mmm... i'm not 100% sure i understand your need. Would it be possible for you to post an example, or perhaps explain this in a different way so as to clarify the visual effect?

I'm guessing that all you'll need is to apply a Radial gradient, but i'd like to be sure i'm thinking of the right effect.

Are you saying that you'd like to create a circle, that fades in from the inside to the outside? So that the inner area is soft and the outer edge is hard/clean...?