Using IR mark on lens.

Discussion in '35mm Cameras and Accessories' started by Mike Kennedy, Oct 8, 2007.

  1. Mike Kennedy

    Mike Kennedy Member

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    My second time out with Kodak HIE will be this morning.I'll be using a Nikon FE with a Tamron SP 28-80mm lens.
    Do I focus on my subject ,then turn the lens to the IR mark on the lens barrel?

    Thanks
     
  2. Steve Smith

    Steve Smith Member

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    I think you've got it.

    Focus as normal then move the distance from the normal focussing mark to the IR mark.


    Steve.
     
  3. Mike Kennedy

    Mike Kennedy Member

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    Thanks Steve.
     
  4. Toffle

    Toffle Member

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    I don't have a lens with an IR mark. What then?
     
  5. Ray Heath

    Ray Heath Member

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    g'day all

    no ir mark? the difference is very slight, use f8 or smaller and don't worry about changing focus
     
  6. keithwms

    keithwms Member

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    Stop down cautiously! Think in terms of the hyperfocal method- remember the basics about how to maximize DOF, and then stop down a bit more if you are worried. This logic works well for landscape, but for closeups, it's better to use a lens with an IR mark or better yet use an apo lens.
     
  7. Toffle

    Toffle Member

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    This is much as I had guessed.

    I just checked the lens for my GS-1 (which I haven't used for IR) it has an IR focus mark a couple of degrees on the far side of the hyperfocal scale. By extension to my SLR lenses without a mark, would I then focus a couple of degrees on the near side? (or have I got this all backwards?)

    Cheers,
     
  8. AgX

    AgX Member

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    First, there are are lenses which are said to need no correction as catadioptric lenses and those lenses special made to cope with a broad spectrum.

    Further, in literature it is stated that with lenses up to 80mm and medium aperture no focus correction will be necessary. (Old literature, prime lenses thus…)

    A common (old) rule would be:
    additional extention = focal length + focal length/250

    To get optimum results:
    Make test photos with a target placed at your typical oject distance.
    Use a caliper to measure the added extension (or more practible: marks for barrel turning)
     
  9. Kiron Kid

    Kiron Kid Member

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    If you're using a medium or deep red filter, there's no need to adjust your focus. If you're using an opaque filter, and there's no IR markings on your lens, focus slightly in front of your subject (about 10%). You'll want to use f/8 or smaller.

    Kiron Kid
     
    Last edited by a moderator: Oct 7, 2008
  10. keithwms

    keithwms Member

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    I usually just focus a bit closer and then stop down roughly enough to preserve the DOF that I would normally have wanted before refocussing. This is the procedure I use for near-IR film (Rollei 400) with a #87 filter. I guess sfx will be quite similar, though for medium format I use lenses with IR dots so I can't say from experience. Ilford, why isn't sfx available in sheets!!!!

    AgX I am trying to guess where that number 250 comes from, do you have a quick answer? I can't think of any universal reason why it should be so. I mean, doesn't it depend critically on the sensitivity of the film, the cutoff of the filter, and last but not least how much ED glass the lens has versus non-ED? Is it because the index of refraction for glass is all roughly the same?
     
  11. Xmas

    Xmas Member

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    If you can recall the experiment in school, the prism bends the light, but the red and blue are bent by different angles, hence you can see the spectra of white light.
    There are (were) two classes crown and flint which dispersed the light by different amounts, separated the colours by greater/lesser amounts(dispersion) then Leitz discovered anamalous dispersion glasses, as calcite was a pig to manufacture, calcite has anamolous dispersion...
    So there are lots of different glasses, some very high refractive, i.e. bending the light by large amount, and variable in dispersion, there are catalogues of different glasses to feed into the computer design lens software.

    Noel
    P.S. Some one must be an optics engineer...
     
  12. keithwms

    keithwms Member

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    Thanks, I know there are different glasses, my question was where the number 250 comes from, so that I can ascertain whether the expression is generally useful. My guess is that it assumes a specific change in the index of refraction as a funciton of wavelength.

    Anyway here is my answer...

    [​IMG]
     
    Last edited by a moderator: Oct 22, 2007
  13. Kiron Kid

    Kiron Kid Member

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    If you're using a non-opaque filter, there is no need to adjust your focus point. Just use a fairly small aperture. If you are using an opaque filter and your lens has no IR markings, focus slightly in front of the subject matter. I routinely meter, focus and expose my Kodak HIE through a deep red #29 filter, and get consistentlyy good results.

    Kiron Kid
     
    Last edited by a moderator: Oct 24, 2007
  14. AgX

    AgX Member

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    keithwms,

    Sorry for not answering you. I can no longer search for my own postings (and the follow up) here.

    Yes, you are right. I based that number 250 on old literature where numbers between 200 and 300 were stated. Obviously it is a matter of refraction related to effective (filtering) wavelenght. Keep in mind that there are specially constructed lenses which do not need adjustments.
     
  15. keithwms

    keithwms Member

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    Thanks. Indeed, I recently started using APO process lenses for IR and have found no need for refocus.
     
  16. Ed Sukach

    Ed Sukach Member

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    Trying for simplicity:

    Glasses, and other optical materials; Crystalline Quartz, and plastics, have widely different Indices of Refraction.

    The working Index of Refraction WILL change, slightly, with the wavelength of light concerned.

    Lens systems with many "first surface mirrors" (reflective) will be affected LESS - in general - than those depending on refraction. Less refraction, less concern.

    "Apochromatic" lenses are - usually - more "highly corrected" in regard to chromatic abberation. Or so it says. sometimes I wonder. They are certainly NOT "miracle" lenses, completely immune to the laws and effects of optical theory.

    Whether or not try to compensate for "focusing shift" with IR film ... It depends a great deal on the character of light involved and the sensitivity of the film receiving that light. With the films whose sensitivity "peak" is closer to that of the visible spectrum (Konica IR, SFX ...) and where focusing is done in a SLR complete with a "mild" IR filter (Wratten #25, or like that ..), I'd forget trying to "compensate". Through experience, I've found that has caused far more out-of-focus-ness than it has corrected.

    Kodak HIE, first focusing without a filter, and adding filter attenuating most visible light, should be another ball game entirely. There, the IR Index Marks seem to make sense.