A particular field is a particular subject of study or type of activity
e.g. “each of the authors of tapes is an expert in his field “
You also can use “Academic discipline/ Field of Study “In formal and collegiate expressions
An academic discipline, or field of study, is a branch of knowledge that is taught and researched at the college or university level. Disciplines are defined (in part), and recognized by the academic journals in which research is published, and the learned societies and academic departments or faculties to which their practitioners belong’
If you want 2 Indicate your field of study in a common way use study
He studied at various universities before gaining a doctorate in 1964.