Doesn't the F5 have the option of letting you rewind the film manually? If so then you feel the winder go slack as the film finally comes off the receiving spool. At that stage there is only the orginally exposed leader still exposed and you can safely open the back. If you can't rewind manually then I'd buy a film retriever. It's a worthwhile investment. My Pentax unfortunately does rewind the film back into the cassette so I have no choice but to use a retriever. Anyway I prefer to load onto a developing reel from the cassette without levering off the cassette's end and a film retriever is the only way to do this.

A film retriever is quite cheap and it should last half way to forever

pentaxuser