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Thread: Fresnel screen focus distance is different from ground glass

  1. #31

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    Re: Fresnel screen focus distance is different from ground glass

    Many years ago when Hasselblad introduced their Acute Matte focusing Screen, there were other aftermarket brands of focusing screens available. Much hemming and hawing happened over the cost of the Acute Matte. We decided to try out the others in camera one day. Using a Hasselblad 2000FCW body, 100mm Planar lens, A12 film back one roll was carefully test exposed with the Acute Matte, another identical roll was exposed as in as much the same as the first roll. Color transparency results indicated the after market focus screen was slightly off focus while the Hasselblad Acute Matte was spot on and brighter using a PME prism as a viewing aid. That was the first and last time any deviation from OEM focusing screens was tried.

    This is also why the insistence on the genuine original focusing screen during the Master Technika days and same instance on using only Sinar fresnel GG on a Sinar camera. Sticking to this Orthodoxy has produced zero focusing problems over decades of using these cameras.

    All it takes is one wasted roll of film or wasted sheet of film due to what appears to be a simple thing, but the results are not worth the risk or believed-perceived improvement. Stay with the OEM when possible and respect the folks who designed and built the focusing system. More often than not. they really DO know what they are doing.


    Bernice



    Quote Originally Posted by Bob Salomon View Post

    Then there was Mary Ellen Mark who had a similar problem with her a Rolleis. We went to SoHo and looked at her cameras, left her with loaners and brought them to service who couldn’t find a problem. Her complaint was that they were sharp head on but soft when angled.
    Service sent them to the factory in Germany and they sent them to Zeiss in Oberkochen and they discovered a third party focusing screen was improperly positioned in the camera!

  2. #32
    Tin Can's Avatar
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    Re: Fresnel screen focus distance is different from ground glass

    Gotta Agree

    If possible I strive to restore anything to stock or OE as possible

    Not a fan of Modders, who butcher OE, our old joke was, 'I can make it better...'

    2 hours ago I got a NOS porcelain lamp socket that exactly replaces a damaged one in an Elwood enlarger. I will rewire with new cable.
    Tin Can

  3. #33

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    Re: Fresnel screen focus distance is different from ground glass

    Quote Originally Posted by Bernice Loui View Post
    Many years ago when Hasselblad introduced their Acute Matte focusing Screen, there were other aftermarket brands of focusing screens available. Much hemming and hawing happened over the cost of the Acute Matte. We decided to try out the others in camera one day. Using a Hasselblad 2000FCW body, 100mm Planar lens, A12 film back one roll was carefully test exposed with the Acute Matte, another identical roll was exposed as in as much the same as the first roll. Color transparency results indicated the after market focus screen was slightly off focus while the Hasselblad Acute Matte was spot on and brighter using a PME prism as a viewing aid. That was the first and last time any deviation from OEM focusing screens was tried.

    This is also why the insistence on the genuine original focusing screen during the Master Technika days and same instance on using only Sinar fresnel GG on a Sinar camera. Sticking to this Orthodoxy has produced zero focusing problems over decades of using these cameras.

    All it takes is one wasted roll of film or wasted sheet of film due to what appears to be a simple thing, but the results are not worth the risk or believed-perceived improvement. Stay with the OEM when possible and respect the folks who designed and built the focusing system. More often than not. they really DO know what they are doing.


    Bernice
    The Master Technika is decades old. Over that time span Linhof has changed their gg and their Fresnel. The ones supplied in the last few years are much brighter and even then the older ones. But their positioning in the back has not changed.

    The Acute matte screen was a Minolta product and was sold by both Hasselblad and Rollei under license for medium format.
    Again the screen position in the cameras was identical to their other screens.

    As for focus aberrations and Fresnels Nikon made a series of K screens that were optimized for different focal length ranges. Wista offers 2 different Fresnel screens. One for the normal range of short to portrait lengths and a second for long focal lengths.

  4. #34
    3D-Stereo-Aeropanoramas
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    Re: Fresnel screen focus distance is different from ground glass-Ron Wisners position

    viewing screens for largeformat camera wisner 13.pdf
    Ron Wisner has a very precise statement about Fresnel and Groundglasses. see pdf above.
    Link at the end here is dead. got the pdf from archive. its also stored in filesection of FB Largeformat Photography.
    Viewfinder screens GG fresnel 4x5 back

    https://www.largeformatphotography.info/gg.html letzter link wisner tot/last link wisner dead:

