When I read the exif data that ARKit what is SubjectArea? Why does LensSpecification repeat the 4.2 (the focal I think) and the 1.6 (the aperture I think)?
"SubjectArea": ( 2013, 1511, 2116, 1270 )
"LensSpecification": ( 4.2, 4.2, 1.6, 1.6 )
The subject area is defined as rectangle with center coordinate and its dimensions. In this case, the center is at (2013, 1511) with rectangle dimensions 2116 x 1270.
For more details, you may refer to Exif standard tags.