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

[George L Smyth <GLSmyth@xxxxxxx>]

Ken Sinclair wrote:
> 
> 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.

The depth of field for a given circle of confusion will most definitely change as a
result of a change of focal length, so perhaps we are talking about something
different (your sentence above doesn't make any sense to me).  You can visit
http://oh114.wpi.edu/cgi-bin/htdof for a visual illustration of this.

> 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.

Believe me, it is much easier to move the lens of a LF camera forward than it is to
move the film plane back. <g>  In past posts I have suggested that one extend the
lens "slightly" forward to get the infrared portion in focus.  However, for my 150mm
lens, that 1/4% means extending the bellows about a third of a millimeter.  Not much,
and certainly covered when stopping the lens down to f16.  With an open lens this
focusing business becomes a bit more crucial with subjects near the camera.  One of
my girlfriend's favorite pictures of herself is a Konica IR shot in the roll back of
my view camera.  I opened up the lens and focused on her, so she is slightly out of
focus (it covers her wrinkles<g>) (the focal point can be seen on the ground).  For
subjects well away from the camera, DOF works just fine.  An example of this is "The
Gazebo At Quiet Waters Park" at http://www2.ari.net/glsmyth/east.htm.

george

- -- 
 Handmade Photographic Images     
  http://www2.ari.net/glsmyth/
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End of Infrared-Digest V0 #278
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