    https://web.archive.org/web/20080228...om/viewing.htm
    no need to download- pdf attached here. see first line.
    What is the Proper Placement of the Fresnel?
    One subject which comes up often is the correct position of the fresnel lens. In years past, several arrangements have been used, including placing the fresnel behind the ground glass, in front of the ground glass, and incorporated into a ground glass made of plastic. However, there is only one correct arrangement. The ground glass surface should face the camera lens, and the fresnel is placed behind it, on the outside of the camera, toward the photographer. The textured surface of the fresnel should be placed against the ground glass. There is a particular reason for this arrangement. In manufacturing cameras and film holders, one overriding concern is the correct position of the focal surfaces of the respective parts. In the film holder this is the position of the septum, against which the film rests, and in the camera this is the position of the diffusion surface, or ground side of the ground glass upon which one focuses the camera. Nothing is more important than the proper registration and agreement of these two elements. If the fresnel were placed in front of the ground glass, interposed between the lens and the ground glass surface, even if it were still physically in the same position occupied by the film plane, a lack of registration can occur. This can be explained by considering the effect of a parallel sided glass plate such as a glass filter upon a beam of light (fig.?). Rays passing through such a plate are displaced by about one third the thickness of the plate, and depending upon the angle when passing through the plate, will be displaced laterally. .Furthermore, the greater the angle, such as in a wide angle lens, the greater the effect, resulting in an apparent curvature of the image when no such real curvature exists. The result will be erroneous focusing of the edges of the image.
    ------
    https://www.photo.net/discuss/thread...or-tlr.105798/
    Depending on the way the orignal screen is mounted your TLR viewing lens will need readjustment. If the original screen lies flat on a frame under it and you can mount the fresnel screen above it without lifting the original screen this will not be necessary. On the east german Reflekta II TLRs the original screen is mounted with four brackets UNDERNEATH its frame. The brackets are just long enough to hold a fresnel lens, too, but of course the original screen will be in a somewhat lower position.
    www.stereopan.org
    3DStereo-Aeropanorama-Jungfraujoch

  5. #35
    3D-Stereo-Aeropanoramas
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    Re: Fresnel screen focus distance is different from ground glass

    not really LF but anyway interesting since the same problems.

    1. have both brightscreens by rick oleson.
    http://rick_oleson.tripod.com/index-175.html
    bought for kiev 60. its 1.56mm(micrometer measured) same thickness like HB-V without fresnel ontop of GG. for K60 its needs shim 1.7 minus 1.56mm. I mention this since its a microprism -fresnel-combination, one version with split.
    And its sensational to focus. But i have got problem when using as GG. see next

    2. was checking and comparing Mamyia Universal Press/Super 23 cameras with calibrated 150mm blue with new Dora Goodman Zone Z1(light and 200series bodies). i used multiple Groundglasses. on Super 23-G the original. in Filmbacks a 57 x83mm standard GG. Tony Sansone advised to use one back with GG permamently attached. on the other hand the original Rightangle viewfinder with fresnel is sensational.
    2b.I was checking Zone Z1 for correct focus. 200series is 0,84mm too short needs shim. 3m distance was 2.8m on scale.
    then i tried to use rick olesons brightscreen. it seems too tiny to mount in 6x7-back. i am not sure if this fact led to misfocus. it was laying flat but angled. and i had misfocus. beside that i had this problem: distance set with macroslider and laser. focus checke with helicoid. was fantastically working with brightscreen. best focussing aid i had. seems sort of acumatte. But: when trying to finefocus with macroslider it was not possible. couldnt see a difference. with GG i could.

    3. Here is an interesting statement by Tony Sansone: The Mamyia Universal Press/Super 23-guru- we discussed this camerasystem:

    "Beware of adaptations with fresnel lens. Remember the incidence of refraction. The focused image is somewhere inside the plastic fresnel, not on the surface. Person who makes the fresnel focus screen with glass at rear must understand this and compensate for it. Very . very few do that".
    maybe someone could explain refraction further.

    4. Brightscreen can be cut to a larger size than 6x6 but the origina not to 4x5:
    Oleson: "And finally, if you need a larger screen, the largest that I can make is 68mm x 78mm for either the BrightScreen MP or the BrightScreen MPD (the cost of these is $80 including shipping). If you need something larger than that, I do have a small number of 103mm x 133mm BrightScreens with no focus aid in the center, which can be cut down to fit larger cameras.... if you need one of those larger special sizes the cost is $100 including shipping."
    The question is if it has also a hole in center to see GG underneath. will ask him. see 5.

    5. got a special chinese 4x5-fresnel 100 x 122mm with 15mm hole by 3s-krpc ebay. want to use it ontop of Groundglass of Polaroid MP3(made ready for outdoors). since GG there must anyway be replaced i will get it from the same seller(18°, Cosmos Circle and more stuff)

    6. Mamyia Quick Roll Graflok 23-4x5 sliding back. When i had no good groundglass i removed the 6x9-GG from this sliding back. it is sandwiched with fresnel and focussing is a joy. old HB-V-GG is very bad then(is also a sandwich but very coarse). I also check focus by moving it to filmholder. then i forget where i had to put Fresnel. i remembered surface was glossy so eighter glossy GG or Fresnel(structure towards lens then. After reading through all the discussions i was unsure. didnt recheck in camera till then. nobody answered in FB.
    But i am sure Fresnel is inbetween lens and GG. GG ontop.
    On the other hand: Mamyia Universal Press Groundglass-rightangle viewfinder here-has fresnel ontop. i could focus without loupe. its the same construction like the chinese one cosmos circle/3s-krpc. So we can expect really high quality there and its made of very solid material-cannot be cut. he told.

    7. Since Tony Sansone already offered and wanted to recreate 4x5-adapter for MUP: Aaron Smith can already deliver 4x5-adapter for Mamiya Universal Press. I have his G-adapter which is not only very precise but better than the original or Super 23-G. no problem using Mamiya RB 67 Pro S.Slit must be prolonged. We adjusted blades for M RB67/70 back and Horseman 16EX should also work-will be verified.
    Smith can also deliver universal GG-holder-blade-mounted not slipped in hooks. but original G-GG will work(must be taped since no hooks on the adapter)
    www.stereopan.org
    3DStereo-Aeropanorama-Jungfraujoch

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