Psychophysics is a multidisciplinary field featuring people with a Physics, Psychology, Medicine and/or Biology background.
OK, here follows a lot of personal account to illustrate where psychophysicists may come from.
Traditionally, psychophysics is a part of Psychology.
I do a lot of psychophysics and I'm a Biologist by training (MSc), but learned psychophysics during a postdoc. My supervisor teaching me psychophysics was a physicist (MSc) who acquired their skills during their PhD (in visual sciences).
My current supervisor is a physicist by training (MSc), but has done heaps of psychophysics as part of his post-grad Audiology training.
Many of my psychophysics colleagues are in fact physicists.