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View Full Version : How efficient is a ground glass?



Mark Sawyer
1-Jun-2008, 13:01
In other words, what percentage of light coming from the lens is passed through to the other side? I'm asking this in terms of the various "bright screens" and applications that claim to increase brightness up to two stops. It's hard to believe they can be 4 times as efficient; that would mean that, at best, a normal gg is only 25% efficient. Or are all these "improved screens" just channeling the light directionally, creating a brighter hot spot?

How about it? Are there any raw numbers on what percentage of light is passed through a typical ground glass?

Walter Calahan
1-Jun-2008, 14:03
I get 48 mpg with my gg.

Grin.

As long as I can see to focus, I ask no questions.

Bob Salomon
1-Jun-2008, 14:08
Old GG or modern one? With a modern fresnel or an old one? With an enhanced system or not?

Linhof makes a Focusing/Metering Bellows that allows one to measure exposure through the ground glass. To do so you must use the fresnel screen and then take a meter reading of a grey card through the gg and a reading normally of a grey card. The difference between the two readings is the light loss of the gg and fresnel combination. Usually this is less then one stop with a modern gg/fresnel. That factor remains constant for all lenses as long as the gg/fresnel are not changed.

BennehBoy
1-Jun-2008, 14:08
I think the main problem with GG is how it is actually ground, some grits will pit the surface (the grain effect you see) and this reduces the efficiency of the GG because only the areas between the pits are actually scattering light. Better GG is made using grit that does not cause or has a reduced pitting effect, it takes much longer to grind hence the cost.

That's just my lay understanding, hopefully someone more knowledgeable will have a better answer.

Dave Parker
1-Jun-2008, 14:32
There are so many ways to grind glass, that it is very difficult to compare the various results, and as far as measuring light transmission, then you open a whole new can of worms...I don't even think you can fairly compare the various enhanced screens with true ground glass, they are two entirely different types of products!

All focusing solutions have their up sides and they also have their downsides....there is just no way to compare them.

I have seen sandblasted glass, ball bearing glass, acid etched glass, and enhanced screens, and they are completely different and have their pluses and minuses....

Dave

Mark Sawyer
1-Jun-2008, 15:15
...Usually this is less then one stop with a modern gg/fresnel.

Aha! So a "modern ground glass" in combination with a fresnel lens, (I'm presuming that's what you mean, Bob), is losing less than a stop, or less than 50% of the light?

Just trying to figure out how efficient the system is, and yes, I know there are many systems, each with their own effect. Personally, I don't like fresnel lenses despite the increased brightness, as they add a visual effect I find disturbing, though I allow that other people might find it not at all bothersome.

I'm most interested in finding out how efficient a good, conventional, modern ground glass is, (say, a Satin Snow. which I like very much). Could I use the system Bob mentioned on any gg system and get an accurate answer?

Mark Sawyer
1-Jun-2008, 15:19
As long as I can see to focus, I ask no questions.


Ever try to focus using a lens with a maximum aperture of f/22? Actually, I'd like a brighter screen for composing; actual fine focusing is always through a loupe, which is much more forgiving of a darker image...

Bob Salomon
1-Jun-2008, 15:27
"Could I use the system Bob mentioned on any gg system and get an accurate answer"

Yes

Peter K
1-Jun-2008, 15:34
How about it? Are there any raw numbers on what percentage of light is passed through a typical ground glass?
There is a paper written by a Zeiss scientist about measuring the efficiency of ground glasses. "Photometric tasks of optical instruments" by G. Hansen, "Optik" Vol. 1 part 1, pp. 227-295, 1946. He describes a method to compare different groudglasses and diffusor screens. But I've never seen how the "4 times increase in brigthness" are measured. So I cannot believe this numbers.

It's interesting to think about the need for brighter screens increased since LCD displays where common. But a ground glass can never be as bright as such a display. And many famous photographers made their images since 1839 with unoptimized screens.;)

Peter K