INTRODUCTION
A
panorama is a wide or expansive view. In still photography
a panoramic image is one that is wider -sometimes much wider-
than normal. Without a specialized camera, panoramas are formed
by creating a mosaic of two or more images in a process called
stitching. Although images are generally stitched horizontally
they can also be stitched vertically or in both directions.
In
order for two images to stitch together properly they must
be taken with the entrance pupil of the lens in the same position
in space. If this requirement is not met then objects at different
distances from the camera will move in relation to each other
making stitching impossible. This requirement can be relaxed
only for those situations where the entire scene is fairly
far away and that is why the demonstration shots above were
made in close quarters and not of a landscape.
ENTRANCE PUPIL
Images
for a horizontal panorama are usually collected by panning
the camera on a leveled tripod. But the camera is not rotated
about the tripod socket but rather about the entrance pupil
of the lens. The location of the entrance pupil is a function
of the focal length and focus position of the lens. If you
have made an investment in a panoramic head then generally
locating the entrance pupil is done by trial and error.
A pair of objects, one close by and one distant are sighted
through the viewfinder and the panoramic head offset is adjusted
until these objects no longer move relative to each other
when the camera is panned. However, if you're constructing
your own panoramic setup with a fixed offset then you need
to determine the offset beforehand. This tip provides a technique
for locating the entrance pupil without requiring a fancy
panoramic head.
A
METHOD
I
want to share this method that I used to locate the rotation
point for my 12-24mm @ 12mm and it worked quite well.
As an example I decided to apply the same technique to the
question posted in the Panoramas forum:: Rotation
Point for AF-S 18-70mm DX lens?
I
will be referring to the following diagram.
I took an ordinary 8 1/2x11 inch sheet of paper.
I drew two isosceles triangles with a base of 120mm and heights
of 180mm and 240mm. These triangles represent 1/2 the horizontal
field of view at infinity at 18mm and 24mm focal lengths respectively.
(Notice 120mm is 10*(24mm/2), 180mm is 10*18mm, and 240mm
is 10*24mm.)
-
I
set the Focus-mode selector on my D70 body to M.
-
I set the focus on the lens to infinity.
-
I
place the paper on a flat brightly lit surface.
-
I
set the aperture for f/8 and the focal length to 12mm.
-
I
turn the grid lines on in the viewfinder.
-
With
the camera turned on and the depth of field preview button
depressed, I position the camera on the paper so the long
sides of the 18mm triangle matched the 1/4 and 3/4 grid
lines.
-
I
mark the position of the front edge of the lens on the paper.
-
I
turn that camera over and measured the distance from the
front edge of the lens to the middle of the tripod socket.
-
I
measure the distance from the marked position on the paper
to the triangle vertex.
-
The
difference in these measurements is the distance from the
tripod socket to the entrance pupil. I repeated the procedure
at 24mm.
According
to my results the distances from the tripod socket forward
to the entrance pupil is 74mm at 18mm focal length and 65mm
at 24mm focal length.
BTW, the 12-24mm at 12mm has a socket-to-node distance of
100mm.
Editors
note:
How
do you set your gear for this without much investment?
Simply,
with a long plate such as the Kirk
LRP-1
As
you rotate over the entrance pupil, two planes must
be leveled for horizontal panoramas like the one sample
above, so that images line up when stitched together.
First,
the panning or rotation plane must be leveled to the
horizon, and then the film or sensor plane must also
be leveled and perpendicular to the plane of rotation.
The inexpensive way to that is with a simple double
bubble level. |
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