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Thread: Stupid fresnel focusing question

  1. #1
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    Stupid fresnel focusing question

    Hi all,
    This is a stupid question regarding a problem I'm encountering after switching from my Horseman 45FA to Linhof Technika V. The 45FA has a ground glass and fresnel which I believe, but aren't certain of, is two separate pieces. I have never had a problem focusing when using it. The screen is bright and the grain fine, with the image sharp on the GG. I use a lupe and have always had it set at the "zero" focus position. When using the lupe on the Technika yesterday (the first outing I've had with it) I found it incredibly hard to find a sharp plane of focus. My lupe just didn't seem sharp and I have come to wonder (it's late here so can't test my theory) that this new set up might require a different setting on my lupe to bring the ground glass and not the fresnel into focus? Does this sound plausible? The only explanation to this is that the Linhof fresnel is thicker than that of the Horseman, therefore emphasizing any margin of focus error with the lupe. Does any of this make sense? Am I going crazy?

    As an aside, and I'm sure some of you with identify with this, it's funny that when out in the field and encountering these problems, my brain just didn't click to what might have been the cause. I'm so used to one way of doing something that I persevered and pushed on, despite frustrations. I did turn the lupe upside down and wondered why the focused image was so much sharper, but even then I didn't think to adjust the lupe focus for normal viewing, if indeed this was the problem...

    T

  2. #2
    Senior Member Lars's Avatar
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    Re: Stupid fresnel focusing question

    Is the fresnel on the technica behind or in front of the GG? Personally I think it makes more sense to have the fresnel behind. This means the distance from loupe to etched surface is increased.
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    Re: Stupid fresnel focusing question

    My fresnel is behind the GG, on the lupe side.

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    Sr. Administrator Jack's Avatar
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    Re: Stupid fresnel focusing question

    Like Lars said, you need to adjust your loupe shorter to compensate for the added thickness of the fresnel lifting it further away from the GG.
    Jack
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    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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    Re: Stupid fresnel focusing question

    Thanks guys,
    I tried doing this yesterday but it didn't seem to make much of a difference, if a difference at all. I'm thinking that the Linhof GG might actually be coarser than that on my Horseman and therefore my 8x lupe is too strong to adequately judge relative sharpness. Does this theory sound plausible?
    T

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    Re: Stupid fresnel focusing question

    I've been thinking (obsessing?) about this and can't understand how I can focus my lupe beyond the immediate surface it rests on. I can't remember using a lupe that allows for focusing as such, only negative diopter adjustments. Am I missing something here? When the diopter is screwed all the way in it seems sharpest, unscrewing it just makes things worse. Sorry for what might be a stupid question but not being able to focus the ground glass like I have been able to with my Horseman (without thinking about it!) is driving me bananas!

    To elaborate, without a lens I pointed the camera at a light source to illuminate the GG and rested my lupe on it. The grain was noticeably soft at all settings but best when the eyepiece was screwed all the way in. The fresnel rings were sharp. Turning the lupe around and looking through it backwards, I could bring the grain pattern into perfect focus and the fresnel rings disappeared.

    I can't believe I'm stuck on such a silly technical point but this is really bothering me! HELP!

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    Senior Member Lars's Avatar
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    Re: Stupid fresnel focusing question

    I like loupes 4x-6x for focusing. I have loupes at 4x 6x 8x 10x 22x.

    Oh! Yes focusing loupes have a helical mount that allows for focusing, that must be the culprit. The loupe I use in the field is a Silvestri 6x which has a helical focusing mount.

    So... Perhaps get another loupe? What kind are you using now? I take it it's not fixed but cannot focus past the plane it rests on? Also, are you nearsighted? That would also be a problem. Either way, another loupe should resolve this issue.
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    Re: Stupid fresnel focusing question

    Thanks Lars. Last week I ordered a new Rodenstock 4x lupe out of frustration, thinking that my problem was something to do with too much magnification (my older one is 8x and I've always found it too strong anyway.) I haven't received the Rodenstock yet but now worry it will present me with the same problem. I bought it on recommendation but didn't know the importance of a helical focusing mount and so didn't research that spec. In fact, I can't find any information listed now that hints at wether it has one or not.

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    Sr. Administrator Jack's Avatar
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    Re: Stupid fresnel focusing question

    Anything over 6x gets difficult to use on coarse GG's -- the grain is so contrasty you can no longer distinguish image detail. I regularly used an 8x or 10x on my fine GG's and loved it. ALL of my loupes had focus helicals; I would not even consider using a loupe I could not adjust on a GG. It might be worth investigating a precision GG and doing away with the fresnel, and then getting a good, focusable loupe.
    Jack
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    Re: Stupid fresnel focusing question

    Thanks, jack. I think on further inspection the problem lies in the fact that my current loupe doesn't allow for focusing fast the surface it sits on. It has a diopter adjustment but not a focus as such. Turning the loupe upside down, I'm able to find an extremely sharp image so I don't think it's a GG or fresnel quality issue I'd hope not considering they're both brand new Linhof parts! I'm scared now that I'll receive the Rodenstock 4x Loupe I ordered and be presented with the same focusing problem.

    Ah, I feel really stupid asking these questions, they seem so basic!

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