In a Camera's properties you can "Reset to Default" on "Focal Length"
because "lens" is a direct member of Camera and it has "50.0f" as its
default value in DNA_camera_defaults.h. However change the "Len Unit"
to "Field of View" and what you are changing is "angle". This is not
a member of Camera but is a calculated value based on sensor size. So
"reset to default" makes this "0", which is invalid and otherwise not
possible to enter. This PR uses RNA_def_property_float_default to set
a default value of 0.6911504f radians which is the 39.6° FOV for the
default 50mm lens.
Pull Request: https://projects.blender.org/blender/blender/pulls/132364
e5e45dc140