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Thread: Loupe focussing issue

  1. #1

    Loupe focussing issue

    Hi there,
    I have an "issue" when focussing with a loupe, which I would like your input on.

    When I focus (using this loupe, https://www.bhphotovideo.com/c/produ...for_Canon.html) I get acceptable focus on the ground glass. But if I retract the loup a distance from the glass (so that there are air between the loupe and the glass, around 1-2 cm) the focus is always sharper.

    Is this a problem in getting the sharpest possible negative or is this as it should be? Should I not worry about this or is a new loupe the way to go? Is it ok to focus the camera with the loupe some distance from the ground glass?

    I "think" I get fairly decent negatives out of the camera, but as I'm new to this I'd like to nail the process/technical stuff as soon as possible so that i can focus (sorry..) on the creative side of things.

    I have attached a typical scan fwiw, maybe you can get some clues from this.

    Click image for larger version. 

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    Best regards,
    Patrik

  2. #2
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Loupe focussing issue

    Quote Originally Posted by patrik_LF View Post
    When I focus (using this loupe, I get acceptable focus on the ground glass. But if I retract the loup[e] a distance from the glass (so that there are air between the loupe and the glass, around 1-2 cm) the focus is always sharper.
    What camera are you focusing?

    That loupe is a dedicated accessory for a specific DSLR camera model with a specific LCD. As such I would expect its default focal plane to be some distance in front of the loupe's base, allowing for the thickness of the cover glass and protector.

    I would not consider that product appropriate for focusing a view camera.
    Its magnification is only 2.2x. I expect you'd be happier with 5x to 7x for most view cameras.
    Look at products from Silvestri or Wista for dedicated view camera magnifiers.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  3. #3

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    Re: Loupe focussing issue

    Quote Originally Posted by Leigh View Post
    What camera are you focusing?

    That loupe is a dedicated accessory for a specific DSLR camera model with a specific LCD. As such I would expect its default focal plane to be some distance in front of the loupe's base, allowing for the thickness of the cover glass and protector.

    I would not consider that product appropriate for focusing a view camera.
    Its magnification is only 2.2x. I expect you'd be happier with 5x to 7x for most view cameras.
    Look at products from Silvestri or Wista for dedicated view camera magnifiers.

    - Leigh
    Leigh correctly points out that your loupe is not designed to be used resting on the ground glass of a view camera. As you've noticed, you have to hold it away from the surface a bit to get it the right distance from the ground-glass image to focus. Also noted is the rather low magnification of that particular loupe, which is not particularly suited to fine focusing...

    So, a couple of comments:
    Most LFers like loupes in the 4x-8x range to focus with. 2x is really not enough to fine focus with. In fact, you can get to 2x pretty easily with reading glasses. (I use 4-diopter reading glasses to view the ground glass image with and an 8x loupe for fine focusing).

    Also, most like loupes with skirts that rest on the ground glass at the proper focusing distance so they don't have to find focus by moving the magnifier back and forth all the time. Many (and most good) loupes have an adjustment that lets you set the exact length of the skirt, i.e., distance from focus plane to your eye, to match your eyesight. These type of loupes are de rigueur for view-camera focusing. Top-of-the-line loupes like this have skirts that tilt so you can adjust the viewing angle to see better in the corners of the image.

    Some theory:
    Any magnifier in an appropriate strength can do the job focusing on the ground glass. There are, however, inherent problems using just any old magnifier that need to be considered and dealt with if you decide not to go the more traditional route and get a loupe with a skirt. First, as you have discovered, you need to move the magnifier back and forth to find the right position for focusing. Finding this position is not as easy as it may seem, since it's easy to get distracted by things like Fresnel lines, grid lines, or anything else that is in the wrong plane that you might accidentally focus on. That said, if you spend some time figuring this out for your particular applications, it is doable.

    An common myth is that an out-of-focus image from the loupe equals an out-of-focus image on the negative. Not true. You have also discovered that if you have a loupe sitting at the wrong distance from the ground glass, you will never get a really sharp image viewing through the loupe. It will be "more sharp" and "less sharp," just never really correct. In cases like these, however, the "sharpest unsharp image" is the correct focus for the camera. The problem here is that it is not so easy to make fine distinctions between "pretty sharp" and "a bit less sharp" and focusing errors are easy to make. It's always easier to have the loupe at the right distance from the ground glass; a dedicated loupe with a skirt ensures that this is always the case.

