Question: In Judith in Vulgate, why does Jerome transliterate the name "Arphaxad" with 'ph', but he transliterates "Holofernes" with an 'f'?

By the time of Jerome, both 'f' and 'ph' were the same sound, the English /f/, right? And Greek letter φ was pronounced /f/ as well at the time, right?

Regardless, they were arguably the same sound given that Jerome was transliterating φ from the same Greek text two times.

Answer: 
One possible avenue is that Jerome is hearing a difference in the original Hebrew/Aramaic. In Hebrew, Arphaxad is spelled אַרְפַּכְשַׁד. The פַּ has a dagesh, which means that it is pronounced as a bilabial plosive, *p*. With Judith, [Jerome says](https://www.tertullian.org/fathers/jerome_preface_judith.htm#4) he is translating from "Chaldean" (i.e. Aramaic), not Greek. So it doesn't matter then what the Greek is doing; if the Aramaic preserved the plosive /p/ for Arpaxad and the fricative /f/ for Holofernes, that's likely what Jerome followed, at least as best as he understood it.

Possibly too he knew the name before his translation from other sources, as Draconis mentioned. The standard way of writing it in Latin would have been to follow the Greek transcription of Olophernes (same name, different person), so it is uncertain from the evidence available why he would have chosen to forego the -*ph*- and write -*f*- instead.

Jerome is not entirely consistent in his transliteration, at least not regards with the Masoretic vocalization. In the name *Ephraim*, Jerome spells it with a -*ph*- even though the peh lacks the dagesh, yet he spells the name *Phineas* (which has a dagash) with an *F-*, *Finees*. There is no distinction between phi /ph/ and phi /f/ that Jerome would have been hearing, so it is again unknown why he chose to transliterate certain words one way and others another way.

Interestingly, this is not restricted to Hebrew names. In the *Prologus*, he writes the name *Xenofontis*, whereas it's unambiguously [*Xenophontis*](https://latin.packhum.org/search?q=xenopho) in Classical sources. I suppose to be thorough one would have to check the *apparatus criticus* of enough of those references to see if, when, and where there was variation, but at least in the UBS edition, there is no mention of *Xenohpontis* in the manuscripts.

So, even if Draconis' answer might be technically correct (although we can't be sure), it doesn't explain *Finees* or *Ofni* v. *Ephraim*, especially since in some places the latter is spelled *Efraim*. Possibly some of it comes from Old Latin, possibly some from the Greek v. historical texts, possibly from learned sermons of the day, but in the end it doesn't look very consistent to me.

Which is funny, of course, since you can also find Holophernes and Holofernes in English papers and books, sometimes interchangeably by the same author.

Short answer is: We just don't know, but it is consistent with Jerome's inconsistency. **And at the end of the day, it just might come down to only to inconsistencies.**
The answers provided by cmw and Draconis are very learned, but I'm not sure they explain the inconsistent ways of transliterating Phi in the Vulgate. Here is why.

1. Different editions of the Vulgate transliterate Phi in different ways. For example, the Clementine edition has *Arphaxad* and *Holofernes*, which is what FlatAssembler asked about. However, the Stuttgart edition of the Vulgate has *Arfaxat* and *Holofernes*.
2. I am sure that the answers given about Hebrew and Aramaic are correct, but I'm not sure they are pertinant, because Jerome almost always transliterated proper names in the Old Testament from the Septuagint, even when he translated from Hebrew or Aramaic.
3. My own answer is pure speculation. The medieval manuscripts of the Vulgate were written by scribes who were listening to lectors reading from earlier manuscripts. The scribes wrote what they heard, and they heard what the lectors said, not what Jerome wrote. So the details of Jerome's transliteration of Greek Phi have been lost in history.

I suspect that if we want to understand why *F* is here and *PH* is there (and *D* and *T*), we will need to study the state of classical and biblical scholarship during the Early Modern Period when the first bibles where being printed. I am far from an expert in those subjects in that period.