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Infra Red & Focusing

 
 
Bernard Rother
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      01-11-2008
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.
 
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Charles
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      01-11-2008
On Fri, 11 Jan 2008 06:52:08 +0200, Bernard Rother
<(E-Mail Removed)> 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.
 
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Martin Brown
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      01-11-2008
On Jan 11, 4:52*am, Bernard Rother <(E-Mail Removed)>
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
 
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Neil
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      01-11-2008
In message <(E-Mail Removed)>, Bernard Rother
<(E-Mail Removed)> writes
> I don't think I have to compensate for any shift in focus .... do I ?
>


http://photonotes.org/cgi-bin/entry....edcompensation

"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

 
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