Infra Red & Focusing

Discussion in 'Digital Photography' started by Bernard Rother, Jan 11, 2008.

  1. Have just ordered a Cokin R72 Infra Red filter for a 72 Diam Sigma
    18-200, couldn't find a Hoya screw on type of that size here. I remember
    on my older Nikon glass there was a red focusing alignment curve on the
    lens for infra red. How did the point of focus "shift" ? I would have
    thought if I focused on a tree at 200m then 200m would be 200m ?
    On my older 18-135 I focused with the polariser fitted then screwed on
    the IR filter and took various exposures. I don't think I have to
    compensate for any shift in focus .... do I ?

    Bernard.
     
    Bernard Rother, Jan 11, 2008
    #1
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  2. Bernard Rother

    Charles Guest

    On Fri, 11 Jan 2008 06:52:08 +0200, Bernard Rother
    <> wrote:

    >Have just ordered a Cokin R72 Infra Red filter for a 72 Diam Sigma
    >18-200, couldn't find a Hoya screw on type of that size here. I remember
    >on my older Nikon glass there was a red focusing alignment curve on the
    >lens for infra red. How did the point of focus "shift" ? I would have
    >thought if I focused on a tree at 200m then 200m would be 200m ?
    >On my older 18-135 I focused with the polariser fitted then screwed on
    >the IR filter and took various exposures. I don't think I have to
    >compensate for any shift in focus .... do I ?
    >
    >Bernard.



    Good thing with digital, you can try it and see. Check it at various
    zoom settings, results may vary.
     
    Charles, Jan 11, 2008
    #2
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  3. Bernard Rother

    Martin Brown Guest

    On Jan 11, 4:52 am, Bernard Rother <>
    wrote:

    > Have just ordered a Cokin R72 Infra Red filter for a 72 Diam Sigma
    > 18-200, couldn't find a Hoya screw on type of that size here. I remember
    > on my older Nikon glass there was a red focusing alignment curve on the
    > lens for infra red. How did the point of focus "shift" ? I would have
    > thought if I focused on a tree at 200m then 200m would be 200m ?


    When you have an IR filter and IR film in use the lens no longer has
    the standard achromatic properties that it has for visible light. The
    IR wavelength is longer than the lens is calibrated for so the
    effective focal length is slightly different. NB a mirror lens would
    still work OK it is only refractive optics that are affected.

    > On my older 18-135 I focused with the polariser fitted then screwed on
    > the IR filter and took various exposures. I don't think I have to
    > compensate for any shift in focus .... do I ?


    Provided that there is enough signal to noise in the IR image for the
    autofocus system to work then the camera will get a decent in focus
    image with the IR pass filter in place. Otherwise you are into manual
    focus by trial and error.

    Some cameras have IR reject filters in the light path precisely
    because CCDs are rather sensitive to near IR (and the colour filters
    in a Bayer mask can leak a bit in the near IR).

    Regards,
    Martin Brown
     
    Martin Brown, Jan 11, 2008
    #3
  4. Bernard Rother

    Neil Guest

    In message <>, Bernard Rother
    <> writes
    > I don't think I have to compensate for any shift in focus .... do I ?
    >


    http://photonotes.org/cgi-bin/entry.pl?id=Infraredcompensation

    "Note, however, that this type of IR compensation is not needed if
    you’re doing infrared filter with a standard red (Wratten red 25)
    visible light filter and you aren’t doing closeup work. In such cases
    there’s enough visible light making up the image that focus
    compensation probably won’t be needed and may in fact mess up the
    picture."

    Regards
    --
    Neil Pugh
     
    Neil, Jan 11, 2008
    #4
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