How to find the principal point in an image? I need to find the principal point in an image. Its a point where the principal axis intersects the image sensor.
Due to misalignment this point is not at the center of image always(or image sensor). I need to precisely determine its location using any of the optical methods available(if any). Any suggestions are greatly appreciated. 
More information: The camera is giving me a live feed over a screen and I am able to store, analyze any part of it in real time or later.

 A: To do this you would need to take an image of some sort of reference object, like a regular grid pattern. You would also have to know that your imaging system has some measurable field aberrations which are radially symmetric, such as distortion. Then you could do some simple image processing to locate the center of the distortion pattern.
In the case of a regular grid pattern, you would plot the positions of the grid lines in the image. With third order distortion, the grid lines should be spaced quadratically (ie. with the space between each grid line increasing linearly). The grid spacing will reach a max (or min) at the "principal point." (I use scare quotes around "principal point" because the principal point has a specific meaning in geometrical optics, and it is not the meaning used in the question here.)
A: I suggest shining a narrow laser beam along the optical axis. Just don't fry your sensor! Hopefully, the antireflective coating of the lens is not perfect, and you will be able to see on the object side two reflections of the beam: one from each side of the lens. Align the laser in such a way that both reflections come back exactly at the output port of the laser: the beam is then perpendicular to both faces of the lens, and therefore it is exactly along it's optical axis.
Now look at the image: you have one bright spot exactly at the focus.