    Now for the heresy:
    I use loupes that do not rest on the ground glass (mostly because I can't justify the expense of the loupe I'd really like). I do a lot of work with shorter-focal-length lenses and this facilitates focusing in the corners, since I can find the correct angle to hold the loupe fairly quickly and don't have to rely on a tilting skirt. The downside to this is that I have to find the correct loupe-to-ground-glass position every time I focus. I've gotten really good at this, and it is now second nature to me. Others I have talked with find this approach more trouble than it's worth and prefer the loupes with skirts. My personal preference is for an 5x-8x loupe for fine focusing. I have used many different kinds of magnifiers, but now use only a couple types: a high-quality loupe designed for slide viewing (mine is a Peak 8x) and a compact magnifier like the one Sherlock uses in the TV series (see here: http://www.sherlockology.com/props/sherlocks-magnifier ).

    FWIW, I've been drooling over this Silvestri loupe for some time now... http://www.silvestricamera.com/ita/c...ng.asp?ID=5050

    Best,

    Doremus

  4. #4
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Loupe focussing issue

    Quote Originally Posted by Doremus Scudder View Post
    You have also discovered that if you have a loupe sitting at the wrong distance from the ground glass, you will never get a really sharp image viewing through the loupe. It will be "more sharp" and "less sharp," just never really correct.
    The surface on which you want to focus is on the ground side of the ground glass. That's the side toward the lens.

    The best option I've found is to use a ground glass with a grid on its ground side.
    A variable-focus loupe can be adjusted by focusing on that grid.
    If you have no grid, stick a piece of electrical tape on the ground side and focus on its rear edge.

    You won't achieve a sharply-focused image at any point because the image is on a bumpy ("ground") surface.

    The easiest solution is to adjust the focus one direction, going through "best", and on to a noticeable distortion.
    Note the focus adjustment at that point, then go back the other way through "best" to a similar point on the other side.
    Once you've noted those two adjustment points, set the focus adjustment midway between them.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  5. #5

    Re: Loupe focussing issue

    Thanks for your replies.
    I have a Sinar P1 and, judging from your insights, I guess I'll have to get myself a proper loupe.
    Thanks again,
    Patrik

  6. #6

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    Re: Loupe focussing issue

    Many years ago we ordered probably a dozen 8x plastic Lupes. Normally our vendor sent us name brand Agfa or Gepe lupes. Instead we got cheap generic plastic lupes that you had to raise 3 or 4 mm's in order to use them.

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    Re: Loupe focussing issue

    Keep in mind that anyone using reading glasses or bifocals as an optical aid to look at the GG is estimating the position of their heads and doing that without a skirt about their head resting on the GG ( regarding skirts,a kilt might be a good dark cloth. no other skirt jokes please)
    Bill
    "There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

  8. #8

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    Re: Loupe focussing issue

    Quote Originally Posted by cowanw View Post
    Keep in mind that anyone using reading glasses or bifocals as an optical aid to look at the GG is estimating the position of their heads and doing that without a skirt about their head resting on the GG ( regarding skirts,a kilt might be a good dark cloth. no other skirt jokes please)
    Exactly! When using a loupe without a skirt, the procedure is essentially the same; it's just that a smaller movement makes a bigger difference the more magnification you introduce. Head position becomes really critical with the loupe.

    Best,

    Doremus

  9. #9
    Jac@stafford.net's Avatar
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    Re: Loupe focussing issue

    Quote Originally Posted by Doremus Scudder View Post
    [...]
    FWIW, I've been drooling over this Silvestri loupe for some time now... http://www.silvestricamera.com/ita/c...ng.asp?ID=5050

    Best,

    Doremus
    Okay! I hope Santa is reading this.
    ...and this - that loupe needs an extension for those who use a ground glass hood.

  10. #10

    Re: Loupe focussing issue

    Thanks you all for your comments. I have learned that I have to focus on the gg and I have read in other threads here tips on how to accomplish just that, which was an eye-opener. I have ordered a cheap 6x magnifier, I know I should probably have ordered one with variable focus, but they are more expensive. I'll begin this way and apply what I've learned so far.

    Thanks,
    Patrik

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