Thank you and I'll post a few here soon! I'm using to backpacking 40 pounds while hiking at higher altitudes (6,000 feet) without issue. I'm not worried - this is a light setup at 30 lbs, including backpack weight:
FA* 250-600/5.6 (13lbs)
Wimberley WH-200 gimbal head (3lbs)
Gitzo tripod (4-6lbs, depending on model)
A couple of light landscape lenses (DA 12-24, DA 50-200)
I am probably going with Gitzo since that was the only lightweight tripod with a load rating that was double the super telephoto setup. Feisols aren't tall enough for me nor do they have anything rated above 26lbs. Of course, that means extra $$$ for Gitzo, but they can hold a lot of weight and
they are tall enough (I'm 6'3")...
BD: thank you and perhaps some day I'll take the plunge for the FA* 600/4! I agree with your points about the focal - most folks do shoot at the long end. That makes perfect sense since it's simply taking advantage of the extra length at 600mm. The only advantage for the 600/4 is the extra stop of light against the FL range and flexibility for cropping, framing, etc. that the FA* 250-600/5.6 can give the photographer. I've been very frustrated in the past when I carried my FA* 300/2.8 and the Sigma 500/4.5. Twice in 3 days I decided to pack it in and both times I carried the wrong lens back to the SUV first (hidden out of sight from the blind) and the Great Egrets swoop in...
Once they landed further out and all I had was the 300/2.8 and the other time they landed really close and all I had with me was the 500/4.5 - it's a conspiracy!!!
The images I'll post can clearly demonstrate it's usefulness as a birding and wildlife lens, especially with the aforementioned points. I am not in any way, shape or form dismissing your suggestion - I'd love that lens too.
However, for me the one disadvantage (max. F/5.6) is very much outweighed by everything else. Even F/4 is pretty tough to use in lower light levels, so it's a bit of a wash for me = YMMV.
For reference, I thought it might be an interesting comparison on the MFD (minimum focusing distance) for several super telephotos:
Nikkor 600/4 VR: 16.4 feet
Nikkor 500/4 VR: 13.1 feet
Nikkor 400/2.8 VR: 9.5 feet
Canon EF 600mm f/4L IS USM: 18 feet
Canon EF 500mm f/4L IS USM: 14.8 feet
Sigma 500/4.5: 13.1 feet
Sigma 300-800/5.6: 19.68 feet
FA* 600/4: 16.5 feet
(varies - up to $7000)
FA* 250-600/5.6: 11.5 feet (varies - up to $5500)
However, for the FA* 250-600/5.6, the lens' MFD is only 11.5 feet throughout the focal length. That's a huge deal for 600mm, especially when you are in a blind. Quite a feat for Pentax to do that with the FA* 250-600/5.6, given it's design date, etc. A friend who owns a Nikon D3 with the previous version of the 600/4 has mentioned that more than a few times he's lost shots due to the MFD being around 18-19 feet.
I know the FA* 600/4 is still available for special order - are you considering finding one on the used market?