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I gave this a try today. Went to Staples (an office supply place) and got one of these for only $3.99:
Returned home and set up the 8x10 in the yard and pointed it at a mixed sun/shade scene. Took a careful look at the normal ground glass image, then overlayed the fresnel screen on the back of the GG. (Fresnel placed between my eyes and the non-frosted back side of the GG, concentric grooves touching the GG and facing forward towards the lens.)
What a remarkable difference!
As expected, the center spot was no brighter. But the off-axis periphery areas just lit up. It was so nice that I went and found my 4x5 Crown that has a newly installed OEM fresnel (between the GG and the lens, per the original design spec) and visually compared the same scene. The two looked essentially identical.
The Crown fresnel has much finer ridges spaced closer together than the cheap fresnel. But when I tried focusing the cheap fresnel with a small loupe, it wasn't bad. I figure if I can come up with a way to easily pop the screen on and off the GG, then I can use it just for compositional purposes and remove it for final fine focusing.
Ken
"In 1850 it would have been unusual to find someone who had handled a camera or looked at a photograph, but 100 years later the reverse would have been truethe camera had become a ubiquitous device, its techniques manageable by even the clumsiest and least sophisticated person."
Naomi Rosenblum, A World History of Photography, 1984
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I used one of these fresnel magnifiers as the view screen in my DIY 8x10 nested box camera. I took the flat side of the fresnel and sanded it with 400-grit emory paper using a random orbital sander. The results are a ground glass on the front side and a fresnel on the rear side, all using one sheet of plastic.
~Joe
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 Originally Posted by holmburgers
That's what L Gebhardt is saying and it seems like that would be a safe bet. But I'll say, in my limited testing, the difference appears to be nominal.
I'd really like to tackle the calculation, but I'm not sure if I'm up to it at the moment... still sipping my coffee.
Let's imagine we have a 135mm lens at f/4.7 (Raptar, Optar, Graftar) and our subject is only 2 feet from the lens (a fairly critical circumstance I would suspect). What difference in the subject distance would be necessitated by a .003" change at the plane of focus; again, 3 mils being a liberal estimate of the fresnel thickness.
I don't know the answer or exactly how much difference it makes, but I do know that the thickness of the fresnel is not the issue, or at least not the only issue. The fresnel is a magnifying lens in its own right that adds its own focal length into the optical mix.
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Ken, I'm so glad that you gave it a try and moreover that it worked so well.
There's too much theorizing and optical-hypothesizing about these Fresnels (though I'm sure some considerations are valid); in reality you can stick one on your ground glass and it'll make a marked difference.
From the film shooters will rise a well developed practice of the alternative processes that, in time, will be adopted in the age of the digital image to free it from the extreme boringness of pressing print.
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Chris ,
I looking some optic design books and all I understand that , if you put a lens distant and back of a lens , second lens focus the first inside of it , not in front or backside of the second lens. If your fresnel is thin enough you will lose sharpness approximetaly half of the thickness of fresnel.
Umut
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My sheet magnifiers measure about 10 inches in focal length and about .022" thick. If placed between the ground glass and lens, this thickness would certainly make a difference in accurate focusing at large lens apertures. The pitch of most of the sheet magnifiers was 50 lines/inch; painfully coarse. One measured 100 lines/inch, and was slightly dimmer. A Speed Graphic fresnel measured about 200 lines/inch. Not all fresnels, even of identical pitch and focal length, are created equal. Some will work better than others with a ground glass.
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If you know the focal length of the fresnel (e.g. use the 10" Jim suggests) and the focal length of the lens, you can convert them to dioptres, add them together and convert back to focal length and compare it to the focal length of just the lens to find the difference in ground glass position:
Assuming a 150mm lens, this is 0.15m in focal length or 1/0.15 Dioptres. A 10" lens is 0.254m or 1/0.254 Dioptres.
So 6.6667D + 3.937D = 10.604D or 0.094m or 94mm so 56mmm closer. Or in other words, my theory is nonsense!
(I couldn't not post it after going through all the maths!).
Steve.
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