Undistort images or not before finding the Fundamental/Essential Matrix?

edit

I am quite confused right now. In order to find the Fundamental Matrix and the Essential Matrix, my common way is by first, undistort the images before did the other processes like detecting keypoints, matching the keypoints, find the Fundamental Matrix and then, the Essential Matrix. Is this correct? Can I not undistort the images in order to find the Fundamental Matrix and the Essential Matrix?

Another question is, as for the function findEssentialMat of the OpenCV, does it operate on the undistorted points, or distorted points, or both?