Re: infrared focus correction

[Dieter Lefeling <lefeling@xxxxxxxxxxxx>]

Cameron Shaw wrote:
> 
> I found in focus tests with HIE and #25 filter, using Nikon F4 ( with non APO
> lens85mm f2 ) that the focus correction markings on the lens gave too much
> correction. Then I realised that focusing through the #25 red filter my eye was
> already making some correction.
> So IMHO focus correction is too small to be needed in this situation. Maybe if
> the focus was first set with #25 and then a #87 filter was used some of the
> correction would have been made so with a reasonable depth of field no focus
> correction would be needed) Of course the halation effect softens the edges.
> 
> So my question is please, for non APO lenses what wavelength does the
> marked focus correction correspond to, xxx nm ?

Hi Cameron,

your findings exactly match mine. Please read my "Do it different" post
from last Friday, 1:21 MET. Here I mentioned some reasons why focus
correction indeed might be obsolete when you focus through a #25 filter.

Short summary: A part of the spectrum that HIE records with a #25 filter
(600-900 nm) is already used for focusing (600-700 nm), so that at least
that part is perfectly in focus. The rest (700-900 nm) depends on your
lens' color correction.

I don't know to which wavelength Nikon's IR-index is calibrated, but for
Canon lenses it's 800 nm. I assume Nikon will do it at least similarly.
This means, the index represents the focus correction that's needed if
you focus without a filter ("white light" - usually 550 nm) and the
picture is exposed with 800 nm (e.g. through a #87). Since things are
different with a #25 red filter, focus correction usually is obsolete.
At least that's also my impression with lenses up to 85mm.

Dieter


*
****
*******
******************************************************
*  To remove yourself from this list, send:          *
*         UNSUBSCRIBE INFRARED                       *
*       to                                           *
*         MAJORDOMO@xxxxx                            *
*----------------------------------------------------*
*   For the IR-FAQ, IR-Gallery and heaps of links:   *
*  http://www.a1.nl/phomepag/markerink/mainpage.htm  *
******************************************************

------------------------------

End of Infrared-Digest V0 #412
******************************