Finding The Point...
Some call it the "Nodal
Point", "Entrance Pupil" or
Whatever it is...
Every VR photographers' initial goal...
...is finding "it"...
What is "it"?
NOT the authority on this subject matter nor will I try. I'm
NOT going to try to define it. It is an unending debate
as to what and where this really is or what it should be called.
There are numerous articles written on this one subject alone. I simply know that we, as VR photographers, have to set our camera's
to "IT". So I'll skip this definition and get on
with the tutorial. I'll call it the "point" for
Why find this "point"?
Because this "point" has a
named "parallax". Now you ask what is parallax?
Parallax occurs, in panoramic photography, when the camera is not
positioned correctly in this "point" when the images are taken.
Parallax is clearly visible during the stitching process where the
images overlaps and do not line up correctly.
"point" - The Grid... revisited...
Searching on the web
you'll find that there are numerous ways of finding this "point".
You'll probably find one that will better suit your needs.
This method I find is time
consuming than most but probably the most accurate. It's called "The GRID" method by
This was originally posted in September 2004. I could have simply linked to his site and/or
file to illustrate this but how would I know if it works if I don't
try it for myself? You can "GOOGLE" this method -- type in
"alain hamblenne, the grid". This is a review of his
Why a grid? Simple. A
grid has both vertical and horizontal lines that crosses each other
and acts as reference points with the background. Obviously
there will be lots of reference points to refer to.
Ever played Battleship as a kid?
You're basically doing the same thing here. You're matching
points on a grid to an external reference point; the background...
or in the game Battleship, your points of reference are your hits on
your grid with the location of your opponents ships on the other
The equipment mentioned below are
for demonstration purposes.
Different combination of
equipment can be used for the same method.
This method is a step by step
follow through of Alain's "GRID" method. Here is a link of the
Camera - D70s with Nikkor
10.5mm f/2.8 AF DX Fisheye lens
VR head - Nodal Ninja 3
Tripod - Manfrotto
055CL Classic Tripod
Grid - This can be an oven
rack, shelving rack, fencing, furnace filter frame, old animal cage
frame, etc. Basically anything that resembles a grid.
The smaller the thickness of the grid the better. It also must
be sturdy enough.
Don't use a flimsy chicken wire
or plastic snow fences
unless you can secure it firmly. They're perfectly
manufactured grids but too
I used a furnace filter frame
purchased from a local home hardware store.
It's plastic and lightweight. ...and if you're wondering what
I did with the actual filter element... I replaced the existing
filter in the furnace
of course.... two in one usage... ;-)
It has 1" grid squares.
The size is 19" H x 24" W. It's not solid but sturdy enough
for this demonstration.
Mount the grid securely in front
of an area with lots of solid details. Such as the front of a
house with windows, living room, etc. The details in
the background will be used as reference points during this "quest".
Mount the grid high enough so the camera lens is approximately at
the center of the grid..
Use whatever method to hold the
grid securely. Use ropes, clamps, stands, etc. If
you're doing this outside make sure your grid is solidly secured
from the wind.
GRID NOTE: The GRID
and camera must stay secure at all times when shooting the test
shots. The Grid DOES NOT have to be leveled.
1. Make sure the VR head is
perfectly leveled. Use whatever tools you have. A level
from your carpentry tools, a hot shoe bubble, a tripod head leveler,
A slight deviation from Alain's method:
When mounting the
vertical arm of the VR head, align the arm so it's intentionally
OFF centered to one side. Do not try to center the lens at the
axis point of the tripod. This way you know you only have to
move the vertical arm to one side until you hit the "point".
Mount the camera so the front
of lens is just behind the pivotal axis of the upper rail.
Let's pretend we don't know that this "point" is approximately
the gold band of the Nikkor fisheye. So we'll play along.
3. On the NN3, use a small rotational
15° degree detent plate and rotate every 30°.
4. When using a zoom lens
make sure you shoot only at one focal length.
5. Set the camera as close
as you can to the grid with enough space to rotate the camera every
three shots. Here I have the front of the lens' built in lens
hood approximately 2 1/4" away from the grid.
Adjusting Horizontal Rotation
Shoot three images through the
grid at 30° increments apart.
I intentionally did not align the lens with the VR head's axis
obviously the images will be misaligned as shown above.
You can clearly see the objects
pointed out are misaligned with the grid: the picture frame,
hockey helmet, baseball cap, sea shell, bike helmet and light grids
on the floor.
The lower rail was set to 60mm
and upper rail to 85mm on the Nodal Ninja 3.
NOTE: For obvious
reasons I added the objects for reference points.
Adjust the vertical arm closer to
the center and move the camera position forward and repeat the steps above until the
matches the gird.
The LCD screen of the DSLR conveniently gives instant
results. Simply zoom in with each set of three shots.
I moved the vertical arm and
camera position on the upper arm at 1mm increments.
Above you can see the picture
frame, climbing helmet, baseball cap, sea shell, bike helmet and
light grids are correctly aligned with the grid.
Adjusting Vertical Rotation
To test for vertical rotation
alignment shoot 3 images rotating only the upper arm. Shoot
Use the demarcation points on the
vertical arm's pivotal point to rotate for 30° increments.
If you've done this correctly the
reference points should match perfectly during vertical rotations.
Above, you can see both picture
frames, climbing helmet, baseball cap, sea shell and bike helmet are
aligned with the grid.
To further test this method, and
to actually see if I've done this correctly, the image below was
stitched using PTGUI at it's default settings.
Even with the grid at
2 1/4" away from the lens' hood and objects at a distance the images
manages to stitch perfectly.
To no surprise Nodal Ninja's
recommended rail settings for this camera and lens combination, 55
mm lower rail and 80 mm on the upper rail, are
Alain Hamblenne's method is truly
accurate. As he says, "Proof by sight". Having both
vertical and horizontal reference points is much easier to adjust
the camera lens' position and to determine whether or not you are
off from the "point".
Alain Hamblenne's web
Again, his tutorial can be
If you have found
Alain's technique helpful you can contact him at:
OUTLINE Graphic Designers
Rue Destriveaux 10 - B-4000 Liège - Belgique
+32 (0)475 76 35 73 - ahamblenne (AIM)
Other topics you
may be interested in...
Shaving the Nikon 10.5 Fisheye Lens