|Exif TIFF Tag FocalPlaneXResolution|
TIFF Tag Reference
TIFF Tag FocalPlaneXResolution
Indicates the number of pixels in the image width (X) direction per FocalPlaneResolutionUnit on the camera focal plane.
The FocalPlaneResolution tags record the actual focal plane resolutions of the main image which is written as a file after processing instead of the pixel resolution of the image sensor in the camera. It should be noted carefully that the data from the image sensor is resampled.
These tags are used at the same time as a FocalLength tag when the angle of field of the recorded image is to be calculated precisely.