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Thread: focusing loupe for ground glass

  1. #1

    focusing loupe for ground glass

    Hi, I have a flatbed 5x4 with a regular ground glass. For fine focusing I use a 4x slide loupe. But I think this is not going to give me correct focus since the image is formed on the other side of the glass and I am focussing the shiny side of the glass (thickness of glass is about 2mm).

    Any suggestions on how to get around this. In India we dont get loupes meant for ground glass. Can I file off 2mm from the existing loupe - will it work fine.

    Thanks

    Dileep

  2. #2
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    focusing loupe for ground glass

    Scribe a fine line around the bottom of the loupe using the ground glass as your thickness gauge. You can then file down to the correct focus by taking out material to this line. Make sure to protect the loupe's lense with some tape so you don't scratch it while working on it. If you take off too much, a layer or two of tape will work as a shim to give correct focus.

  3. #3

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    focusing loupe for ground glass

    dileep,

    I'm not sure I understand---you should be focusing on the aerial image thats projected on the inside surface of the ground glass, right?The thickness of the glass I don't think should matter, since you're concerned with whats on the "other side." How do your prints turn out? If they are slightly out of focus, it could be due to other issues, such as the film not being held in the same plane as the inside of the ground glass side of your ground glass. Take the back off your camera and with a film holder in place, with the dark side pulled out and a piece of sacrificial film in place take a measurement from the bearing surface of the back(the part that goes against the camera body) to the film plane. Take the holder out and measure again, only this time measure to the ground surface of the ground glass. If there is an appreciable difference, the problem could be your film holder or camera back. I hope this helps.
    I steal time at 1/125th of a second, so I don't consider my photography to be Fine Art as much as it is petty larceny.
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  4. #4

    focusing loupe for ground glass

    Your slide viewer will work adequately and give you the correct sharpness. If it's sharp on the glass, it's sharp on the film. You'll be hard pressed to see the surface of the glass on your side anyway due to the image on the otherside. I use an old 80mm enlarging lens ($12)as a focuser and so don't have the worry of putting the focuser on the surface and I can see clearly and brightly all the way to the corners even on my 90/f8 lens. My loupe is more versatile than my buddy's Toyo loupe too as he can't see squat out at the periphery of the GG.

  5. #5

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    focusing loupe for ground glass

    Dileep,

    You ARE focusing on the inside of the ground glass when you use your loupe--that is where the image is formed. No trimming is required. Any loupe will work.

    If you focus your camera and the image is sharp on the ground glass... but your negatives are out of focus, there is a problem elsewhere. Are your negatives consistently out of focus?

  6. #6

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    focusing loupe for ground glass

    I see your point: as your loupe is designed to focus at where the end of the barrel stops, you think this may cause a problem given that the image you are focussing on is 2mm further away.

    As long as you can focus on the grain of the ground glass surface, you have no problems. By definition, if you can bring the surface on which the image is focussed into focus, then if the image is also in focus, you have no problem. If you can *not* focus on the ground screen surface grain, then you may try reducing the length of the loupe's barrel as you suggest, but this is very unlikely, especially with a relatively low power as x4 loupe - depth of field will most likely take care of the 2mm.

    Cheers,

  7. #7

    focusing loupe for ground glass

    Using a cheap loupe at college, I found there was fall off due to the convex shape, and therefore could only use the centre of the loupe as a reliable reference for focussing. Looking in corners of the ground glass was difficult too, as the loupe was large and round.

    I solved these problems by investing in a German made x8 Linen Tester (Kaiser) which was very flat and square, which meant I could get right into the corners of the groundglass. It was also collapsable which made it easy for transportation and storage when on the move.

  8. #8

    focusing loupe for ground glass

    Thanks everyone. Actually I got confused about this because somewhere on this forum I read that you cant use a slide viewing loupe for focussing a groundglass.

    My pictures are very sharp - I got a little worried because I normally stop down to about f16 which may actually cover an error.

    John thanks for the method of checking if the film and the back are on the same plane - I'll try that out in a couple of days.

    Dileep

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