How to control soft focus filters?

[Garbz]

I have a 62mm B+W Soft focus filter. I've never had the chance to properly use it yet, and infact untill a few days ago I didn't know what a gem I had (someone in the photoclub shouted oh my god you have a B+W filter!)

Anyway I've tried it once and the effect was useless so I guess I did something wrong. It's on a tokina wide-zoom lense 28-70mm f4. That's the only lense I have with a 62mm rim.

How does aperature effect the soft focus.
Also how does zoom effect it? Is it better to stand close with wider angles, or be far away and zoom.

Basically i'm wondering how to get the strongest and weakest effect from this filter.