I've been playing around with my new 50mm prime lens. For portrait it seems that about F5 is perfect to give a well blurred background while still sharp on the subject. Here's a test shot of Annika at the park.

Unfortunately it was extremely cold that day, after just having a nice week in the 60's it was about 33 degrees so we only stayed at the park a few minutes. This was the best of the shots.

The Olympus Zuiko Digital F/2.0 is a sweet little prime lens. Being "prime" means that it doesn't zoom. Ever. It's locked in at 50mm - which on the Olympus 4/3 system is equivalent to a 100mm lens on any other camera. This photo was taken at around 6 feet from the subject. This lens will also do macro photography, i.e. flowers, bugs, dust mites, etc. lol.

This shot was taken at F/5.0 which seems to be the sweet spot for portrait photography with this lens. It will go all the way out to F/2.0 but the depth of field drops off significantly.