Welcome to RootsChat.
I think the word 'interchangeable' might not be the right term to use.
At this date, spelling was not uniform, particularly where name were concerned. Many people could not read or write and so were not able to check how the name had been recorded. In the middle of a name like Cam(p)field, the 'p' could well have been silent and therefore easily missed.
Robert Nicholas Camfield was baptised on 17 June1838 at St Thomas Portsmouth, son of George & Ann. Their other children were: Ann (1826), Eliza Sarah (1828), Mary Ann (1830), Hannah (1834), George (1832), Elizabeth (1836), Jane (1841).
George Camfield married Ann Kerry or Kerray on 14 August 1825 at St Mary Portsea.
Hope that helps.