Mon 06-May-13 12:49 AM | edited Mon 06-May-13 01:00 AM by Leonard62
Whatever you choose to enter for focal length is for your benefit. It will only show up in the EXIF data so choose something you'll recognize. Since you're using a 2X extender most likely using the lens at the long end I would use 600mm. As far as the aperture setting it's also for your benefit. With the 2X extender I would choose f11. It won't affect the exposure.
edit: I just took a photo with my AI 28mm f3.5 set to f3.5. I kept the non-CPU entry the same as the last lens I used which was 500mm and f8. The exposure was perfect even though the EXIF data says I shot a 500mm lens at f8.