Re: Is there a definitive focal length for tight head shots...
I'll go with whatever lens I have on hand. Because I shoot for myself and really couldn't care less if others like or hate my work and how they're portrayed, when given the luxury of choosing FL (most of my photos are done without planning) I'll choose based on the current lighting and characteristics of the lens. I don't find myself going much longer than 105mm on 35mm. I usually carry a 50mm as my 2nd lens to the 25.
Basically I don't seek to flatter the subject in their eyes – I seek to record how I see the subject at the moment and how I want the photo to turn out.
Which is why I'm reluctant to make photos for people (but have no problems making photos of them).