Originally posted by pschlute
It is a heavy beast and would need a good tripod or monopod. I used it with the latter recently and it balances very well with the K1.
This is the nub of it IMO.
The DFA 150-450 and the Bigma or Little Bigma all weigh upwards of 2kgs - 2325g for the DFA with hood and tripod foot. The DA*300 f4 weighs 1070g. Add the TC and you are around 1200g. There's a world of difference between a 1.2kg lens and a 2.3kg lens.
Not to mention that the DFA costs about twice as much too.
From the shots I've seen, they both have superb IQ. You would expect the DFA to have better AF - other users seem to have the same experience as @pschlute
So the big question is whether the extra cost and the extra weight are worth it to you, for the (considerable) extra versatility and slightly greater reach, and for better AF performance.
Originally posted by Adam
The FA* 400mm is nice and compact, sharp, and fairly affordable if you can find one! That would be my recommendation.
There's also the Sigma 400mm f5.6 tele macro (the version that takes 77mm filters). Or the Sigma 100-300 f4 + TC. Or (my favourite) the FA*300 f4.5 + TC. But none of these is WR, or has the quiet autofocus that @jddwoods is after.