Correct Cropping When Removing Lens Distortion


#1

I have a series of clips which I am tracking and matchmoving (in Boujou) to be used for VFX shots.

I am using Nuke to correct the lens distortion prior to tracking (using values calculated for each lens). The undistortion in Nuke increases the size of the image as the corners are stretched out.

I have the focal length and film back size recorded for each clip (to use in matchmoving). If my understanding is correct, cropping a plate is equivalent to changing the focal length compared to the original shot.

So my question is: with the undistorted plate, is the expanded size correct for the focal length it was shot at, OR do I need to crop to the original size in order to match the real focal length?