FOV for different resolution

Hi,

According to OpenCV documentation: 

https://docs.opencv.org/2.4/modules/calib3d/doc/camera_calibration_and_3d_reconstruction.html

"If, for example, a camera has been calibrated on images of 320 x 240 resolution, absolutely the same distortion coefficients can be used for 640 x 480 images from the same camera while , , , and  need to be scaled appropriately."

However, I found the following profiles do not follow this rule especially for X direction as the resolution changes from 640 to 1280, the fx does not double.

I understand the reason (based on OpenCV documentation) we need to scale the fx is because we use the "pixel" as the unit. In fact, the physical size of the imaging sensor does not change as we change resolution.

From the following profile, I guess the reason that the fx does not scale accordingly is because the physical size of the image sense also change as we change the resolution.

Am I right?

Thank you!

Yixun

 Intrinsic of "Color" / 640x480 / {YUYV/RGB8/BGR8/RGBA8/BGRA8/Y16}
  Width:        640
  Height:       480
  PPX:          316.995513916016
  PPY:          248.528915405273
  Fx:           608.773010253906
  Fy:           607.541870117188
  Distortion:   Inverse Brown Conrady
  Coeffs:       0       0       0       0       0
  FOV (deg):    55.46 x 43.1

 Intrinsic of "Color" / 1280x720 / {YUYV/RGB8/BGR8/RGBA8/BGRA8/Y16}
  Width:        1280
  Height:       720
  PPX:          635.493286132813
  PPY:          372.793365478516
  Fx:           913.159484863281
  Fy:           911.312805175781
  Distortion:   Inverse Brown Conrady
  Coeffs:       0       0       0       0       0
  FOV (deg):    70.05 x 43.1