Hello Olympians,
The reason I have my #87 filter behind the mirror and in front
of the shutter is simply to allow me to use the viewfinder.
The #87 filter is visually opaque so one can't take advantage of
the Single Lens Reflex design unless the filter is behind the mirror.
There are actually several ways to mount the filter behind the mirror
but I won't go into it here.
I learned of this technique from an article by Andrew Davidhazy
called Action Infrared Photography.
I have devoted my OM1n to this type of photography.
There is a web site run by W.J. Markerink that has lots of info
on both the theoretical and practical aspects of infrared photography
http://www.a1.nl/phomepag/markerink/mainpage.htm
I urge anyone who is interested in infrared photography, filters
and the like to check out the site.
I'm sorry if I caused any confusion by my musings about the lack of
an infrared focus mark on my 35-70 Zuiko zoom.
Hank Hogan
Marco wrote:
>
> On Mer, 4 feb 1998 14:24, Gene Mayeda <mailto:gmayeda@xxxxxxxxxxxxxxxxx>
> wrote:
> >How does one install the #87 filter behind the lens and in front of the
> >shutter and can you still see through the lens or not;that is since the
> >#87 is opaque to visible light is this a possible way to use this filter
> >without having to remove it in order to use the viewfinder? I do not
> >understand why this results in correct infrared focus unless you are
> >just using the depth of field that you get with the aperture setting
> >that you are using.If this is the case then it would not matter where
> >the filter is placed as far as focus is concerned.It seems to me that
> >you are claiming that the #87 can refract infrared wavelengths to the
> >correct focus.I was not aware of this focus correcting ability of the
> >#87 filter.
> >-Gene Mayeda
> >
>
> Every lens focuses different wavelenghts light at different distances (that
> is called axial chromatic aberration). In common lenses, exp. long lenses,
> there is a considerable gap between blue and green from one side and red to
> the other. So most lenses have a red dot on the barrel in order to correct
> focus when using infrared or b&w red filters. It is a reference for the red
> focusing plane. 87 filter cannot correct focus, the fact is this: that red
> focuses on a different plane that other colours. Some lenses have not red
> dots because are corrected for all the three colours, that is are APO
> lenses.
>
> btw: all my prime lenses have a red dot, while no zoom lens has...
>
> Marco
>
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