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Thread: Meniscus lens iris "sweet spot"???

  1. #1

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    Meniscus lens iris "sweet spot"???

    Hi,
    I'm putting together a couple of single meniscus camera lenses (about 250mm) for 4X5 shooting, and wanted to know if anyone has tried to optimize the distance from the front stop aperture and the concave element surface???

    I poked around online, and there was mention there is a balance between spherical aberration and rectilinear distortion there somewhere... My last attempt came out great simply taking a 270mm element and positioning it as close to the iris as physically possible, but wanted to know if there was a variation of "special" effect at different spacing distances???

    Then there's the addition of the "magic" spit Steichen used early in his career for that dreamy soft-focus stuff??? ;-)

    Thanks,

    Steve K

  2. #2
    jp's Avatar
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    Re: Meniscus lens iris "sweet spot"???

    I have built meniscus lenses (spaghetti measure lens) and bought meniscus lenses (Reinhold wollaston meniscus, Kodak 305 portrait (doublet), tube for spencer portland) and used some others including other smaller spencer portland and bigger kodak portrait. Seen enough that I can roughly guess where the iris would go ahead the lens.

    Most are positioned quite a ways ahead of the lens but not far enough to interfere with their not that wide field of view. Sorry I don't have math other than focal length / front opening approximately equals f-stop...

  3. #3

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    Re: Meniscus lens iris "sweet spot"???

    I think I read online that Thomas Dallmeyer had a "rule of thumb" of iris/lens distance of 1/5 of the FL, but this would surely limit IC, but haven't seen many "soup can" iris' on lenses, but haven't seen too many meniscus lenses sold either!!!

    Spherical aberration does not bother me (used plenty of SF lenses) but rectilinear distortion kinda sucks...

    Steve K

  4. #4

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    Re: Meniscus lens iris "sweet spot"???

    Lens design books have discussions of this. In Warren Smith's "Modern Optical Engineering" he works it out that the ideal position for the stop is about 1/6 of the focal length in front of the lens, and the meniscus lens should be concave towards the subject. (This is for a specific example but should roughly hold for other lenses.) This position of the stop sets the coma to roughly zero, and (overcorrected) astigmatism balances curvature of field.

    For a simple lens, curvature of field will likely limit usable image circle before vignetting by the stop. I have a Brownie Special camera that had a curved film plane presumably to follow the field curvature of its meniscus lens (other than that, it's not actually that special).

  5. #5

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    Re: Meniscus lens iris "sweet spot"???

    Thomas Grubb pondered this back in the late 1850s and came up with a 'portable' (pillbox style mount) which gave reasonable definition and coverage when the stop was pushed as far forward as it was designed to go, but which also enabled the stop to be positioned closer to the lens for greater coverage.

    To quote: "The lens had a special mount by which the distance of the stop from the lens could be varied. By bringing [the] stop nearer the lens, the distortion was reduced at the expense of increased curvature of the field. On the other hand, when the distortion was not material, for instance for landscape work, the field could be flattened by increasing the distance of the stop from the lens."

    There were numerous contemporary reports in the press about the ability to increase coverage (producing a wide-angle if used on a larger format) by varying the stop position, which many photographers found to be advantageous despite the distortion. So I suppose the solution is to try it empirically to decide what compromise works for the specific lens you have.

    Another quote about the Grubb mount: "This mount is now almost exclusively used with the Aplanatic Lens (the Camera being such as supplies the means for focusing). The advantages of this Mount are not merely those of superior lightness and compactness; it supplies the means of readily adjusting the distance between lens and stop, so that either the more generally useful field shall be covered with the maximum distinctness and brilliancy, or a much larger field, extending to 60 or even 70 degrees, can (with the Aplanatic Lens) be covered, using a stop of lesser aperture. This power of including so large an angle is not only useful for obtaining Panoramic Pictures, but also for including the entire of a lofty structure, while avoiding the tilting of the Camera and the inevitable consequent distortion."

  6. #6
    Ron (Netherlands)'s Avatar
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    Re: Meniscus lens iris "sweet spot"???

    read through this site which contains some formulas : https://sites.google.com/view/wollas...cape-lens/home
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    Re: Meniscus lens iris "sweet spot"???

    Quote Originally Posted by Ron (Netherlands) View Post
    read through this site which contains some formulas : https://sites.google.com/view/wollas...cape-lens/home
    Thanks for posting this site .... Great info !!!

  8. #8
    Nodda Duma's Avatar
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    Re: Meniscus lens iris "sweet spot"???

    The lens distance balances field curvature and astigmatism, with some effect on coma. Spherical aberration is corrected by the lens shape.

    Coma, lateral color and distortion are corrected by stopping the lens down
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  9. #9

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    Re: Meniscus lens iris "sweet spot"???

    Great answers, thanks guys!!!!

    Steve K

  10. #10

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    Re: Meniscus lens iris "sweet spot"???

    I'm the guy that wrote the web page on the Wollaston Landscape Lens someone gave a link to above. https://sites.google.com/view/wollas...cape-lens/home

    Would you tell me a little more about the lens you are using and where you got it? Diameter, focal length, shape, image size, etc. Where did you end up putting the stop? Thanks.

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