Re: Refocusing for IR photography using Nikon ED glass lenses.

[Ken Sinclair <photo1@xxxxxxxxxxxxxxx>]

GLSmyth@xxxxxxx wrote

>The shorter the focal length. the greater the depth of field.  The above
>sounds
>backwards to me.
>
>As I normally shoot with the lens stopped down to f11-f16, depth of field
>takes care
>of everything and I do not refocus.

I would have preferred the statement "...the shorter the the focal length
the greater the
APPARENT depth of field", since we seem to be discussing "accuracy" in
written statements.

According to all that I have read (and tried to practice) is that at any
given F-stop, the DOF with all lenses will be the same as long as the
position of the camera remains constant.

It has been described that to focus IR with "normal" glass lenses, the
focal plane of the IR image requires that the rear nodal point of the lens
in use, be moved forward, or the in Large Format cameras, the film plane be
moved further back one quarter percent of the distance from rear nodal
point of the lens plus any extension for 'close' focussing) to bring the IR
image into focus on the film plane. In some cases there may be enough
"depth of focus" on the lens (at the film plane, at the given F-stop) that
will bring the IR image in focus on the surface of the film. I would
hesitate to imply that it depends (sorely) on the F-stop indicated on the
lens.
Non-glass lenses such as the specialised fluorite lenses need no such
correction when recording images outside the normal hominid-visible range.

Ken



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End of Infrared-Digest V0 #277
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