ground glass film plane

[Clay Turtle]

Boy, that sure sounds easy doesn't it? I wondered about the gg thickness so I tried some substitute glass while I was having a piece of glass (satin glass) prepared which may give you other considerations to take into account in preparation of your gg.
In another thread I mentioned using a substitute glass #1 (s.g. #1 http://www.largeformatphotography.in...ad.php?t=37482) which was a piece of glass designed for framing. It was about the same thickness & was coated to protect the print (art) from adverse effects of the sun.
The second piece or substitute glass (s.g. #2) was much thicker, in fact it was so thick that when I put it into the frame, it actually extended above the frame by a height that was close to the original G.G.! I was using these as a means to do some shooting with the camera I am (was) building, to see how well the adaptations I was making worked as I went through the remodeling process. S.G. #2 was stock ground glass from a factory, a stock item. The first time I used it, I came to the crux of a real problem with the glass => grain size. I went down to the pier & they were preparing for an triathlon to be held about a week later so I wanted to make some test exposures of the participants as they made their preparations for the real deal.Swimmers would enter the water swim the length of the pier to a buoy, then turn & swim to the far shore where some steps had been placed in the water so they could get up the retaining wall. I set up along the entrance (drive out) to the pier intent of shooting those who were just swimming the first leg (foreground running parallel) but allowing for those in the lead (second leg running perpendicular) as a group shot. First problem even with the 500mm len, the swimmers offered little more than their heads to focus on so I tried focusing on the kayaks which were being used to escort. The grain in that area turned out to be as large as the kayak that I was trying to focus on!
So much for the substitute glass but how does the thickness of the glass (gg) itself effect the focus? See attached photos, note taped glass is s.g #1, the cracked glass si s.g #2 ( broke while mounting glass as I reversed the part which held glass in frame) & the mounted glass is Satin Glass via Dave.
Note the piece [part] used to secure glass, I certainly didn't want a repeat preformance which broke the substitute glass.

[ic-racer]

If I just wanted to purchase a new ground glass, one that was very bright, dose any one have any suggestions?

kenn

Standard ground glass is not that much different in brightness from one piece to the next. A better way to get a brighter image is a taking lens with a wider viewing aperture. What lens are you using now? I suspect a new ground glass won't have much affect in terms of brightness unless the existing one is irreversibly blackened by dirt or fungus.

[Dave Parker]

ice-racer,

You will find quite a bit of difference in the various standard ground glasses, as vast majority of the older cameras have sand blasted glass in them, which is dim, as it diffuses light differently than ground glass. There are many ways to do ground glass and sand blasting over the years has been one of the most common ways it was made, I have also seen ball bearing glass, which in my opinion is dim as well, there is acid etched, which can be very bright, and then their is true ground glass, which if done with the proper materials, can be quite bright, due to even illumination across the surface of the glass. Another thing that can effect brightness in standard ground glass, is how are the edges of the screen finished, how are the corners clipped and finished. One of the biggest things that affect standard ground glass, is they get dirty, and contaminated by environmental issues which become embedded in the pores of the glass, I have looked at thousands of screens over the years, that looked dim and actually brightened up considerably when they were cleaned properly..the first thing I always suggest is clean the screen, with a grease cutting liquid soap, hot water and a scotch brite pad, this will clean any contaminates out of the pours and allow the screen to perform at its best...then make a choice if you want to get a new screen.

A key to how well glass performs, is how well does it evenly diffuse the light hitting it?

*Standard disclaimer, do not use scotch brite on plastic screens or fresnels*

Dave

[Bobf]

<snip>

*Standard disclaimer, do not use scotch brite on plastic screens or fresnels*

Dave

...or on the side of the screen that has the grid printed on it... don't ask how I know this ...

[Dave Parker]

...or on the side of the screen that has the grid printed on it... don't ask how I know this ...

Yup, that to Bob, many grids are just screen printed and baked on, so they will also come off!



