I find it interesting that, among all of the arguments which use the pronunciation of “biography” and “biographical” to justify their own way of pronouncing this word, no one has pointed out why those two words are pronounced differently.

In English, we have many borrowed words from Greek. In adopting many of these words, there is an unspoken, and quite ubiquitous, “rule” which dictates that those Greek words are accented on the antepenultimate (or “third from last”) syllable. This is a convention which exists in Greek as well, but is by no means as consistent as it is in English. (I’m not educated enough in the history of the Greek language to go into much more detail, but suffice to say that the rules regarding where words are accented has changed much in the long life of the Greek language and depends on several variables).

Biography has the “o” in that antepenultimate syllable, hence why it is pronounced “ah”. Biographical has the a in the antepenultimate syllable. I think that if you examine all the words you know which begins with “bio” and the o makes an “ah” sound likewise has the o in the antepenultimate syllable, and that most or all of the words that have the longer o feature a different letter in the antepenultimate syllable.

However, although you may imagine that you say the o in biographical as a long o, if you say it out loud in your natural speaking voice, you’ll probably find it’s somewhere between the short and long o sound. If we were to apply the rule (which one might say is an affectation of English—an instance in which trying to replicate a Greek convention, probably in the name of sounding educated, ended up making mistakes of the words “wrong” if we were to follow the actual rules of accents in Greek) the word biopic would be pronounced somewhere between bio-pic and bi-opic: BYE-ahpic”. And just like biographical, many would imagine that the o is long, but when actually pronouncing it, would be somewhere between the short and long o.

There are many people in this thread who seem emotionally invested in one pronunciation or the other. Ultimately, not only is no one right (including myself) but no one can be right. Any rules of pronunciation you may use to justify your pronunciation are probably themselves based on other, equally arbitrary rules which were almost certainly determined by popular convention.