Originally posted by Mike.P® I had a Sigma 150-600mm contemporary arrive a couple of days ago to try on my 7D. I know it's half the price of the Pentax lens but IMO this is one of those times where I think you do actually get double the performance for your money. I recently tried the sport version as well but that was (A) much to heavy to be shooting at an airshow all day and (B) not much better than the 100-400mm I am thinking about replacing.
In fact I have to say that IMO the 50-500mm OS is a much better lens IQ wise.
Thank you for your information. I've never had the chance to shoot with any of the Tamron or Sigma 150-600s. In fact the 600mm long end is very tempting, but from what I've read only the Sigma S is really good at this focal length (unless seriously stopped down) in the entire frame, but as you said it is too heavy to consider buying it! Moreover the DFA 150-450 is usable with the HD 1.4X converter and thus it offers even more reach (630mm) at the cost of -2/3 of a stop light.
For the DFA I still have no experience how it performs with the K-5IIs at aviation photography, but I will soon find that out. I agree with you that the Bigma 50-500 OS is a really nice lens. If you use it around f9 you get very good results and it offers the advantage of a 10X zoom, so with one body and this lens you have great versatility. What I would prefer is that Sigma would upgrade the 50-500 with better glass and construction quality that would give an even better Bigma. I was expecting that the 150-600 would surely be better than the 50-500 and I'm surprised by your comment. The 50-150 makes HUGE difference when you are close to a landing strip or you shoot formations, planes at the static display etc.
How does the DFA compare to the 100-400? This is an interesting comparison especially with the new II version. Also with the 80-400 from Nikon. These lenses are direct competitors but I have never used any of them and the comparison is difficult as the cameras are different as well, so it would be better if you compare different systems not just the lenses.