I've been dabbling with bird photography over the past month or so. The 300 f4 has done a sterling job, but, a 500-600 lens would be more suitable.
I've been offered a Canon 500mm f4.5, which appears to be in very good condition (it was previously owned by a private individual, so, well looked after). The price is around $3,000 which is about standard here in Australia. A new 500mm f4 is, well, worth a small car, and I have one of those (the car)
How does the IQ compare between the 300 f4 plus 1.4 TC? I realise that combination doesn't quite make the 500mm mark, but the extra reach is nice.
I'm using a 5D m3.