Originally posted by thibs 1/ Judging based a DA*400 based on the FA*400 is misleading, especialy with lenses not meant to be used on digital.
2/ I never said it wasn't better to first introduce a 300/2.8 + TC and then a 400/5.6. Of course the 300+TC would buy time very easily.
3/ Provided they do their job correctly with the new lenses which I beleive, I see no reason for a 400 being less sharp than a 300+TC.
Oops! I wasn't trying to be argumentative, honest!
1/ I was comparing the FA*300 with the FA*400, not a DA* to FA*, so it was "apples with apples". Even with film on my MZ-S, the FA*300 + TC was better than the FA*400.
2/ I believe a 400 f5.6 would not be as sensible as it was in the film (FF) days, I think a 400 f4.0 (or even f4.5, to keep the front filter size below 100mm, 95mm is available from Hoya & B+W) would be a much better choice as a 400mm lens to introduce later on. In the meantime a quality 1.5x TC to use on the 200 & 300 DA* lenses would be a very popular and sensible step, as with SDM, there wouldn't be any slow down of the AF.
3/ Pentax have always made superb 300mm lenses (both the FA*300's [f4.5 & f2.8] are superb on digital) and "reasonable" 400mm lenses, I agree that, in theory, a quality 400mm is possible, I just think it would be more sensible to design one faster than f5.6, that's all.
Speaking personally, I would rather have the 60-250 f4.0 zoom, plus a cracking 300mm f4.0 and a really good 1.5x TC as a lightweight (relatively!) long lens setup. That is my ideal birding, aircraft and motorsport combination. My estimated cost for such a combination would be around $2 - 2.5k. ($1100 + $1100 + $300), which I consider quite enough! For less money the Bigma is the way to go and hard to beat, especially for convenience.
An alternative if you are a prime only user, is to have a 200 f2.8, a 1.5TC and a 400mm f4.0. That covers everything from 200mm (at f2.8) through 300mm and 400mm (at f4.0), to 600mm (at f5.6) where it is still fast enough and still autofocus okay. (I find my FA*300 f4.5 with the TC [= f6.3] still autofocuses quite well, even in average lighting, it's just slow focussing due to the added friction of the screw drive through the TC slowing the AF down).
The downside of the last combo would be that the cost would be considerably more, my estimate would be nearer $4 - 5k, as I would expect a 400mm f4.0 alone being similar to a 300mm f2.8, i.e. around the $2.5 - 3k mark.
Remember that Pentax are currently targeting the mid band sector, at least for the next 2 or 3 years, probably more, where any lens or body over $1500/2000 would be out of that arena.