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paul ron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, April 02, 2004 - 11:10 am: | 
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Does anyone know anything about the science of fresnel lenses used in focusing screens in medium format cameas? I know what they do and how they do it ut what i am looking for is to put a finner fresnel lens in my camera since it is so hard to focus with the course screen I now have in it which was a dissapointment at $160 used.
Where can I buy this focusing lens as a generic material, probably also used by OEM manufacturers, instead of purchasing it as "Beatiescreens" and "BosScreens" or "Maxwell" screens? i am sure they get it from someplace in different grades and types.
Thanks fo your help. |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, April 02, 2004 - 01:12 pm: |
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I am afraid there is no way to purchase the 'raw material'. Today fresnel screens are injection molded, and one part of the die has the (negative of) the fresnel structure on it - similar to the grooves pressed on a vinyl disc. All the manufacturers you named seem to have a proprietary process to form and to mold these fresnel structures.
I know that Edmund Optic sells fresnel lenses in different formats but I think they are too coarse to be used as a focussing screen for 35mm cameras.
BTW, I am not a SLR specialist but I think the easiest and most precise way to focus is using a split-image screen. That's why I replaced the screen (fresnel screen with microprism ring and mat spot) on my Kowa SET by a split image screen cannibalized from probably the junkiest SLR ever made (Huashan DF3 from China). |
paul ron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, April 03, 2004 - 01:34 pm: |
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I am not looking top by acrylic resin, just the lens itself in difering line counts and styles for critical focusing. I magnify my view screeen and fine the comercial view screens are too course to be accurate so I want to erplace the lens with a finner one over my ground glass. |
rick oleson
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, April 04, 2004 - 04:16 pm: |
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I have found a number of sources for Fresnel sheet stock by searching on Google, but I don't have the URLs handy (it's at work). Try that though, see what you can find. I think Edmund Scientific may also have some stock.
Be aware though that most of this stuff is COARSER, not finer, than your camera screen and may not help you. Also, it comes in different focal lengths.... you probably want something fairly close to your viewing lens FL (say, 80-160mm); much longer will be less effective at brightening the edges.
In the 50s when they were selling drop-in fresnel screens for this purpose, many of then had a round hole cut in the center to permit precise focusing and saved the fresnel rings for the outer area (as many factory screens do too). You may find this to be a way to get the effect you need.....
 = |
paul ron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, April 07, 2004 - 08:30 am: |
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I have been involved in camera repair as well as professional photography and have been suffering with focusing problems all these years. I spent thousands of dolars on this problem only to be temporarily satisfied. I thought it was about time in 2004 that technology should be good enough to get over this. I have seen Maxwell, Bosscreens, and the plethora of others that are just an expensive compromise and not a cure at all. Is this as good as it gets? It stinks! I thought if i could find a source i could custom make my own to fit my needs. I am sure there is a source for these lenses in different degrees of courseness and focal range someplace since Bill Maxwel a one man operation has found. He isn't making his own lenses. I'm sure there is a Chineese manufacturer someplace selling em. I just need to find the source. |
