Re: 4x5 field camera considerations - Reply

[Scott Donkin <photos@xxxxxxxxxx>]

At 01:35 AM 05/12/96 +0000, you wrote:
>focussing the IR image is one quarter of one percent of the
>distance from the rear nodal point of the lens to the film.
>
>ie move your film BACK that calculated distance.


Some people here might want to hear about how to find the Rear Nodal Point
(RNP).

For the purposes of this discussion, lets say the RNP is the point at which
converging light rays "cross over",  inverting the image that will be formed
at the image plane (film).  There is a better description of this, but  it
aint necessary for the following info.

The RNP does not necessarily fall in the middle of your lens.  To find where
the RNP of a particular lens is, start by visually focussing at infinity.
Now take a rule and measure a distance from your film plane forward a
distance equal to the focal length of your lens.  This point is the RNP.
Ie, with a 50mm lens, the RNP will fall 50mm forward of the film plane when
the lens is focussed for infinity.  

>From here on, when you go to extend your lens by the 1/4 of 1 percent for IR
focussing (works well for me when I use my 4x5" IR stock), use the RNP as
the point from which your measurements are taken.

As a side note, any of you wishing to do a panoramic shot, using a standard
format of film (ie, not a panoramic format), a fair degree of success can be
had by using the following method....

You may have noticed that taking a shot, panning the camera a bit, taking
the next shot, and so on, then cutting and pasting your prints does not work
too well.  That is, your prints do not line up properly, and you get an
horizon that is not smooth.

Try this.  Work out where your RNP falls, then build / make a device that
ensures that the RNP is the pivot point at which your camera pans when on a
tripod.  One such device that does work well for this, and allows a certain
degree of adjustment, is the bracket for a Metz 45 series flash (or similar).

Any Hasselblad owners will find that the tripod lount hole on the bottom of
the camera is about 80mm forward of the film plane, so you will be just
about right for this exercise (depending on your tripod) when using the 80mm
lens.

Once you have taken your series of frames (camera exposure, and subsequently
enlarger and dev times, must remain the same for all prints for eveness of
density), cut your prints and mount them end to end for a seemingly (near)
perfect panorama!

If anyone wants the above explained a little bit more in depth, E-mail me.


Cya...

   ______
   Sco||                E-mail:   photos@xxxxxxxxxx
                       Fidonet:   3:713/914.11

..due to the current economic situation, the light at the end of the tunnel
has been turned off until further notice.


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Topic No. 5