Issue with focusing projection through a condenser lens

Discussion in 'Photography' started by Doc, Nov 19, 2006.

  1. Doc

    Doc Guest

    I've set up a rig to do "screenless" capture of film through a 4 1/2"
    condenser lens which is then bounced off a front surface mirror at 45 deg
    into the cam. Generally it seems to work pretty well, however, I find that I
    get a mild blurriness on the outer periphery of the image. I've tried
    various combinations of zooming of the projected image with the projecter
    onto smaller/larger areas of the lens and the corresponding zoom in/out of
    the capturing cam but can't seem to make it go away. Yes, I could zoom in on
    only part of the image effectively cropping it, but my goal is to capture
    the whole image.

    Is there a way to correct this or is it unavoidable?


    Thanks
     
    Doc, Nov 19, 2006
    #1
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  2. Doc

    Dave Schaack Guest

    What you are seeing is the effect of the combined full field aberrations of
    the "condenser" lens and the camera lens. You could try using a different
    lens for the "condenser", one better suited to this application. However,
    in general, you will see this effect for the combination of any two lenses
    not designed to work together in this way.
     
    Dave Schaack, Nov 19, 2006
    #2
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  3. Doc

    Colin_D Guest

    Optically, a condenser lens is very poor. It is not designed to focus
    images, only to 'condense' the divergent light from the lamp, i.e. to
    'bend' the light at the edges back to illuminate the slide/negative.

    As such, it is not corrected for chromatic aberration (color fringing,
    miniature rainbows around images), and most other aberrations as well.

    So, it's unavoidable. You will need to use a scanner, or a lightbox and
    a macro lens on the camera to do it properly.

    Colin D.
     
    Colin_D, Nov 19, 2006
    #3
  4. Doc

    Pat Guest

    Might be worth a try to put a piece of paper between the lens and the
    film to diffuse the light.
     
    Pat, Nov 19, 2006
    #4
  5. Doc

    Doc Guest

    Not following you. Between which lens and the film? Between the projector
    lens and the film obviously isn't going to work, nor between the projected
    image and the condenser lens, which obviously would block the image from
    reaching the lens.

    Do you mean between the lamp and the film? If so, I've already got an opal
    diffuser in place.
     
    Doc, Nov 20, 2006
    #5
  6. Doc

    Doc Guest

    Hmm... Can you elaborate on a lightbox? Or point me to a URL of an example
    of what you're talking about? I assume the cam has to be such that it's
    built to accept a macro lens? I'm currently using a Sony Digital8 Camcorder.
     
    Doc, Nov 20, 2006
    #6
  7. Doc

    dj_nme Guest

    A lightbox has a panel on the top (or front, if it's wall mounted) made
    from translucent material with a diffuse light source behind it.
    Basicaly the idea is to provide a glowing surface that tranparencies
    (such a slides or Xray film) can be viewed on with requiring a projector
    and a screen.
     
    dj_nme, Nov 20, 2006
    #7
  8. Doc

    dj_nme Guest

    Between lamp and film would be the logical place, but it is the
    attempted cheap alternative to opal glass anyway...

    My solution to your problem would be to find a much larger lens to act
    as a relay lens.
    Or alternatively, use a macro bellows to move the lens on your DSLR
    further from the camera body to allow closer focusing and greater
    enlargement than is possible with the focusing helical on its own.
    Another alternative is to get some "macro" filters, they are lenses that
    screw into the filter thread on the lens and allow closer focusing as
    well (it acts like "reading glasses" for your camera lens).
     
    dj_nme, Nov 20, 2006
    #8
  9. Try an achromatic lens instead of a condenser lens. Much less aberration.
    Should cost you only $6 or $12 at surplusshed.com for a suitable specimen.
     
    Richard J Kinch, Nov 20, 2006
    #9
  10. Doc

    Doc Guest

    Something that I'm not sure I made clear, I'm capturing motion film - 8mm.
    I'm under the impression DSLR (digital single lens reflex?) means a still
    camera? I'm using a Digital8 camcorder as my "interface".
     
