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Thread: DIY Meniscus

  1. #21

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    Quote Originally Posted by Terence
    Greene suggests using :
    [A] A focal length equal to 0.828 x Film Diagonal.
    [B] A lens diameter equal to 1/5 to 1/7 of the focal length.
    [C] A stop of between f/16 and f/32, located 1/6 to 1/7 of the focal length in front of the lens.
    Thanks for looking that up for me. I take it that the [A] calculation was for minimum coverage ? If so, then the widest lens would be a FL=135mm, and 22.5mm in diameter to cover 4x5. Hopefully this is the case, as I am well within the guidelines with a FL=235mm, and a dia of 60mm.
    As far as the stop placement, would this be true of all focal lengths, or just the minimum coverage lens ?

    Thanks again.

  2. #22

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    Quote Originally Posted by SteveH
    Dan,
    We're both right I believe. A meniscus can be either singular, or achromatic. Either way, I have seen the doublet called a 'meniscus' in various texts.<snip>
    Thanks for the reply.

    I think you're mistaken. Per my dictionary, a meniscus lens is a concavo-convex lens. Not bi-convex. Shape matters ... And so do names.

  3. #23

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    Dan,
    Thanks, I agree completely with you now. I was confused by the wording of old lenses, and looking at diagrams of the imagon.
    The major reason why I stuck with the thought was from reading this @ the LF board
    Quote Originally Posted by Ernest Purdum
    "Meniscus" refers to the lenses shape, so it can be one piece of glass with considerable chromatic aberration (simple meniscus), or can be color corrected by using an additional element (achromatic meniscus). When P&S used the term "semi-achromat", I'd guess they were saying that the designer purposefully left some chromatic aberration uncorrected. Gundlach, on the other hand, presumably corrected the chromatic aberration as much as possible.
    So, I incorrectly put 2 and 2 together. When I saw the diagram of an imagon (which I thought to be considered a meniscus lens), I saw the doublet and figured the concave lens to be the additional element that Mr. Purdum was refering to. I now see that a 'true' achromatic meniscus is actually 3 lenses.

    Thanks again

  4. #24

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    It was based on rough guidelines he pulled from early photography texts. It allows for a fairly conservative 45 degree angle of view. A longer lens would be better corrected as you'd be using more of the central area of the image circle.

    The stop placement remains the same, at least for slightly wide and slightly long lenses.

    I have to go back and look. After a good night's sleep it seems like it should be FL/0.828, which would give 196mm for 4x5. I remember thinking it was slightly longer than normal "normal". Not that it matters as you already have a lens picked out, but this would make yours only slightly "long".

  5. #25

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    Terence,
    Ok, that's fine, thanks. I just wanted to make sure that the guidelines he layed out were for minimum coverage.

  6. #26

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    One design flaw that my drawing may/may not have is the way the lens faces. In looking throught the LVM, I only see two lenses that would be somewhat simular in construction - the Agfa Igenar f/8.8 105mm, and the Imagon. The agfa is setup how the lens in my drawing is; whereas the imagon has the thinner lens in the front.
    I am thinking that I will have to wait and see what the lens profile actually looks like before I can make this determination; or trial and error. Unfortunately, all that I know is that the lens is 235mm in focal length, 60mm in diameter, and is a cemented achromat.

  7. #27

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    Also, I believe the landscape-type lenses have the concave surface facing away from the film.

  8. #28

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    Ok...
    I found an old brass projection lens today, and picked it up. Upon taking it apart, I found it to have a 240mm achromat in the front, and a 145 in the rear. The one in the front is quite large (64mm) in diameter, and is of a good focal length for me; so I'm going to use it.
    I revised the plans, and have posted them. The nice thing is that I also have the proper sized lock ring for the lens, so that saves me from making one.

    Here is a URL to some photos of the projector lens:
    http://www.flickr.com/photos/veedubr...7594220442744/

    Hopefully I'll get a chance to make the barrel this weekend. I don't see it taking more than 3-4 hours to setup, turn, and thread. Im not sure what Im going to do about affixing it to a lens board...Most likely I'll just thread it in.

    Regards,
    Attached Thumbnails Attached Thumbnails meniscus2.jpg   meniscus2_dims.jpg  

  9. #29

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    Nice. I've threaded several old brass lenses into tempered hardboard lens boards (made from 99 cent clipboards). Works great for relatively light lenses.

  10. #30

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    SteveH,

    Wouldn't it be easy enough to temp. mount the lens to a piece of black PVC pipe with a slot cut in the bottom, make a stop plate with a small handle and move it back and forth to find the optimal spacing between pincushion =|= barrel distortion? The old formuale were just approx. starting points.

    Just a thought.

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