Originally posted by Ontarian50 Around here, where I speak with a large number of bird photographers, a 400mm f2.8 isn't on their radar. Those who shoot Canon or Nikon aren't putting one on their dream list.
What they do want is the similar sized and priced 600mm f4.0. That's their dream lens, while they make do with the Tamron or Sigma 150-600mm zooms.
The 400mm 2.8's are primarily indoor sports lenses. If you shoot football, baseball, or track and field events, that lens is on your list.
About the only argument for a bird photographer to make for a 400mm f2.8 is that it can take a 2x converter to make a 800mm f5.6, if you don't mind the loss of light and some optical loss. But the 600mm f4.0 can do the same with a 1.4x converter. So....?
So as the earlier commenters suggested, unless Pentax is prepared to release a 12fps speed demon body with state of the art autofocus, and go after the pro sports market, there's not much reason to make a new 400mm f2.8.
Bringing back the 600mm f4.0 would be a better bet and use of resources, methinks. Even then the market for that would be pretty small.
For birding and wildlife the 600/4 is the most useful. When I bought my Canon 600/4 IS II I looked at the 400/2.8 also but he extra reach is used more than an extra stop of light is needed and then being able to get to 840/5.6 with the 1.4tc and 1200/8 with the 2x tc is great.
The 2x does soften the image some so I only use it when I have no other options. I was in Yellowstone in September and saw a brown bear on an elk carcass and it didn't like the ravens getting close so I wasn't going to try to get closer, I just put the 2x on and took what I could at a safe distance.