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Thread: Noob Loupe Questions - Focusing v Slide, Magnification

  1. #11
    Peter De Smidt's Avatar
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    Re: Noob Loupe Questions - Focusing v Slide, Magnification

    If you have close up reading glasses, give them a try.
    “You often feel tired, not because you've done too much, but because you've done too little of what sparks a light in you.”
    ― Alexander Den Heijer, Nothing You Don't Already Know

  2. #12

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    Re: Noob Loupe Questions - Focusing v Slide, Magnification

    Quote Originally Posted by Bob Salomon View Post
    Providing that your thread counter isn’t grossly defective it is focused on the surface that you have placed it on.
    Bob, at least my thread counter is not focused on the bottom surface, but some 5mm farther, I guess this allows it to be used in more situations and perhaps to be used by some people requiring some kind of glasses and not wearing it.

    ...so I have to separate it a bit from the gg to see the grain in perfect focus.

    To use a thread counter as a focusing loupe one has to clamp it with fingertips a bit under it, and using the fingers to adjust focus until one can see the grain in the other side of the gg, when grain is seen perfect then at the same time we have to check the image focus at the same time.

    It's not convenient, it's not as fast, but it also works.


    Quote Originally Posted by Peter De Smidt View Post
    If you have close up reading glasses, give them a try.
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  3. #13

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    Re: Noob Loupe Questions - Focusing v Slide, Magnification

    Quote Originally Posted by Pere Casals View Post
    Bob, at least my thread counter is not focused on the bottom surface, but some 5mm farther, I guess this allows it to be used in more situations and perhaps to be used by some people requiring some kind of glasses and not wearing it.

    ...so I have to separate it a bit from the gg to see the grain in perfect focus.

    To use a thread counter as a focusing loupe one has to clamp it with fingertips a bit under it, and using the fingers to adjust focus until one can see the grain in the other side of the gg, when grain is seen perfect then at the same time we have to check the image focus at the same time.

    It's not convenient, it's not as fast, but it also works.




    Click image for larger version. 

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    Do it the right way, it is faster, easier, more precise and if your hand shakes it makes no difference!!

  4. #14

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    Re: Noob Loupe Questions - Focusing v Slide, Magnification

    Quote Originally Posted by Pfsor View Post
    No, they cannot. If your loupe is not correctly focused on the plane that needs to be in focus, your eyes cannot correct for that. Why? Simply because if your loupe sends the incorrect picture to your eye any focusing in your eye cannot compensate for the lost optical information of the first optical system that sends the picture (fuzzy one) to your secondary optical system (the eye). What you say is completely wrong from the optical point of view.

    Were it as you say, we could easily focus a non sharp negative in the printing optical system to get somehow a sharp print - and we all know, it is not possible. Only digital system can do such a trick, the human eye cannot.
    I think you're misunderstanding me. A biconvex lens, like in my thread counter, doesn't have a set focal point. So you can't say that it's focused on the rear of the ground glass without also defining a set viewing point. It's a simple lens system and obeys the Thin Lens Equation (the inverse of the focal length is equal to the sum of the inverse of the subject distance and inverse of the image projection distance). So in the case of the thread counter, if it is indeed focused on the top of the ground glass from where your eye is, all you'd have to do is move your eye a little bit closer to bring the other side of the ground glass into focus.

    That, however doesn't take into effect your eyes ability to focus. Your eye works differently. Instead of changing the focal point by changing the relationship of the distances, it changes it's shape, and thus focal length. It obeys the same Thin Lens Equation, however.

    Now anyone who has had experience with a simple lens, such as a magnifying glass, knows that there is not just one point in space that allows your eye to focus on whatever is being magnified when viewed through the glass. In other words, you don't have perfectly maintain the distances between your subject, magnifying glass, and eye in order to take advantage of it's magnification effects. They do, however, have to be within a useable range. At certain distance relationships, all you'll see is a blurry mess. But at many other relationships of distance, you can see a clear image. This is because the two lenses (the one in the magnifying glass and the one in your eye) work together. As long as the image projected from the magnifying glass is able to be focused by the lens in your eye onto your retina, you'll see a clear image. If, however, the lens in your eye can isn't capable of focusing the image from the combined lenses (magnifying glass and eye) onto your cornea, it will be blurry.

