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Finding the "Point"



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Finding The Point...



Some call it the "Nodal Point", "Entrance Pupil" or

"No-Parallax Point". 


Whatever it is...

Every VR photographers' initial goal... 

...is finding "it"...




What is "it"? 


I'm 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 and discussion 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 lens to "IT".  So I'll skip this definition and get on with the tutorial.  I'll call it the "point" for simplicity sake.




Why find this "point"?


Because this "point" has a foe 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.






Finding that "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 Alain Hamblenne.  This was originally posted in September 2004.  I could have simply linked to his site and/or his PDF 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 technique.


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 side.  Simple?



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 original "GRID" tutorial





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 flimsy.


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.


The Grid.






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, etc.


2.  NOTE:  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 behind 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.

i.e. 24mm.


5.  Set the camera as close as you can to the grid with enough space to rotate the camera every 30³ for 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 Point


Shoot three images through the grid at 30° increments apart.



Shoot sequence




Since 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 reference points 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 Point


To test for vertical rotation alignment shoot 3 images rotating only the upper arm.  Shoot three images.


Use the demarcation points on the vertical arm's pivotal point to rotate for 30° increments.



Shoot sequence.



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.





Stitch Test


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 perfect.





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 site is www.outline.be.

Again, his tutorial can be found here.


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)

studio@outline.be www.outline.be




Other topics you may be interested in...


Shaving the Nikon 10.5 Fisheye Lens