Dave

[ic-racer]

ice-racer,

as vast majority of the older cameras have sand blasted glass in them, which is dim, as it diffuses light differently than ground glass. There are many ways to do ground glass and sand blasting over the years has been one of the most common ways it was made, I have also seen ball bearing glass, which in my opinion is dim as well, there is acid etched, which can be very bright, and then their is true ground glass, which if done with the proper materials, can be quite bright,

Ok I agree with you on those difference, but I suspect those different methods of ground glass preparation just produce different degrees of scatter and have little effect on the transmission. Since energy needs to be conserved, light passing through a ground glass is either reflected, absorbed or transmitted. So, preparation of the 'ground' surface to produce more transmission would need to decrease absorption and reflection. Goniophotometer measurements on ground glass produced by the above mentioned techniques would probably show that the 'brighter' glass is just 'less diffuse.'

[Dave Parker]

Well Ice,

After being a ground glass manufacture for a number of years now, I can say, I pretty much know what I am talking about. And there are a number of things that come into play when your talking about "Brite" ground glass. We are pretty much splitting hairs here. One of the biggest pre-conceived notions in LF photography is brighter is better, which I disagree with, you have to have a focus screen with even illumination, I find this to be far more beneficial to being able to compose, I can make extremely bright ground glass, but then I run into problems with focused "hot spots" which detract from being able to compose the image..As I said before, the most important thing I have found, is make sure your glass is CLEAN that way you get the best even illumination possible, which is really an aid in composing and focusing the image.

Of course when you talk about enhanced screens, that is a whole different set of rules.

Dave Parker
Satin Snow Ground Glass

[ic-racer]

Well Ice,

After being a ground glass manufacture for a number of years now, I can say, I pretty much know what I am talking about.

I think we are saying the same thing. I have seen your website and it looks like you have a good product at a excellent price. It would be interesting to see goniophotometer readings of your glass vs. vintage, conventional or competitor (ie e-bay) ground glass to quantify things like transmission, 'hot spots' and such. However, your glass looks affordable enough to just buy and try out.

(I'm a supporter of small business, especially in the field of analog photography, and my comments were intended only to discuss physics of light transmission and such BTW, I have never seen goniophotometer results on any ground glass, that's why I wrote ' it would be interesting to see...' because this ground glass 'brightness' discussion comes up all the time )

[Clay Turtle]

I've just begun to use my grandfathers wooden 4x5/5x7 view camera (KODAK). I've noticed that even thought the image on the ground glass seems to be tac sharp the negatives are slightly soft. I do not believe this is the original ground glass and was wondering if the alignment is off and how do I check it? I would also like to replace the ground glass with a newer and brighter one, any suggestions? Is this something I can do myself or is it something that needs to be adjusted by a professional?

kenn

As my previous post link, I had been using the 4x5 back from the Bender kit as it fit into the opening on the 5x7, having it fall out prompted me to use an adapter reducing the 5x7 to hold a 4x5 holder. If the depth (distance) was wrong, then I would expect to find the point of focus (depth of field) to have changed but there would still be a point of sharp focus (relating to the difference in distance). I knew a guy shooting medium format that claimed that he had a len that had been designed to produce a soft focus when set to a particular stop (stopping down) by changing an element distance . . . I don't know if this is true or not but I am wondering how the thickness of the gg effects the focus?
The scene in the last post was shot with a thick sg yet scanning it, I could not find a point of sharp focus or determine a depth of field. I went as far as having scanned by a lab because the scanner I was using had a TMA (transparency mounting adapter) for the 4x5 but not for the 5x7 format. After pouring over these scans to no avail, I started to wonder if perhaps I should have been looking for a point of soft (disfocus) focus & a (unfocused) depth of field?

[Peter K]

(I'm a supporter of small business, especially in the field of analog photography, and my comments were intended only to discuss physics of light transmission and such BTW, I have never seen goniophotometer results on any ground glass, that's why I wrote ' it would be interesting to see...' because this ground glass 'brightness' discussion comes up all the time )

This was discussed long times ago by Gerhard Hansen from Carl Zeiss in "Photometrische Angaben bei optischen Instrumenten" Optik I (1946) pp. 227-267 and 269-295. Hansen investigated different ground glasses, also with goniophotometer results