Why is ground glass

[Pfiltz]

ground? Probably a really easy answer, but why do LF / ULF have ground glass for backs, instead of lets say, "clear glass"?

TIA

 

[pdeeh]

how would you focus using perfectly transparent glass?

 

[Vaughn]

The light has to hit something for it to form an image. We focus the light, via the lens, so that a sharp image (or at least the image we want) is on the GG, which occupies the same plane the film will be on.

 

[Ken Nadvornick]

Yes, for focusing and composition (it's also your viewfinder). When you pull the back away and slide your film holder into place for the exposure the film plane then occupies exactly the same position as the ground (inside) surface of the glass. So if it's in focus on the glass, it's in focus on the film. Or should be, anyway, if everything is correctly aligned.

Ken

 

[Pfiltz]

ahhhhhhhh

got it....

ty

 

[Gerald C Koch]

Clear glass would not form an image.

 

[Pfiltz]

I should have thought about it more, before asking, because it sounds so simple...

 

[Ken Nadvornick]

No, because like most things it's only simple if you already know the answer. And because you asked, there's probably a hundred others out there who just had their exact same question answered. They were just to timid to ask it themselves.



Ken

 

[BrianShaw]

There is no such thing as a stupid question...

 

[Vaughn]

There is no such thing as a stupid question...

But on the other hand, stupid answers...

(for example mine -- neither very clear, precise, nor informative)

 

[Pfiltz]

LOL@Vaughn...

It's all good mate.

 

[AgX]

how would you focus using perfectly transparent glass?

With an optical system that brings the aerial image in the focus plane into my eyes. Good for focusing, not practical for viewing large format images.

A ground glass disperses the incoming rays, making them directly visible at the focus plan.
However the dispersion takes place around the angle of the incoming rays, and is typically not sufficient to give a bright enough image at the edges of the frame. In order to avoid to avoid bending ones head continuously from one side to the other, a field lens is used to make the outgoing rays bend towards the photographer's head. Typically these lenses are flat lenses of the Fresnell type. Also the coarseness of the ground glass is of influence: the more course the better the dispersion and the more even the illumination, but the less the resolving power and the lower the luminance.

 

[lxdude]

With an optical system that brings the aerial in the film plane into my eyes. Good for focusing, not practical for viewing large format images.

To add to this, some focusing screens do offer an aerial focusing spot, or a clear screen with parallax focusing. Good for very high magnification or other situations where there is extremely shallow DoF, and especially useful where any groundglass would scatter too much light.

 

[Marc B.]

The ground surface on the GG is often referred to as, 'the tooth.'

3-M, 'Scotch Magic Tape' has a tooth:
Dead Link Removed
http://www.flickr.com/photos/midnighttoadstool/3475037320/in/photostream/
http://www.flickr.com/photos/midnighttoadstool/3475037434/in/photostream/

Slightly milky, Vinyl page protectors/document sleeves have a tooth:
http://salihonbashome.blogspot.com/2011/08/grinding-ground-glass.html

 

[AgX]

But I don't think "tooth" is a technical term.

 

[Old-N-Feeble]

The question is too diffuse to answer... or is it too direct?

 

[horacekenneth]

I was taught that there are stupid questions and that if I didn't open my mouth right away I could've answered it for myself. Then again gg still seems a little bit like magic to me, but of course it is.

 

[lxdude]

The question is too diffuse to answer... or is it too direct?

Now that you've made light of it, I'll just say it's good to screen the answers. Not to say you're out of the depth of the field, but focusing on the subject, it's clear that clear is not as a rule the clearer choice- usually a matte glass is a better ground for focusing.

 

[Old-N-Feeble]

Oh crap... now I'm visually kunfyoozed.

 

[mark]

Actually I think it is ground because the grinding produces a finer image in the end than chemical etching. At least this is what was explained to me by a person who ground the glass.

 

[AgX]

But finer means more uneven illumination of the image...

 

[E. von Hoegh]

ground? Probably a really easy answer, but why do LF / ULF have ground glass for backs, instead of lets say, "clear glass"?

TIA

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

 

[Old-N-Feeble]

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

 

[lxdude]

Oh crap... now I'm visually kunfyoozed.

Didn't mean for it to blow up in your face, but you did light diffuse!

 

[ajuk]

Really stupid question, is this a joke?