That is, why are there two spellings of that name? And how many words have a “ph” that makes a “v” sound?
Hate to say it, but Yahoo answers has a great, understandable explanation.
The ph(f)-v change is answered with more examples at English Language usage.
They are in effect the same name. It’s my real-life first name, and I use them interchangeably. (It’s derived from my grandfather’s name, a foreign version of the same). Ultimately, it derives from the Greek Stephanos, the protomaryr mentioned in the Acts of the Apostles, and I have no doubt it was pronounced with an -f- sound, as it is in most of the world. The -v- sound is, I suspect, an English (and later, American sand other colonial) affectation, the same way the English-speaking world voices the “J” in “Jesus” differently from the rest of the world. How this evolved into a -v- sound I do not know, but it’s woreth noting that my grandfather’s name didn’t use an -f- pronunciation, either, so variations of the name go beyond English-speaking eccentricity.
“Steve” seems to me clearly a familiar form based on the -v- pronunciation of the letters in “Stephen”. Its dereivation is obvious, and no further explanation is necessary.
As for changes from -f- sound to -v- sound, I can’t think of any for the “ph” spelling right now, but it’s not unusual for plurals – the plural of elf, after all, is elves. And the plural of shelf is shelves. And the plural of wife is wives. And of life is lives. Think a while and you’ll comke up with others.
My first name is Steven, and when I was growing up I was told that Stephan was the ‘old fashioned’ spelling while Steven was the modern spelling.
My best friend and his wife had a child a few years ago and named him ‘Stephan’. They keep calling him my namesake even though the spelling is different. When I asked them why they chose that particular spelling they said because it was more ‘traditional’.
So modern versus traditional spelling, I think that’s the difference here.
“V” is just the voiced version of “f”. They are the voiced and voiceless labiodental* fricatives, respectively. IOW, a “v” is just an “f” with some voice added.
*lips on teeth
“Nephew” has a /v/ in some dialects, though not in mine. The only place I’ve heard it is Britain.
My mother’s rationale for naming my brother Stephen was that she didn’t want him to be called Steve.
Did people call him Steph? 
Well… if that was her main aim, she only picked the second-worst possible choice.
Honestly, that is a little weird, as Stephan is the usual spelling for steh-FAHN, rather than STEE-vehn. (Use fs, as in Steffan, and it becomes STEF-fuhn.)
And, no, I’m not using IPA as it’s a hassle to type out and no one will need it.
My basic assumpition or rule of thumb is that V is 'Mercan, while PH is English / Colonial.
Doesn’t always work, but does provide a guide.
Other than that, is just like checking the cheque into the check account.
No. “Stephen” is very common in the US. You might be onto something with the Geoff/Jeff dichotomy, but ever there, I think it’s more that Geoffrey is more common in the UK than in the US.
I used to wonder about Daedelus.
I suspect its usage stems from the British habit of contracting names (e.g. Brendan become Bren, David becomes Dave, Philip become Phil, Stephen becomes Steve, becomes Steven etc.)
Try saying “stee-fen”. It’s almost impossible not to say “Steven.”
Right, because the first syllable is stressed, with a vowel coda. You can, of course, remove the voicing if you change the name, and say /stƐ ‘fɑn/, which usually just annoys people.
Actually, it isn’t. Tolkien had solid linguistic reasons for choosing elves and elven over other possibilities, but he deliberately replaced elfs and elfin because he thought them evocative of the wee folk, not his classically tragic immortals.
Are you sure about that? I do know that the standard plural of “dwarf” was “dwarfs” until Tolkien came along and decided to use “dwarves” in his work, but I thought “elves” was pretty standard pre-Tolkien.
I don’t pronounce them the same
Stephen to me is pronounced ‘steph-’ (like stephanie) and ‘-en’
That’s how people pronounce Stephen Curry’s name.
I’m not sure what you mean. I actually used to pronounce it “Stee-fen” as a kid until someone pointed out that it’s supposed to be said the same as “Steven.” I mean, “leafing” and “leaving” are not hard to distinguish and say, are they?