    What's happening is the loupe focuses the image on a point behind the biconvex lens and your eye. Then your eye focuses on that point. As long as your eye is able to focus on that point, your image will be in focus. If you move the point of focus of the loupe lower, say the distance of the thickness of the ground glass, then the focal point behind the loupe will move closer to the lens by a small amount. All that is required for your eye to see this image in focus, is to focus a few millimeters further. If you pull the loupe slightly off the ground glass, you can still focus on the ground glass by focusing your eyes even further out, closer to the lens in the loupe. At some point, however, you'll move the point of the projected image from the loupe beyond the area where you're eye is capable of focusing. You can either move it beyond infinity, or beyond your eye's nearest focal point. But as long as the loupe projects and image within your eyes focusing ability, the image can be in focus.

  5. #15

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    Re: Noob Loupe Questions - Focusing v Slide, Magnification

    Quote Originally Posted by Bob Salomon View Post
    Do it the right way, it is faster, easier, more precise and if your hand shakes it makes no difference!!
    Of course, a good focusing loupe it's a must have tool, a Pro would had not think it twice, but an amateur that's engaging LF has a lot of things to acquire and a good loupe it's not cheap. Perhaps it's better to buy it later but buying a good one.

  6. #16

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    Re: Noob Loupe Questions - Focusing v Slide, Magnification

    Quote Originally Posted by jim10219 View Post
    So in the case of the thread counter, if it is indeed focused on the top of the ground glass from where your eye is, all you'd have to do is move your eye a little bit closer to bring the other side of the ground glass into focus.
    No... nearing the eye to the thread conuter won't modify the focus, you have to separate the loupe more or less from the ground glass to see the sharpest.

    Check this, place your thread counter on a newspaper and you'll see how probably you will have to separate the base of the loupe a few mm from it to see the sharpest image.

    Don't tell me that you were not doing that when focusing the view camera with the thread counter !!!

  7. #17
    Tin Can's Avatar
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    Re: Noob Loupe Questions - Focusing v Slide, Magnification

    If we see the GG grain and the imposed image by any means are we not seeing usable focus?

    Is it possible to find a fake focus from the user/rear side of the GG?

    And how would that be possible?
    Tin Can

  8. #18

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    Re: Noob Loupe Questions - Focusing v Slide, Magnification

    Quote Originally Posted by jim10219 View Post
    I think you're misunderstanding me. A biconvex lens, like in my thread counter, doesn't have a set focal point.

    ...
    Now I understand what you say! Your thread counting loupe is just a biconvex lens. Frankly, I did not expect that - the last time I saw a thread counting loupe, it was a composed cell lens, even apo corrected. Now I understand you - we were just speaking about two optically different things, called the same name. So it is the case where we both are right! Cheers!

  9. #19
    Jac@stafford.net's Avatar
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    Re: Noob Loupe Questions - Focusing v Slide, Magnification

    I have 72 year-old eyes. My loupe is long to work under a hood. It has a diopter adjustment (if that is the correct term). When I find where it focuses upon the glass grain, I lock it in to avoid mistakes in the future. Keep it simple.

  10. #20

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    Re: Noob Loupe Questions - Focusing v Slide, Magnification

    Quote Originally Posted by Pere Casals View Post
    No... nearing the eye to the thread conuter won't modify the focus, you have to separate the loupe more or less from the ground glass to see the sharpest.

    Check this, place your thread counter on a newspaper and you'll see how probably you will have to separate the base of the loupe a few mm from it to see the sharpest image.

    Don't tell me that you were not doing that when focusing the view camera with the thread counter !!!
    I think he's talking about the depth of focus of his lens and the ability to focus with your eyes on its different edge - the closer or the more distant one. And that ability is possibly dependent on the lens - eye distance. That's how I understand (or misunderstand) him.

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