Shun Cheung , Dec 27, 1997; 10:09 a.m.
This topic has been discussed many times. While I am sure that some individuals would disagree with me, but from my personal experience with a 500mm/f8 N Nikkor, I would say as strongly as avoid 500mm/f8 mirror lenses. Most of those are really like 500mm/f11 lenses. If you put a 2X tele-convertor on your 300mm/f5.6, it is like a 600mm/f11. A 500mm/f11 will give you pretty much the same problems as your 300mm/f5.6 with a 2X: the view finder will be very dark and it will be very hard to focus. And you'll need fast film to achieve a fast shutter speed to get sharp pictures. Personally, I have no problems with those donut-shaped out-of-focus bright spots, so that is a non-issue to me.
I am sure that some folks have success with mirror lenses. You could be one of those, but I wish someone had told me to avoid mirror lenses before I bought mine.