Originally posted by noelpolar I have a F*300/4.5 and a DFA 150-450.... a well know person on this forum "Mikesf" sold his F*300/4.5 and recently got a DFA 150-450. He's written a review of the 150-450 in the lens database that you should have a look at....about 4 reviews down..... I concur with his comments... the F*300/4.5 is a special lens.
HD Pentax-D FA 150-450mm F4.5-5.6 ED DC AW Reviews - D FA Zoom Lenses - Pentax Lens Reviews & Lens Database
Of note...I do believe the DFA 150-450's light transmittance is a stop to stop and a half down on the F*300 for a given aperture.... I really notice this with the K3.... not so much with the K-1 as it's high ISO performance compensates (at the cost of reach of course).
I have no fact base to go on, however, I wouldn't be surprise if the DFA at 400-450 provides better IQ than the FA400 and loses one stop of light in the process.
As I do not have the DFA150-450 or FA*400 I cannot comment directly on this question.
However, I have compared the F*300 with the FA*250-600 and F*600. The F*300 is better than the FA*250-600 at commensurate focal length. The F*600 is better than an equivalent crop from the F*300, but the differences are minor. So, in regards to noelpolar's observations I whole heartedly concur on the 'specialness' pf the F*300.
I would be very surprised if the DFA150-450 has better IQ than the F*300. Even with a high quality teleconverter, I would be surprised to find the DFA150-450 has better IQ than the F*300.
This all said, I would be thinking the IQ differences across all mentioned lenses are minor. But, when it comes to portability, WOW, the F*300 is extraordinary!
If I were to speculate, I would suggest the FA*400 is likely to have better IQ and portability, but will require post processing as Adam pointed out.