Canon FD 1.4 50mm as a prime

I have the original 50mm FD 1.4.

As a rough guide for whether you need f1.4:

For head shots, f1.4 might be a bit extreme - unless shooting square, it is difficult to get both eyes in focus. f1.8 is probably safer.

f1.4 really comes into its own for half length or full body shots, where it can really pull a figure away from the background while retaining critical focus on the whole face.