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Thread: Single element lens design questions...

  1. #1
    Scott --'s Avatar
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    Single element lens design questions...

    Okay, I'm ordering glass from Surplus Shed. Designing a single-element lens, to be used on 8x10 with no aperture. Need some help deciding what, though. Magnifying glasses are double convex, but Surplus shed has positive (and negative) meniscus lenses, achromatic, etc, etc. What's a good choice? And is there any way to predict coverage? Anything else I should be considering, too?

    Thanks for any help.
    Scott

  2. #2

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    Re: Single element lens design questions...

    Convex - concave (meniscus) lenses have better spherical aberration properties then double convex.

    Achromatic lenses are corrected for chromatic aberration.

    The lenses illuminate quite a big angle. 'Coverage' depends on what area you want to be reasonably sharp and reasonbly evenly illuminated. If you are considering this type of lenses you probably don't expect a high quality image. About 50-60 degrees would probably work quite fine.

    If it were me, I'd order a convex-concave positive miniscus of 300-400 mm focal length.

    Make sure to put the concave side toward the front and put the aperture in front of the lens.

  3. #3

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    Re: Single element lens design questions...

    Now I don't wish to promote e**y. But there is a very interesting Germany lister where the key search "achromat" would find him. He describes his surplus products very well as regards diameter, focal length, clarity and colour and coating. I think you can guess where his lenses come from (Z,S) and they often have slight edge damage. His listing is different in a big way so you will know when you have found him.

  4. #4

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    Re: Single element lens design questions...

    An achromat would be very desirable. Non-achromats focus different colors onto different planes. In most cases, the image from an achromat would be preferable.

    You have some control over coverage and softness when dealing with a simple lens. If you stick your aperture well out in front, you have a narrow angle lens. If the tube is short, you have a wide angle lens, but you may not like the results very much. I would suggest trying to find a lens with a focal length on the long side for your format. As an example, 210mm or a little more if you use 4" X 5". Washer stops slid into the lens tube,and held in place by a snap ring, are the easiest answer to the
    aperture requirement. You'll want several so as to find out what softness you desire.

  5. #5

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    Re: Single element lens design questions...

    Yes he is still there - he has the same surname as the "boss" of cafe Central Perk!
    Current offerings in "our" sizes are. D42mm - EFL285mm, 34mm - 285mm, 33mm - 400mm. Very simple German.

  6. #6

    Re: Single element lens design questions...

    Yes he sells some Zeiss (Jena) optics, but his ads are full of - anyway. The optics are good and that is what counts. Search for "HQO".
    Klaus

    http://www.macrolenses.de for macro and special lens info
    http://www.pbase.com/kds315/ for UV Images and lens/filter info
    http://photographyoftheinvisibleworld.blogspot.com/ my UV diary

  7. #7
    Scott --'s Avatar
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    Re: Single element lens design questions...

    Thanks for the input, guys. Let me ask you this: I'm thinking a positive meniscus lens; there's a 308mm FL lens, 66.5mm diameter. Gives me a little faster than an f/5 lens. I'm not planning on using an aperture - wide open only. Now, I'm pondering on barrel design for this lens. I want to use a Packard with it, but other than that one restriction, I have no idea how long a barrel it needs, or how inset the lens should be.

    Ideas?

  8. #8
    Mark Sawyer's Avatar
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    Re: Single element lens design questions...

    The "barrel" will function mainly as a lens hood, but will also vignette if it goes too long. I'd say an inch to an inch-and-a-half extension front and rear for 8x10. (Regarding movements, keep in mind you probably want to keep the optical axis pointed at the center of the film, as the lens will likely be visibly sharpest in the middle of its image circle.)
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  9. #9

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    Re: Single element lens design questions...

    I just bought a box of stuff that has some 'real' lens barrels in it. If you get an idea of what you diameter/length need, I'll check and see if any of it might suit you. That way you might have an aperture or an existing opening for waterhouse stops at least.

    Richard

  10. #10

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    Re: Single element lens design questions...

    If you rig up a temporary cardboard or plastic tube with a telescoping extension, you can see how long a tube you can use without vignetting. Does your groundglass have clipped corners? If not, remove it for the moment. Look through the lens from each corner and make sure you can't see the tube.

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