Strange values for matrix camera and distortion values

edit

I am trying to calibrate a fisheye camera. In order to do that I load a video take with that camera and takes frames where appears a cheesboard pattern. In order to find patterns I use:

bool found = findChessboardCorners(image_to_show, board_sz, corners, CALIB_CB_ADAPTIVE_THRESH | CALIB_CB_NORMALIZE_IMAGE | CALIB_CB_FAST_CHECK);
        if(found)
        {
            cornerSubPix(gray_image, Mat(corners), Size(11, 11), Size(-1, -1), TermCriteria(CV_TERMCRIT_EPS | CV_TERMCRIT_ITER, 30, 0.001));
            drawChessboardCorners(gray_image, board_sz, Mat(corners), found);
        }

when I take 15 pattern I calibrate the camera as follow:

double error = fisheye::calibrate(object_points, image_points, image_to_show.size(), intrinsic, distCoeffs, rvecs, tvecs, fisheye::CALIB_RECOMPUTE_EXTRINSIC | fisheye::CALIB_CHECK_COND | fisheye::CALIB_FIX_SKEW);

In this case the error is: 1.6847, then I print the intrinsic matrix and distortion coeficients and I get

Intrinsic Matrix
[251.48312, 0, 471.14383;
 0, 255.30005, 501.81467;
 0, 0, 1]

Distortion coefficients
[0.055155002;
 0.07189583;
 -0.082159713;
 0.026405662]

I think this values are wrong because the image resolution is 2048x2048, si cx and cy should be 1024 but the calibration method returns 471 and 501 for that variables.

then I try

fisheye::undistortImage(image_to_show, imageUndistorted, intrinsic, distCoeffs);

But I get a black image in imageUndistorted

This is the undistorted image that I get after calibration

Edit: I modify my program to take images instead of video stream and now I am using a flat chessboard pattern.

If I use the fisheye::undistortImage the result is the same (more or less) than before, but if I use

Mat newCamMat;
Mat view, result, map1, map2;
fisheye::estimateNewCameraMatrixForUndistortRectify(intrinsic, distCoeffs, image_to_show.size(), Matx33d::eye(), newCamMat, 1);
fisheye::initUndistortRectifyMap(intrinsic, distCoeffs, Matx33d::eye(), newCamMat, image_to_show.size(), CV_16SC2, map1, map2);
remap(image_to_show, result, map1, map2, INTER_LINEAR);

I get something similar to this

.

so, seems that fisheye::undistortImage doesn't work well. The result that I have obtanined is closer to solution but It still seems wrong. How can I get a better result?