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Thread: Loupe project -- cheap and easy for the handy and thrifty

  1. #11
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    Re: Loupe project -- cheap and easy for the handy and thrifty

    When I focus my adjustable loupe, I make an "X" with a lead pencil on the "lens side" of the ground glass and then focus on that. For me, that was the fastest way to focus the loupe on the grain of the ground glass. I leave the X on the ground glass for checking the loupe's focus, and don't leave the gaffers tape on it more than about a year to avoid tape residue on the loupe.

    As for varying the focus of your well-crafted loupe, perhaps the answer is to use inner and outer tubes that are taped together after achieving the proper focus on the ground glass.

    Keith

  2. #12

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    Guelph, Ontario, Canada
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    Re: Loupe project -- cheap and easy for the handy and thrifty

    Bob: On my Toyo D45M ground glass / Fresnel sandwich, the inscribed side of the Fresnel (the one with the grooves) faces towards the eye, meaning that the "rings" and the "grain" of the glass are just about in the same plane. Once the "rings" are in focus, I just have to pull back a tiny amount to have the grain of the glass be really sharp. That will make it easy to determine the final length of the loupe.

    Keith: I had that same idea as I mulled this over. I'm going to add a small additional ring at the glass end of the loupe. Once I figure out the exact position I need, I'll fix it down. If I had multiple cameras this could be problematic because I'd have to take it apart to adjust for another camera, but I just use the one currently. Your "X" idea is clever. I'll have to take the sandwich apart to do that, but it's the work of a few minutes.

  3. #13

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    Re: Loupe project -- cheap and easy for the handy and thrifty

    I edited Post #4 to add corrected focusing information. It's not shown in the original pictures, but the solution was simple. I trimmed a bit off the bottom, then made a 1" collar out of the plastic. The collar fits snugly around the base. With the collar sliding freely, I found the correct length and taped it down securely. As my eyes get worse, or if I switch cameras, I can now adjust the loop. A big thanks to Bob for catching this error in my original design.

  4. #14

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    Re: Loupe project -- cheap and easy for the handy and thrifty

    There's a myth that a loupe needs to be focused on the ground glass. This assumes that the human eye works like a piece of film. Unlike film, the eye can shift it's focus forward and back to compensate for focus, with a limited range. If it didn't, then the ground glass would only ever be in focus when you eye was placed at exactly one spot. So even if you held your eye directly against the eyepiece, the thickness of your brow could throw the focus off.

    This isn't to say that adjustable loupes are a bad idea. They can allow you to adjust the focus to allow you to see better close up. So if you were farsighted and couldn't see sharply through a normal loupe, you might be able to use an adjustable loupe to alter the focal point to compensate for your eye's inability to focus close up.

  5. #15

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    Re: Loupe project -- cheap and easy for the handy and thrifty

    Quote Originally Posted by jim10219 View Post
    There's a myth that a loupe needs to be focused on the ground glass. This assumes that the human eye works like a piece of film. Unlike film, the eye can shift it's focus forward and back to compensate for focus, with a limited range. If it didn't, then the ground glass would only ever be in focus when you eye was placed at exactly one spot. So even if you held your eye directly against the eyepiece, the thickness of your brow could throw the focus off.

    This isn't to say that adjustable loupes are a bad idea. They can allow you to adjust the focus to allow you to see better close up. So if you were farsighted and couldn't see sharply through a normal loupe, you might be able to use an adjustable loupe to alter the focal point to compensate for your eye's inability to focus close up.
    Sorry, you are trying to justify the wrong way to use a loupe.
    Proper procedure is to focus it on the gg grain. Not on the top!

  6. #16

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    Jul 2014
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    Re: Loupe project -- cheap and easy for the handy and thrifty

    For checking the focus on my GG I ordered some +7 reading glasses (143mm) from Amazon. Pull them down on my nose to set the camera, push them back up to check the focus on the screen. If I remember fight they were $20.

    They have them on ebay also.

  7. #17

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    Re: Loupe project -- cheap and easy for the handy and thrifty

    Quote Originally Posted by Bob Salomon View Post
    Sorry, you are trying to justify the wrong way to use a loupe.
    Proper procedure is to focus it on the gg grain. Not on the top!
    I think you misunderstood me. YOUR EYES need to focus on the ground glass grain, but that doesn't mean the loupe needs to be. Focusing on the top of the ground glass serves no purpose unless you want to look at your own reflection (or any guidelines you have there). And that's certainly not a suggestion I made.

    Let's do an example. Take a pair of reading glasses. Do you need to keep the newspaper at a precise distance to keep the print in focus, or does it just need to be kept within a usable range? It just needs to be kept within a range, obviously, because your eyes can adjust their focus to compensate for the changing focal points created by shifting the distance between the glasses and newspaper. If you took the reading glasses, however, and built a camera that used a lens from those glasses, you'd discover that the image would only focus at one particular spot, and to change that spot, you'd either need to change the distance between the lens and newspaper, or the lens and film plane. That's because, unlike the adjustable lenses in our eyes (which can be pulled or relaxed by muscles to adjust their focal length), the film has no way to focus light on it's own. So the entire job of focusing light must be done solely by the camera lens (and aperture). That's why the lens on the front of the camera has to be focused precisely onto a specific point (film plan or ground glass), and the lens on the back of the camera (the loupe) just has to be focused within a usable range (because it's not the only lens in the system, since our eye is also a part of this optical system). Hence why you don't need to focus the loupe onto the ground glass grain for the ground glass grain to be in focus.

