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Thread: Correctly focusing using a loupe?

  1. #1

    Join Date
    Jul 2010
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    537

    Correctly focusing using a loupe?

    I Am using a wista field 45dx. it has a fresnel in front of the ground glass (which is not gridded). I have noticed that there generally seems to be a small but meaningful variation in sharpness between my negatives (shows up in scans at ~2400) shot with the same lens at same aperture (usually f22). Ie i have what I perceive really sharp negatives and sometimes sharp but a little less so. I am using a 6x loupe, and I wonder whether my focusing is not critical enough, thus accounting for the difference. The loupe has adjustable focus, so I wonder exactly what i should use as a reference point in adusting to loupe focus, and what exactly i should be trying to get the image shap on, the fresnel ? The ground glass texture? Somewhere in between? The ground glass does have an etched mark for the 6x9, which I have been using as my loupe focus reference point...

  2. #2

    Join Date
    May 2009
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    Norwalk Ca.
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    696

    Re: Correctly focusing using a loupe?

    If the fresnel and ground glass are factory installed I would focus on the image on the ground glass.don't know if the 6x9 marks are on the ground side of glass or the smooth non image forming side of glass,that might make a difference.

    The adjustment on the Lupe could be a dropper adjustment for glass wear's not sure though.

    Mike

  3. #3

    Join Date
    Jul 2010
    Posts
    537

    Re: Correctly focusing using a loupe?

    Mike, I think the 6x9 mark are on the outer most surface of the glass (closest to the viewer) but still part of the glass itself. So how can you tell where the ground glass stops and starts using the loupe, and what should be used tO reference the loupe?

  4. #4

    Join Date
    Sep 1998
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    Oregon now (formerly Austria)
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    3,397

    Re: Correctly focusing using a loupe?

    I'll chime in here since no one is answering.

    The lens image is projected onto the frosted surface of the "ground glass" portion of the Fresnel sandwich (many times it is plastic, but no matter). This is where you want to focus.

    Finding the frosted surface with the Wista factory Fresnel screen is easy, since there is a center spot that is without Fresnel enhancement. Just adjust your focusing loupe to this (if you have one). Another way to calibrate a loupe (if you don't have the center spot) is to take the lens off the camera, point it at a light source and then search with your loupe till the frosted surface comes sharply into focus. Once you've done this a time or two, it becomes pretty easy to recognize the frosted surface.

    Once you have your loupe adjusted properly, tape it in position and you are good to go.

    That said, many of us use different techniques and loupes that are not adjustable. I have found that it is fairly easy to recognize the plane to focus on, and use a 8x or 10x loupe reversed, i.e., with nothing at all touching the ground glass. This allows me to tip the loupe to align it better with the angle of incidence and get a brighter image to focus on. Using short lenses (65mm-90mm) and the Wista Fresnel results in a hot spot and dark edges. Having the loupe at 90° to the ground glass in this situation doesn't use all the available light.

    Keep in mind also that even if your loupe is not precisely focused on the plane the image is projected on, the image will still be sharpest when it is in focus. That means you can use a slightly out-of-focus loupe to still find the sharpest focus, you just don't get to see all the details through the loupe.

    Hope this helps,

    Doremus Scudder

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