    Doc, Nov 20, 2006
    #10
  11. Doc

    Doc Guest

    Any suggestions for size?
     
    Doc, Nov 20, 2006
    #11
  12. Doc

    Doc Guest

    Actually, size and any other pertinent parameters? Again, I'm capturing the
    image from an 8mm movie projector.

    Thanks
     
    Doc, Nov 20, 2006
    #12
  13. Doc

    dj_nme Guest

    This is the first time you've mentioned that you're trying to capture a
    movie film with a camcorder.
    Perhaps you could project the film onto an ordinary projection screen
    and then point the video camera at the screen, with the camera as close
    as possible to and parallel to the projector lens.
    Perhaps one of the video tranfer boxes that are listed on eBay would be
    the way to go?
    <http://search.ebay.com/projector-vi...mZR10QQsaslcZ2QQsbrexpZWD2SQQssPageNameZWD2S>
     
    dj_nme, Nov 20, 2006
    #13
  14. Doc

    Doc Guest

    Thanks. I have a custom-made rig on that order that actually works
    better than the typical "conversion cube" - better projection material
    with no hotspot. But it seems eliminating the transfer screen would
    give a couple of notches better resolution, no grain from the screen
    etc.
     
    Doc, Nov 20, 2006
    #14
  15. Doc

    dj_nme Guest

    My idea of "fine tuning" your current set-up is to use a much larger
    realy lens, perhaps two or three time bigger in diameter than the size
    of the image.
    That would be so that the image isn't passing through the periphery of
    the lens were there is more of a "prism effect" (chromeric abheration)
    or other distortions that are lesser towards the centre of most lenses.
    The rest of the lens could be masked off behind a piece of black cardboard.
     
    dj_nme, Nov 20, 2006
    #15
  16. Doc

    stauffer Guest

    Why do you want to do screenless transfer? I have used one of those
    cheap rear projection setups you can buy that has a folding mirror and
    translucent screen. Works pretty well. The translucent screen does
    create a bit of a "hot spot" in the center, but it is not bad. I have
    been thinking of replacing the stock plastic screen with a real ground
    glass screen (it is only about 12 x whatever, but haven't got around to
    it yet.

    The slight hot spot is the only problem I have. The resolution I get
    is better than the 8mm and Hi8 films I am copying.
     
    stauffer, Nov 20, 2006
    #16
  17. Doc

    Jukka Aho Guest

    There is a hotspot removal filter for VirtualDub:

    <http://neuron2.net/hotspot/hotspot.html>
     
    Jukka Aho, Nov 20, 2006
    #17
  18. Richard J Kinch, Nov 21, 2006
    #18
  19. Doc

    Doc Guest

    One issue is that you lose a slight amount of sharpness. You can minimize
    this with obsessive focusing of both projector and cam but you still lose
    some. While you also lose some due to the limits of the camcorder,
    eliminating the plastic screen eliminates 50% of the problem. And in really
    light scenes, grain in the plastic screen is visible. The casual observer
    might not even notice it but I do.
     
    Doc, Nov 21, 2006
    #19
  20. Doc

    Doc Guest

    Hey I appreciate the lead.

    Couple of questions:

    Is the placement of the mirror is going to be more crucial with the
    achromatic lens? My current setup has the apex of the condenser lens at
    about 6" from the lens of the projector and the mirror angles out at 45deg
    from about 1" from the plane of the edge of the lens, butted up against the
    backside of the board the lens is mounted to.

    I take it there are various configurations of achromatic lenses, how would I
    determine what would be the right "kind" of achromatic lens(es) to focus the
    image at the distance I need - say between 6" - 12" out and whether I need
    2? How would you use 2 in series - would they be flipped around from each
    other or oriented the same direction?

    Also, currently the projected image is probably only around 1 1/2" - 2" in
    the center of the condenser. Do you feel I'll see a noticeable difference
    using achromatic lenses?
     
    Doc, Nov 21, 2006
    #20
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