Infrared-how to adjust zoom with no markings?

Discussion in '35mm Cameras and Accessories' started by metod, Jul 18, 2006.

  1. metod

    metod Member

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    I generally use my old Canon AE1 with 50 and 28 mm lenses for infrared film. I’d like experiment a bit with a longer zoom, which I have only on my Canon Rebel. Of course, the zoom does not have any infrared markings on it. Is there a rule of thumb for adjusting the focus when using longer lens (say 100-300 range) for infrared shooting?

    Thanks.
     
  2. kb244

    kb244 Member

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    Generally the longer the zoom, the more you have to shift (so if its 10 foot and you have to shift to 8 to be right on a 50mm, you'll probally have to shift it to 6 or 5 on a 200mm ).

    I don't know of any exact science to how to actually figure out the real distance, I just know on my Tamron 70-210 for my Canon FD, 70 was just a nudge shift, where as 210 was more dramatic.
     
  3. colrehogan

    colrehogan Member

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    Does your Canon rebel use an IR sensor to count the sprockets between frames? What IR film were you planning to use/do you use?
     
  4. kb244

    kb244 Member

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    Even if it does, might not be a big problem as noted by one of my writers.
    IR in a Canon A2
     
  5. Kino

    Kino Member

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    Get a Sony or similar camcorder that has "night vision". Confirm that the camera sees IR by pointing it at a remote control for a TV and pushing some button; you should see the LED remote flash through the viewfinder.

    Place the remote behind some hard-edged object at a given distance and tape one of the buttons down to pulse continuously. Paint the object with the IR light.

    Place the slr on a tripod at the given distance to the film plane you wish to check. Load dummy load, open back, open shutter on "T" with cable release. Turn out all lights and look at film plane through base using the Camcorder and night vision. Focus image painted in IR on film emulsion. Turn on light and compare difference between what you have in IR and what works in visual spectrum. Try a variety of distances to try to get a feeling for what offset works.

    Frank
     
  6. Ole

    Ole Moderator Staff Member Moderator

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    The IR markings are for far IR - the kind only Kodak HIE is sensitive to. With any other film (and HIE with most filters, too) it can safely be ignored.

    If there is significant focus difference between visible light and near IR, that lens is so badly colour corrected it will be unable to take a sharp picture with any film.
     
  7. metod

    metod Member

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    Thanks guys. Rebel IIS is one of the older generations of EOS and use mechanical sprockets for frame counting. That is at least what I've read on the web. I'll give it a try and bracket more than usual with the zoom and see how it goes.

    Metod
     
  8. TheFlyingCamera

    TheFlyingCamera Membership Council Council

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    The quick and dirty solution to avoid the problem is to shoot everything at F16 or smaller. That will cover your DOF sufficiently that even if your focus is off a bit you'll still be in the DOF of the aperture.
     
  9. colrehogan

    colrehogan Member

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  10. Sean

    Sean Admin Staff Member Admin

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    I heard a while back that Apochromatic lenses do not need IR adjustments, is this true? If so you might be able to find a apochromatic zoom lens..
     
  11. Ole

    Ole Moderator Staff Member Moderator

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    All the various chromatic corrections are good only in the visible range. APO or not makes no difference.

    But as I said above, for the near-IR we're dealing with unless using Kodak HIE and some really esoteric filters, the "IR" is close enough to red light that we can ignore all corrections.

    One of my pet peeves is my Russian fisheye lens. Due to the construction, you can only use the filters enclosed with the lens: Clear, yellow, Green, Red.
    It's got an IR mark, so far off that if it were correct for IR, an unfiltered shot would look like it were shot through a stained-glass window. Note that there is no IR filter enclosed...
     
  12. Fotohuis

    Fotohuis Member

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    I heard another explanation from an optical product specialist in Solms (Leica). Their APO lenses are corrected over 900nm so do not need any focus correction.

    The normal focus correction (red dot) on lenses is valid around 800nm. Due to the fact that most photographers are using wide angle lenses for IR, the problem is not so big because in most cases it's indeed in the DOF.

    Best regards,

    Robert