Does anybody know what is Distortion Grid and how it is used

Hello Nirakar,
 
HAPPY NEW YEAR! :-)
 
A "Distortion Grid" is usually a sheet of white paper (3'x4') with a black rectilingar grid on it (4"x4" ), which should be exposed with every lens at a known distance (i.ex. 50x focal length).
It gives information about lens distortion, therefore its name, since straight lines become bent ("cussion" or "barril" ).
It usually varies for zoom-lenses at different focal lengths, and may also do so for different distances!
You may want to recreate this distortion in your 3D-render, to perfectly match the life action footage.
Also, you can easily check the distortion value calculated by MM, since applied to the grid, this should become perfectly straight.
PS: To easily adjust the center of the grid to the center of the optical axes of your camera, I fix a mirror in the center of the grid: when I see the lens in the mirror through the camera, I know it's aligned.
I hope this answered the question.
 
Best regards
 
Tim
 
PS: Here an example: http://www.movematchers.net/forum_ [...] ongrid.jpg