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Thread: ULF lenses

  1. #1
    Guest
    Guest

    ULF lenses

    Hi,

    if theres one thing harder to find than a cheap 8x10 lens its a cheap 10x12 or 20x24 lens!

    since we are exploring homemade lenses, why not think big?! lets design a monster, for pennies and bugout all the smug sods on the "large format forum"

  2. #2

    ULF lenses

    If you don't mind a very long focal length, almost any conglomeration of glass will cover an acre. Generally, it's pretty simple to make a symmetrical lens cover in a "normal" focal length.

    One design to consider is the Periskop -- it's a symmetrical pair of meniscus achromats with a stop (and often a shutter) halfway between. If the focal length is long enough, once can dispense with achromaticity; I've owned a couple Speedex Jr. 6x6 cm folders with this type of lens using single-glass elements, and at approximately 80 mm focal length, they're sharp enough to sustain 8x12 inch enlargements from a 35 mm frame size crop well off center.

    So, what you want to cover, say, 20x24, is a pair of fairly deep meniscus lenses of about 62 inch focal length (to give the final combination a focal length around 31 inches); that'd be around +5/8 diopter (+1/2 should work fine, just be a little longer focal length lens). Spectacle lens blanks might work, but you'd probably be ahead with something more deeply curved. The classic Periscop has the lenses separated by their inner radius of curvature, which is usually about 1/8 to 1/6 of the focal length, and is usually stopped to f/11 or smaller.

    The advantage of a symmetrical lens is that all geometric distortions are cancelled and the field is flat, as a consequence of the glass geometry. With a little fiddling with spacing and stop size, almost any symmetrical pair of postive lenses can be used (a pair of cheap telescope objectives, for instance, from the 60 mm refractors commonly found in junk shops; those are usually about 400 mm focal length, so a pair would give 200 mm and should nicely cover 8x10 with a stop to f/11 or smaller). The result, in a size that's only ever contact printed and normally viewed from well past arm's length, ought to compete nicely with multi-kilobuck modern glass.

  3. #3
    Guest
    Guest

    ULF lenses

    thanks much, ImageMaker, for your detailed and always useful posts!

    i'm inspired by the 20x24 prints produced by Poleriod's monster camera-for-rent both using pinholes and lenses. im not sure when i will get roung to putting one together but its nice to know that its within the bounds of reasonable possibity.

  4. #4

    ULF lenses

    One thing to consider with a 20x24 camera is the price of film. Even paper in that size isn't anything I'd call "cheap", but starting with paper is probably preferred. An alternative would be to (in the dark, or using ortho under safelight) mount multiple smaller sheets at various locations to verify things like flatness of focal plane and image plane coverage using film that doesn't cost an arm and a leg per exposure (20x24 film is going to cost you a *minimum* of about $20/sheet). You could even make a virtue of this economy, making your final 20x24 image from an array of 30 4x5 sheets instead of a single full frame film...

  5. #5

    ULF lenses

    20x24 graphic arts film can be had cheaper than that...

    Kodak still makes large rolls of graphic arts film under the name of Polychrome, out of Kansas.

  6. #6

    ULF lenses

    Mes-

    I posted some suggestions for long fl lenses but it's hard to catch up with you...the ULF ?'s are appearing in multiple locations!

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