DIY lens- periscope question

Discussion in 'Large Format Cameras and Accessories' started by Niebylsamwlesie, Oct 5, 2013.

  1. Niebylsamwlesie

    Niebylsamwlesie Member

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    Hi

    I have a simple question. I've got two +1 lenses and I'd like to make periscope type lens +2 diopters.
    It is quite simple, just two parallel lenses, but what should be distance between them?

    greetings

    JW
     
  2. Maris

    Maris Member

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    I'm not sure it is possible to get a +2 lens out of two +1 lenses except by putting the lenses in contact. Gullstrand's Equation seems to apply here.
     
  3. Niebylsamwlesie

    Niebylsamwlesie Member

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    periscope.jpg
    This is periscope type lens. In my case both lenses are +1 diopters. as a whole, this couple has +2 diopters. My question is about distance between these tho lenses.
     
  4. Jim Jones

    Jim Jones Subscriber

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  5. wombat2go

    wombat2go Subscriber

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    And you can model it with OpticalRayTracer.jar , a GPL download and very simple to use.
     
  6. Steve Smith

    Steve Smith Subscriber

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    Does it need to be exactly +2 Dioptres? Put them as close together as you can and I'm sure it will be accurate enough.


    Steve.
     
  7. Niebylsamwlesie

    Niebylsamwlesie Member

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    Thanks a lot. This is what I really needed :smile:)

    greetings

    JW
     
  8. Jim Jones

    Jim Jones Subscriber

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    Sometimes the answer to a simple question is more complicated than the answer to a complicated question. Is the focal length of the proposed Periscope critical? As you can see from Gullstrand's Equation the focal length will be slightly less than 500 mm for practical element separation. Will a shutter be used between the elements? What is the required image size? Periscopes have been designed to cover up to 70 degrees at f/30. In primitive cameras, the coverage was less, and the aperture larger. Will the chromatic aberration inherent in the Periscope be a problem? For black & white photography, performance can be improved by color filters. How will you mount the lenses? Some of us are experienced in improvising lenses and lens accessories.
     
  9. Mustafa Umut Sarac

    Mustafa Umut Sarac Member

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    APUG is blind to optical science IMHO. For such forums , these questions are complex , some other forums , they are very easy to answer , nobody bothers. I could not find a sweet spot either.
     
  10. Niebylsamwlesie

    Niebylsamwlesie Member

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    The focal length is not critical. 500mm +/- 10mm is no problem. There is no shutter inside. Just simple couple of lenses and removable stops inside. The image size is 35x35cm. My periscope lens is done yet. I'l try it when the camera will be ready to use. My LFC camera is DIY as well.


    thanks a lot

    JW
     
  11. Old Fart

    Old Fart Member

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    I use the R.D. Gray Periscope No. 6 on my 8x10. I'm curious if the lens design being described in this thread is of the same family as the R. D. Gray by the same name? I'm curious as I've traveled these forums for a number of years and have never heard mention of the Periscope lens. Now it's the talk of the town. ;-)

    Basic sales info for the R.D. Grays No. 1 through No. 10 (ef 2-3/8" thru 36") can be found here . . .

    http://books.google.com/books?id=Xa...6AEwAg#v=onepage&q=r d gray periscope&f=false
     
  12. Dan Fromm

    Dan Fromm Member

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    Here are prescriptions for two Steinheil Aplanat Periskop lenses:

    wide angle (90 degrees): http://www.dioptrique.info/OBJECTIFS1/00022/00022.HTM

    narrow angle (45 degrees): http://www.dioptrique.info/OBJECTIFS4/00181/00181.HTM

    Also see Eric's comments on Periskops at http://www.dioptrique.info/base/complements/periskop.HTM

    Both lenses are symmetrical, both are, by modern standards, dreadful. If you want to calculate the performance of lenses for which you have prescriptions, you can download his program 030 from here: http://www.dioptrique.info/base/complements/a_propos.HTM
     
  13. desertrat

    desertrat Subscriber

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    If you want to get a reasonably flat field, the curvatures of the lenses and the spacing between them is important. If the curvatures are fairly steep, you can get a flat field with the elements close together. If the curvatures are less steep, the elements need to be farther apart. If the curvatures are too steep or too shallow, a flat field cannot be obtained.
     
  14. wombat2go

    wombat2go Subscriber

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    Dan, I am modeling ( just as a learning ex) the 90 degree wide angle prescription on the OpticalRayTracer3.2.jar
    The model is predicting an on axis focal length of about 98 mm and the optimum increases to about 110 mm at about 35 degrees off axis ( ie 70 degrees coverage).
    But I am not sure If i read the prescription correctly, in the 12.6 mm distance between the lenses.
    Do you know if that distance is between glass to glass on the axis , or were some other reference datum used in the lenses?
    What frame size would this lens have been intended for?
    What does "no dioptre diaphragme :2 " mean?

    Thanks!
     
  15. Dan Fromm

    Dan Fromm Member

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    The distances are measured on-axis.

    means that the diaphragm goes after the second surface. The lenses are 1.3 mm thick, the distance between them is 12.6 mm. You got it right.

    If you click on "courbes" you'll see some calculated performance data. The news isn't good, horrible curvature of field. But since the OP wants to use a long lens on a small format, i.e., need the lens to cover only a small angle, he may be able to get away with it.

    The calculated spot diagrams are dreadful too. The world gave up on periskop lenses for good reason.
     
  16. wombat2go

    wombat2go Subscriber

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    Thanks Dan,
    I will keep chewing away at the model as it is a simple one to understand. I will try it on 6X7 diagonal with different apertures too.
    I suppose it will be better bang for the buck to get some surplussheds and model them, then try in real life !
     
  17. wombat2go

    wombat2go Subscriber

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    Some screen shots of the rays through the 90mm periscop.
    The rays are from a point at 5 metre.
    At f/40 and 40 degrees ( near edge of 4 x 5) the COC ( spherical + chromatic) was about 200 micrometre. Not too bad?

    The biggest problem was that the focal plane moves as the aperture changes.
    F/40 > F/11 it moved from 98mm to 88mm.
    The second shot is at f/11 and 26 degrees to simulate the lens on 6 x7 with a practical aperture.
    With best focus on the axis, the COC at 26 degrees is about 500 micrometer as shown. No good at all, I think
     

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