Why is ground glass

[Old-N-Feeble]

You can't focus on clear glass. Cut a sheet and try it.

Let it collect dust on one side for a few weeks or months, depending on where you live, and install with the dusty side frontward.

 

[NedL]

OK well I have a related question. I'm planning to build a fixed focus 5x7 camera for paper negatives. The main two features will be:

1. A front "lensboard" so that I can swap a lens and a pinhole.
2. A compartment behind the "film" plane, to hold exposed and unexposed paper negatives, so that it will be easy and quick to swap in my changing bag.

The lens I'm planning to use has a 114 mm focal length and has a scale-focus on the front element. It doesn't quite cover 5x7 but it's pretty close, and the edges are not empty but just vignetted. I think it will look neat and I have the lens available in hand right now.

Last night I started. There was a cardboard box sitting on my desk and I realized it was about 4.5 inches wide. It was white inside. So I punched a hole in the side of the box and stuck the lens in place.

Having never done anything like this before, and never used a view camera, I was like a little kid when I saw the image form inside the box. I pointed it a different objects and lights and out the window and swung it all around. Then I got my daughter and showed her and she thought it was pretty cool too. Now you can all laugh at me: for some reason I was especially thrilled that the image was in color and that the colors were bright and easy to see. It's not that I expected it to be in Black and White or something, but I've been in a B/W mode lately and just didn't think about it ahead of time. I didn't expect the colors to be so bright an noticeable.

Anyway, as I pointed it at different objects, I was able to bring the image into focus by twisting the scale focus. It seems to me that if I'm careful and use a magnifying glass, I ought to be able to build my camera and set the focus just this way -- by looking into the camera from the top and aiming at a bright distant object. I never expected the image to be so bright and clear and easy to see. Is there some reason why I'd need to use ground glass or vellum or waxed paper or something? Something maybe to do with the angle of view? Otherwise I can't think of any reason not to just focus the projected image from the inside on a piece of actual photo paper so the width will be exactly right.

( my normal mode would be to "just do it", but I noticed this thread so you all can warn me if I'm not considering something. )

Ned

 

[E. von Hoegh]

OK well I have a related question. I'm planning to build a fixed focus 5x7 camera for paper negatives. The main two features will be:

1. A front "lensboard" so that I can swap a lens and a pinhole.
2. A compartment behind the "film" plane, to hold exposed and unexposed paper negatives, so that it will be easy and quick to swap in my changing bag.

The lens I'm planning to use has a 114 mm focal length and has a scale-focus on the front element. It doesn't quite cover 5x7 but it's pretty close, and the edges are not empty but just vignetted. I think it will look neat and I have the lens available in hand right now.

Last night I started. There was a cardboard box sitting on my desk and I realized it was about 4.5 inches wide. It was white inside. So I punched a hole in the side of the box and stuck the lens in place.

Having never done anything like this before, and never used a view camera, I was like a little kid when I saw the image form inside the box. I pointed it a different objects and lights and out the window and swung it all around. Then I got my daughter and showed her and she thought it was pretty cool too. Now you can all laugh at me: for some reason I was especially thrilled that the image was in color and that the colors were bright and easy to see. It's not that I expected it to be in Black and White or something, but I've been in a B/W mode lately and just didn't think about it ahead of time. I didn't expect the colors to be so bright an noticeable.

Anyway, as I pointed it at different objects, I was able to bring the image into focus by twisting the scale focus. It seems to me that if I'm careful and use a magnifying glass, I ought to be able to build my camera and set the focus just this way -- by looking into the camera from the top and aiming at a bright distant object. I never expected the image to be so bright and clear and easy to see. Is there some reason why I'd need to use ground glass or vellum or waxed paper or something? Something maybe to do with the angle of view? Otherwise I can't think of any reason not to just focus the projected image from the inside on a piece of actual photo paper so the width will be exactly right.( my normal mode would be to "just do it", but I noticed this thread so you all can warn me if I'm not considering something. )

Ned

You do realise that you will be exposing the paper while you are focussing on it...

The groundglass is used to compose the image, as well as focus. You'll have an easier time composing on the GG, rather than viewing the image at a slant from the top or side.

 

[E. von Hoegh]

Actually, acid etched is better than ground and laser etched is even better. Grinding makes the chips too coarse which decreases fine focusing detail.

You can grind the glass to any degree of fineness you like, it depends mostly on the grit size of the grinding medium and slightly on the method of grinding.

 

[NedL]

You do realise that you will be exposing the paper while you are focussing on it...

The groundglass is used to compose the image, as well as focus. You'll have an easier time composing on the GG, rather than viewing the image at a slant from the top or side.

Ha!! I wasn't clear. The camera will be fixed focus. I thought I'd start by focused on infinity. Over time I can try moving the lens to get a reasonable hyperfocal setting. I meant to use a scrap piece of photo paper to set the focus one time only. After that I would be putting unexposed paper into it inside my changing bag.

Yes trying to focus on "live" paper inside the camera would be an extreme challenge to say the least!

But your point about composing the image is a good one.

 

[E. von Hoegh]

Ha!! I wasn't clear. The camera will be fixed focus. I thought I'd start by focused on infinity. Over time I can try moving the lens to get a reasonable hyperfocal setting. I meant to use a scrap piece of photo paper to set the focus one time only. After that I would be putting unexposed paper into it inside my changing bag.

Yes trying to focus on "live" paper inside the camera would be an extreme challenge to say the least!

Oops. Don't forget a viewfinder, the wire-frame type works well.

 

[NedL]

Oops. Don't forget a viewfinder, the wire-frame type works well.

Thank you! That's a very good idea.