-Mar-2020, 13:39

Originally Posted by Bob Salomon

There are adjusting screws on top and beneath the gg on the 23. If you install a gg that is thicker then the original one and adjust the wrong ones everything will be out of focus.

Ah, that sounds like a different issue unique to that camera and a repairman who made a dumb mistake. Just don't adjust the screws that the frosted side of the gg rests on.