Big Ben's Panorama Tutorials

Lens Calibration

The Process

There's actually nothing fancy about this lens calibration. It's effectively just stitching a single row 360° panorama using more overlap and more control points than you would normally use for a panorama.  PTOptimizer works on a trial and error basis.. i.e. it involves a certain amount guesswork. The more information you can supply, the more accurate a guess becomes, until you eventually provide enough information that will make a correct answer inevitable.  This is the principle on which this test works.

The criteria for collecting images and optimising control points for the test are:

1. Correct placement of the lens over its nodal point.
   This is critical for a lens calibration as any deviation will result in different values for a, b and c. You don't need a full on panorama head. A simple focusing rail will do the job, or even a Metz flash bracket.
   
   
2. The images have at least 50% overlap
   The parameters a, b, c correct for radial distortion in a lens..  i.e. they are used to recalculate the distance of each point from the centre of the image.  To calculate them properly then, we will need to collect data from the centre of the image to the very edge.
   
   
3. A large number of control points are used
   There's not much point shooting with a lot of overlap if you're not going to use it.  Selecting several control points from all over the images produces a better overall result. A large number of control points is also required since a large number of variables will be optimised at once. Using several points will reduce the risk of a bad result.
   
   
4. Several vertical lines are defined using the t1 option for defining control points.
   This ensures that the parameters will properly correct for lens distortion. Without these it is still possible to get a good stitch that may be slightly distorted.