Whar are the implications of using a thicker ground glass than is standard?

[moggi1964]

I am going to create a new GG for my recent find, a half plate field camera from around 1900.

I have measured the current glass as being 1.5mm thick and wondered what the impact would be if I was to replace it with a 2mm or even 3mm thick piece of glass?

I appreciate it will require slight adjustment of the metal glass holders but I am more interested in the effect on accurate focusing. Am I right in thinking that I would be focusing forward of the image plane by the additional 0.5 or 1.5mm?

I am very new to LF so I may not have grasped this correctly.

Thanks.

[Oslolens]

The frosted/ground/matt glass is forward so focus will stay in place.

Unless of course the frosted glass is backwards and still calibrated to give correct focus, then every touch and fingers would have left a print or a scratch, and you would have wanted to turn it already.

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[moggi1964]

The frosted/ground/matt glass is forward so focus will stay in place.

Unless of course the frosted glass is backwards and still calibrated to give correct focus, then every touch and fingers would have left a print or a scratch, and you would have wanted to turn it already.

Sent fra min SM-G975F via Tapatalk

Thank you.

I might be able to get a piece of Borosilicate Glass to replace the piece that is in, it's a little more expensive. In your opinion is it worth it to spend about 25% more on this over plate glass?

[pjd]

Thank you.

I might be able to get a piece of Borosilicate Glass to replace the piece that is in, it's a little more expensive. In your opinion is it worth it to spend about 25% more on this over plate glass?

Maybe Borosilicate is harder to grind - if you are grinding the glass yourself. I bought a borosilicate GG years ago and I think the seller mentioned at the time it takes longer to grind. Since then I've started cutting and grinding my own glass, I use glass from cheap picture frames.

About the earlier questions, thicker glass will make it a bit heavier of course, but the glass should be a bit less likely to break if knocked. I used quite thick glass in a Triple Victo (the glass was just what I had to hand at the time, thick glass from an old picture frame), that hasn't caused any problems.

[Steven Tribe]

The only consequence is that the camera is a little heavier and the brass tab mounting system may need a little adjustment.

[Tin Can]

I have several VGC Cycle Cameras that came to me with very thin OE ground side facing away from lens

Matching period holders collaborate 'T' so I think it was purposeful

Perhaps in 1895 this issue was not settled, I equate this with the Fresnel issue where some cameras varied particularly Horseman as I have example with both OE positions on 8X10

I seldom touch any GG, except with a plastic loupe

The frosted/ground/matt glass is forward so focus will stay in place.

Unless of course the frosted glass is backwards and still calibrated to give correct focus, then every touch and fingers would have left a print or a scratch, and you would have wanted to turn it already.

Sent fra min SM-G975F via Tapatalk

[moggi1964]

Thank you everyone for your thoughtful contributions.

I'll have a go at the Borosilicate and see how it goes. I have ground a few small telescope mirrors over the years and enjoy the relaxing nature of it. I'm not in a rush either as I still need to fix the bellows.

I'll report back of course.

[Doremus Scudder]

Despite what Tin Can says, it seems illogical to me that a camera design would intentionally place the ground side, i.e., the viewing/image forming surface, of the ground glass away from the lens. That would mean light from the lens would have to pass through the clear part of the glass before forming an image, which would result in a focus shift caused by the glass thickness. Yes, that could be designed in, but it's so much easier and intuitive to just have the ground glass facing the lens that I imagine that's what was standard practice both then and now.

So, IF your camera is designed to have the frosted surface of the ground glass facing the lens, then replacing the original ground glass with a thicker one should make no difference as long as the plane of the frosted surface is in the same position as the original.

If, on the other hand, I'm wrong and your camera really was designed with the frosted side of the ground glass facing the photographer, then you need ground glass of exactly the same thickness (and refractive index!) to avoid a focus shift. Still, I doubt that this is the case.

Knowing the original configuration of your camera would certainly help...

Best,

Doremus

[moggi1964]

Despite what Tin Can says, it seems illogical to me that a camera design would intentionally place the ground side, i.e., the viewing/image forming surface, of the ground glass away from the lens. That would mean light from the lens would have to pass through the clear part of the glass before forming an image, which would result in a focus shift caused by the glass thickness. Yes, that could be designed in, but it's so much easier and intuitive to just have the ground glass facing the lens that I imagine that's what was standard practice both then and now.

So, IF your camera is designed to have the frosted surface of the ground glass facing the lens, then replacing the original ground glass with a thicker one should make no difference as long as the plane of the frosted surface is in the same position as the original.

If, on the other hand, I'm wrong and your camera really was designed with the frosted side of the ground glass facing the photographer, then you need ground glass of exactly the same thickness (and refractive index!) to avoid a focus shift. Still, I doubt that this is the case.

Knowing the original configuration of your camera would certainly help...

Best,

Doremus

Well until I get the opportunity to fix and test the camera it's going to be guesswork however from the research I have done, English cameras from that period seem to have gone mainly with frosted towards the lens.

I ordered the glass and I'll start work on the bellows this weekend. If the glass arrives in time I'll also start the 'grinding' process.

Thanks for everyone's helpful contributions.

[mmerig]

[QUOTE= Clip:

Knowing the original configuration of your camera would certainly help...

Best,

Doremus[/QUOTE]
You could mount a plate holder with a plate (or film holder with film) in it, measure the distance to its lens-side surface from a definite edge on the front standard, then see if the distance to the ground-glass surface, front or back, matches the distance to the plate (same idea as with film).