Hi Grumman-cats,
>I have to manual focus when using the 2x magnifier but it usually isn't that tough and with the bright colors of the planes I wouldn't have thought it to be a problem but it was.
Auto-focus needs a lot of light to work correctly, and the magnifier actually increases your effective f-stop value which does not help.
(I have shot pictures at airshows with manual-focus lenses for years, and it's not easy even with a lot of practice.)
Judging from the only slightly blurred propellers, you can still increase your exposure time without fear of camera shake in order to gain some field-of-depth.
I don't know if you have already cropped your pictures, but if not, it might be worth it to leave out the multiplier and shoot with the basic lens only.
That will improve the pictures for two reasons: You will have the benefit of autofocus which is more reliable than manual focusing, and you will have sharper pixels - due to more accurate focusing - even if the plane doesn't fill the frame completely.
In the end, it's not the number of pixels that counts but the number of sharp pixels
Of course, it depends a bit on your camera system, but you could just give it a try at the next airshow for an hour, then use the multiplier for the rest of the show, and compare results. Then decide what works better
Regards,
Henning (HoHun)