When english words are derived from greek origin why is the letter phi written with ph rather than F?

Question

I'm greek and my last name starts with the letter phi in greek. When I write it in english I use ph but a relative of mine uses F in his. We would always debate on which is the correct way. Need some proof on why. There was a letter F in ancient greek which isnt used any more in modern greek which was pronounced wau. I watched a movie the other day about a spelling bee and when the girl mispelled a word that was of greek origin using the letter f she was corrected and was told that any word with greek origin is never is spelled. Wondering what the reason was to use ph rather than f.

Answer 1

There was no letter F in the Greek alphabet so representing the letter phi with the letters ph in English was judged to be orthologically closer to the original Greek.

Answer 2

If you look at the Greek alphabet no letters are written with the letter F. Of course that is just the English translation of the symbols they actually used.