Golden Rule wrote:
I see that Adorama has this tripod on sale with the Sirui K-20X 38mm ballhead. Is this a good tripod for a Nikon D750 with up to a 400mm lens? Thanks for any advice.
I wouldn't suggest it. You need a tripod with thicker legs, especially the top tube. The K20X is pretty light duty. Better to get their K40 - no chance it will slip and it's only about $40 more expensive. As far as a tripod is concerned, I will point you towards the RRS guide to tripod selection:
http://blog.reallyrightstuff.com/choosing-a-tripod-part-1/http://blog.reallyrightstuff.com/choosing-a-tripod-part-2/http://blog.reallyrightstuff.com/choosing-a-tripod-part-3/And the Gitzo tripod catalog - attached below.
On pages 13 through 15 of the Gitzo document it discusses the relationship between torsional rigidity, lens angle of view and top tube diameter. They recommend for a 400mm lens, their Series 4 tripods, which have a top tube diameter of 36.6mm. Nowhere do they mention load capacity. But they do suggest that if you get their tripod with 32.2mm top tube, you should be ok using up to a 300mm lens. I think I remember reading that this is the diameter of the top tube on the Vanguard.
That being said, it might not be the best choice. Especially if you use a TC or intend to get a longer lens down the road. If I am wrong about the top tube diameter and it is actually larger, then you will likely be ok.
I will suggest that you take a look at the Feisol CT-3472. It has a top tube of 37mm, weighs only 3.8 (3 section leg) or 4 lbs, and costs $500 if you get the 3 section legs, or $510 if you get the 4 section legs. I ordered mine in 2007 when it was first introduced, and use it heavily, at least several times a week. I think the Vanguard is 6.5 lbs. Not sure what your use will be, but if you are packing a couple of camera bodies, several lenses, water, lunch etc the 2-1/2 pound difference is significant.
BTW, I got my Feisol after owning a Gitzo series 3 for a month and being disappointed with it's stability. Not that it is inherently unstable - it was more than adequate with lenses up to 300mm, just as their documentation states - but with my 500mm it was a bit unstable. This was not the case with the Feisol, and I currently use it with my 600mm F4, and I am still satisfied. The next step up would be a Really Right Stuff, or a Gitzo Series 5, which would be too heavy and way too expensive - but either would be a bit more stable. Maybe if I hit the lottery one day.