Ive decided to buy the 300 f/4, but should i buy it with the 1.4x or 1.7x? Does 1.7x make a difference in IQ compared to the 1.4x? And how is the AF with the 1.7x
Which camera? I'm going to bet that the 1.4x is a much better choice because it will result in a much more usable AF system. BTW, the 1.7 is actually more of a 1.6... The difference between 420mm (1.4x) and 480mm (1.7x) is not that significant given the other tradeoffs.
I've rented the 300 f/4 and TC's a few times... I thought the 1.7x was the best mix of 'Range' with an acceptable loss in quality (vs. the 1.4 being sharper but not the reach i wanted).
However, i never pulled the trigger buying the 300 f/4 because Nikon is supposed to be releasing a new version soon (WHERE IS IT!!!). The AF speed on the existing 300 f/4 is not what i would call speedy (i was using it on a D7000).
The Lost - if Nikons new lens prices are anything like Canons the new 300mm f4 will be VERY expensive. Plus as its jsut rumour you could be waiting years for that.
From what I've heard the 1.7 is indeed a great compromise between reach gain and image quality loss - I'm honestly surprised Canon and Sigma don't make them
Some of those rumour sites just put up rumours because they can - its just rumour so it doesn't have to be true and they don't actually have to hear anything. There are fairly popular and firm things in the line up that they can expect t obe updated. The 100-400mm from Canon has had rumours of a MII for years now.