  8. #18

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    Re: Loupe project -- cheap and easy for the handy and thrifty

    Quote Originally Posted by jim10219 View Post
    I think you misunderstood me. YOUR EYES need to focus on the ground glass grain, but that doesn't mean the loupe needs to be. Focusing on the top of the ground glass serves no purpose unless you want to look at your own reflection (or any guidelines you have there). And that's certainly not a suggestion I made.

    Let's do an example. Take a pair of reading glasses. Do you need to keep the newspaper at a precise distance to keep the print in focus, or does it just need to be kept within a usable range? It just needs to be kept within a range, obviously, because your eyes can adjust their focus to compensate for the changing focal points created by shifting the distance between the glasses and newspaper. If you took the reading glasses, however, and built a camera that used a lens from those glasses, you'd discover that the image would only focus at one particular spot, and to change that spot, you'd either need to change the distance between the lens and newspaper, or the lens and film plane. That's because, unlike the adjustable lenses in our eyes (which can be pulled or relaxed by muscles to adjust their focal length), the film has no way to focus light on it's own. So the entire job of focusing light must be done solely by the camera lens (and aperture). That's why the lens on the front of the camera has to be focused precisely onto a specific point (film plan or ground glass), and the lens on the back of the camera (the loupe) just has to be focused within a usable range (because it's not the only lens in the system, since our eye is also a part of this optical system). Hence why you don't need to focus the loupe onto the ground glass grain for the ground glass grain to be in focus.
    Jim, obviously you have not used a really good focusing loupe to focus on the grain side of the gg. Had you done so you would see an obvious difference between focusing the loupe on the top side or the grain side of the gg. The difference is as plain as night and day!

    Go get a really good Rodenstock or Schneider loupe, take your lens off the camera, point the camera at a light source and see for yourself!

  9. #19

    Re: Loupe project -- cheap and easy for the handy and thrifty

    Hello,

    I have got difficulties to reproduce the problem of malfocusing on the clear side of the focusing screen. I don't see any projected image there coming from the lens.

    If I understand Bob correctly, he says one can focus on the clear surface of a focusing screen.

    When I try this with a plano-convex loupe from a Heidosmat lens, I can only focus on the grounded recto side of the focusing screen. I am not able to see a picture on the clear verso side, towards the eye. It has to be very weak. And normally I don't focus on bright light sources.

    Do you mean an image resulting in frosting the verso side, towards the eye, by exhaling in cold climate?

    There is a recticulum on the grounded side (Sinar). Of course I can misuse my loupe by holding it too far away from the eye. But if I focus on the clear verso side of the groundglass I don't even see the reticulum on the grounded side sharply. I just can't see a sharp image with an unsharp reticulum. So I tend to think that an adjustable focusing loupe only serves to faciliate watching the loupe-independent image on the grounded side of the focusing screen, e.g. in case of vision disorder.

    I learned that the main reason for a groundglass is that there is a matte surface to focus something at all. If I had a clear glass without grounded surface I would see a brighter image but no focal plane.

    Where am I wrong?

    Tschau

  10. #20

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    Re: Loupe project -- cheap and easy for the handy and thrifty

    Quote Originally Posted by Daniel Casper Lohenstein View Post
    Hello,

    I have got difficulties to reproduce the problem of malfocusing on the clear side of the focusing screen. I don't see any projected image there coming from the lens.

    If I understand Bob correctly, he says one can focus on the clear surface of a focusing screen.

    When I try this with a plano-convex loupe from a Heidosmat lens, I can only focus on the grounded recto side of the focusing screen. I am not able to see a picture on the clear verso side, towards the eye. It has to be very weak. And normally I don't focus on bright light sources.

    Do you mean an image resulting in frosting the verso side, towards the eye, by exhaling in cold climate?

    There is a recticulum on the grounded side (Sinar). Of course I can misuse my loupe by holding it too far away from the eye. But if I focus on the clear verso side of the groundglass I don't even see the reticulum on the grounded side sharply. I just can't see a sharp image with an unsharp reticulum. So I tend to think that an adjustable focusing loupe only serves to faciliate watching the loupe-independent image on the grounded side of the focusing screen, e.g. in case of vision disorder.

    I learned that the main reason for a groundglass is that there is a matte surface to focus something at all. If I had a clear glass without grounded surface I would see a brighter image but no focal plane.

    Where am I wrong?

    Tschau
    You place the loupe on the top side of the gg and take the camera lens off and then move the eyepiece of the loupe, or in this case your lens, until you see the grain side of the loupe in sharp focus. Then you have focused on the grain side of your gg.

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