120-300mm f2.8. A fake 600mm f5.6?

[maisatomai]

When I take my 400mm 5.6l to the bird park, I usually enjoy high shutter and low iso. Was thinking to get this lens which cost 3 times as much. People were saying this can be turned into a 600mm f5.6 by a 2x tc, hence beating 400mm f5.6l hands down.

I was wondering whether it is true as a 400mm f5.6 can be turned into a 560mm f8(40mm less) without losing af by taping. I guess a 1.4x glass will not degrade as much as a 2x. Thus, if I don need the iso and
Shutter speed and zoom flexibility, does
It mean I am better off not upgrading?

 

[gundamseed84]

There is a reason why people pay big bucks and get 600mm primes instead of relying on teleconvertors. You can maintain af if you have 1D series body at f8. Otherwise you probably need do some taping. Also at f8 you lose one stop of light and can only shoot at half the shutter speed.

Getting 120-300 with 2x convertor will sure have a drop in image quality due to 2x. I may be wrong but I reading reviews 120-300 IQ is not stellar in the first place.

I would opt to own sigma 50-500 or 15-500 if I do not need f2.8.