"A" or "The"

Hello, everyone.

When referring to a person, are there any firm guidelines when it comes to the use of "a" or "the" after a comma?

To clarify:

Recently, I was writing an article about Amy Winehouse, and I referred to her like this:

"Amy Winehouse, the troubled singer-songwriter who..."

A few weeks later, however, I was writing an article about a photographer, and it felt wrong to use "the" after the comma, so I wrote:

"Sebastiao Salgado, a celebrated Brazilian photographer who..."

As a result, I am now wondering if there are any solid guidelines? I realise that "a" is used when the subject is one of many, and that "the" is used when there is only one, but there have been times when I have followed these guidelines and the sentence has looked / felt wrong.

Can anyone offer some guidance?