A ground glass discovery and scotch tape

[Douglas Henderson]

Some years ago, I bought a Cambo 8x10 studio camera from a dealer in the mid-west. Although reasonably well packed, the GG was broken during shipping. I just patched it back together with magic tape/scotch tape and made an unexpected discovery.

Where ever the tape lay on the glass on the inside of the ground glass against the ground surface, the resulting view through the ground glass was as crystal clear as if looking out a window. Focusing became a breeze in any light or through any dark filter. I put a small strip of tape in the center of the ground glass (on the inside) and four other strips near the GG corners (too close to the corners precludes seeing the image with a lupe). Focusing center and corner elements of artwork with a lupe of any kind is now so easy and accurate, I wonder why I haven't heard anyone else mention this trick.

Standard procedure now is to strategically place small patches of tape across the ground glass for studio copywork. All 8x10 copy is done using an Apo-Ronar 360mm--corner to corner sharpness is superb since the focus is much closer than infinity. Tape on the GG of a smaller 4x5 Graphic View using an Apo-Ronar 240mm works fine as well.

[aggibson74]

Interesting. If there are grid lines on the ground glass, can you still see them?

Al

[Douglas Henderson]

Al,

The tape doesn't mask the wider view at all--the grid lines are still visible. The taped portions of the ground glass are strikingly brighter, however.

To focus with lupe on the more corner tape patches, you have to set the lupe flat against the GG, but move it around to view the path of light from the lens--the reason you can't use tape right to the 8x10 edges.

I had questions if this distorted the true focus, but the film results show everything tack sharp.

Doug

[tom thomas]

Doug, wouldn't this be the equivalent of a fresnel lens in front of the GG?

Tonm

[Douglas Henderson]

tom,

I've used fresnel lenses--they brighten the image certainly, along with all the added concentric lines. But the small portion of the ground glass covered with a small strip of magic tape produces something more like a crystal clear viewfinder. I'm not sure how such a clear image is being projected through the GG, but it sure is.

You can contrast the GG image and the GG + tape image by viewing across the boundary between the two. Nothing could be much easier (or as easily reversed) as putting it to the test.

I guess I should mention that the tape is not being used together with a fresnel lens. I don't have fresnel lenses on my copy cameras. I doubt tape on the GG would have much benefit with field cameras, as the available light is so much greater and the image is much easier to see. But focusing with 1000 watt lamps in a studio can be a study in frustration--especially trying to see well enough to square the camera and focus over the whole image.

[Lachlan 717]

Douglas,

Do you apply the tape to the smooth or the ground side?

Thanks.

[kev curry]

Douglas, I just took the back off my Tech V and stuck a 10mm square of scotch tape to the inside of the gg.....Shit thats totally nuts man! The gg is ''crystal clear as if looking out a window''...no shit, you got that right!
BTW that's with a fresnel on top of my gg, the difference is eye popping!

[engl]

Interesting idea, I'm going to try it. The GG on my current camera is very poor. It makes sense to me that a material with better diffusion than the current glass would give better "focus snap" in the center, and should make the areas closer to the edges much brighter when looking at the GG "dead on".

The basic idea of "magic tape" is strong diffusion but very good transmission (it is basically clear once on the paper). I'd not be surprised if it is easier to make very fine patterns for diffusion easier the chemical way.

The thickness will create a focus shift. I read the tape is 2.2mil (I'm not sure if this applies to all their "magic tape"), which is 0.05mm. Film holder tolerances are 0.17mm so the shift is significant but not devastating.

[Tracy Storer]

It's brighter because the texture of the tape is finer than the texture of the glass, finer texture=brighter, but also = less even. (particularly with WA lenses)

[engl]

Also, if you wanted to avoid the tiny focus shift, you could put the same tape along the edges of the ground glass. This would move the GG back the same width as the tape thickness.

Since my current GG (from the 50s) is horrible I've considered making my own, but finding the supplies around here is not very easy. If the tape works out good, I guess I could cover the entire side of a piece of glass in magic tape I wonder how it compares to IKEA window frosting film...