Fresnel FL vs Lens FL in focus screens

Discussion in 'Medium Format Cameras and Accessories' started by PaulMD, Mar 6, 2012.

  1. PaulMD

    PaulMD Member

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    I'm crossposting this from a Rolleicord thread. I have a Rolleiflex with a 7.5cm lens in it, and a Fresnel screen cut to fit from a Mamiya RB screen. My local repair guy says this won't work, because the Fresnel lens acts on the optical system, causing a focus shift. Apparently the Mamiya RB series are designed with an 80mm focal length in mind, where the Rolleiflex lens is a 75mm, and this is supposedly enough to throw off focus at one end or the other.

    This seems wrong to me; how would (say) a 65mm lens focus properly on an RB then? Or is are the retrofocal/telephoto lenses calculated to avoid this effect somehow?
     
  2. fotch

    fotch Member

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    I would think they work off of the ground glass, and it would not matter.
     
  3. ic-racer

    ic-racer Member

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    What kind of problems are you having with your fresnel that made you take it to the repair guy? You have put the fresnel on top of the ground glass, and held in place by the little clip? You know the fresnel lenses usually work best when a single eye is centered over the screen. They also work well with the flip up magnifier.

    The Rolleicord fresnels are hard to find and if you do find one, it may not be in good condition. I made a fresnel for my Rolleicord out of a cracked Horseman 6x9 fresnel and it works very well.
     
  4. Dan Daniel

    Dan Daniel Subscriber

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    Does Mamiya sell specific screens to match specific lenses' focal lengths? Are specific screens sold along with each lens? I never heard of this. I've heard of certain focal lengths or apertures working better with certain style screens, but these are general issues, not 5mm specific issues.

    One way I could see this being a problem is if the screen material is very thick. On a camera like a Rolleicord or 'flex without the interchangeable screen system, the screen is indexed to the top surface of the screen while the focusing surface is to the bottom side. So maybe the material is so thick that the viewing lens can't be adjusted enough to accommodate the thickness. This might be the 'Fresnel acting on the optical system' thing that the repair guy means? Or maybe the guy simply doesn't want to bother pulling the leather and front shroud to get to the adjustment screw to re-set the focus of the viewing lens to match the taking lens.

    If you DO find that there is an optical issue, please post about it. I hear people mumble about this every now and then, but haven't heard a real explanation or a real case that can't be explained otherwise.

    ic-racer, as I understand it the Mamiya screen is a complete replacement screen for the ground glass, not an add-on fresnel layer.
     
  5. PaulMD

    PaulMD Member

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    I've got a stock screen in it right now, but it's old and dim. I got a screen cut from a RB67 screen with it, and I could have it installed at the time of servicing. The Mamiya screen has a fresnel molded into it, you do need to have it realigned.
     
  6. Frank Bunnik

    Frank Bunnik Member

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    It will work fine. I used the same screen in my Rolleiflex with 3.5 75mm Planar lens. The ony thing you have to do is make sure that the fresnel lens (the rings) is facing upwards instead of what you would expect (facing downwards). If the fresnel lens (the rings) are facing down then your focus will be off.

    The Mamiya RB screen cut to fit is indeed an all in one screen, not an add on fresnel lens.

    All the best, Frank
    www.frankbunnik.zenfolio.com
     
  7. ic-racer

    ic-racer Member

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    Maybe I'm misunderstanding. The Rolleicord has a glass focus screen and an optional plastic fresnel that fits on top (toward the photographer). So you want to take the glass focusing screen out, and maybe never had the optional fresnel and replaced the glass screen with a combined focusing screen/fresnel from an RB?

    What I did was clean my glass screen and put a frenel-only on top. It really brightened it up a lot. I think they were all dim when new, that is why they had the optional fresnel.

    If you really want to get rid of the glass screen you might consider a combined fresnel/focus screen that is know to work, like the ones available here: http://rick_oleson.tripod.com/index-175.html
     
    Last edited by a moderator: Mar 8, 2012