Low RMS value when calibrating single camera, while high RMS value in stereoCalibrate

Question

I am trying to calibrate a stereo setup consisiting of two identical Canon EOS60D's, which i will be using to do Photogrammetry. To calibrate cameras i am using a high precision circlegrid pattern plate. I was able to detect centre points of circles using cv2.findCirclesGrid, and was able to calibrate a single camera just fine. The problem comes when i try to calibrate both cameras using cv2.stereoCalibrate function in OpenCV. I will elucidate the steps taken and also post some code snippets with outputs i have got. I would be grateful if someone can point out where i am doing wrong.

Steps

1) First step is to detect circlegrid pattern using cv2.findCirclesGrid function in OpenCV.

2) Then using the detected grid pattern from each camera, i am using cv2.calibrateCamera to get camera matrices, and distortion coefficients of both cameras.

3) After getting camera matrices and distortion coefficients of both cameras, i am passing them to cv2.stereoCalibrate function.

Output

output after calibrating Left Camera is as follows

Left Camera Return Value :  0.202690712694
Left Camera Camera Matrix :  [[  2.42046647e+04   0.00000000e+00       1.78281995e+03]
[  0.00000000e+00   2.42115121e+04   2.51720578e+03]
[  0.00000000e+00   0.00000000e+00   1.00000000e+00]]
Left Camera Dist Coeff :  [[-0.09448288 -1.38934436  0.00951455 -0.00568115  0.11248399]]

output after calibrating Right camera is as follows

Right Camera Return value :  0.258429588138
Right Camera Camera Matrix :  [[  2.69094574e+04   0.00000000e+00   1.70580157e+03]
[  0.00000000e+00   2.69534209e+04   2.55185400e+03]
[  0.00000000e+00   0.00000000e+00   1.00000000e+00]]
Right Camera Dist Coeff :  [[-0.1854594   1.89124255  0.00592814 -0.00377697  0.05715002]]

As you can see from the output above, both the return values from cv2.calibrateCamera are reasonable.

output after calibrating stereo setup is as follows

StereoCalib Return Value :  278.98818985
Rotation Vector :  [[ 0.72415334  0.13289823  0.67671265]
[ 0.68078781  0.018949   -0.73223554]
[-0.11013584  0.99094853 -0.07675351]]
Translation Vector  [[-1.20860215]
[ 1.23017549]
[ 2.08116679]]
Essential Matrix :  [[-1.55231939  1.17960457  1.42948401]
[ 1.37397347  1.47424592  1.31558743]
[-1.7136373  -0.18638995  0.05250614]]
Fundamental Matrix :  [[ -3.15554184e-07   2.39721211e-07   6.99264445e-03]
[  2.78844610e-07   2.99110045e-07   5.21248054e-03]
[ -9.54712002e-03  -2.19149124e-03   1.00000000e+00]]

But the return value from stereo calibration is too high.

Params

The flags and criteria i am using in cv2.stereoCalibrate are as follows

flags = cv2.CALIB_FIX_INTRINSIC

criteria = cv2.TERM_CRITERIA_MAX_ITER + cv2.TERM_CRITERIA_EPS, 1000, 1e-6

I would be grateful if somebody can point me where i am doing wrong.