Which is because we're using a carbon-fiber tripod from Feisol, the CT-3441S - with a center column (which itself has two sections). It is clearly the weakest part in our setup. I replaced the "kit" ball-head with an RRS BH-40 head and quick-release plate (and an L-plate for the camera as well - it's expensive stuff, but trust me, it's worth it; I knew the second I operated it for the first time), which turned it into a useful combo in my opinion - at least for travel and hiking, since it's very compact and lightweight.
The compact size of the tripod has one downside though, at least for me: in order to use it at or near eye-level, I have to at least extend the first section of the center column (Shuwen doesn't need to;-). I rarely use the second section, since using the first section introduces enough problems already.
Before I continue: I'm not putting a lot of weight on the thing - no camera/lens combination that we use weighs more than ~2kg/4.5lbs, the heaviest being the D700 (no battery grip) with the 24-70/2.8 - but we rarely use this (portrait) combination from the tripod since that is too static. During my hikes, I find the 24-70/2.8 is just too heavy, so it's the D700 with either the 16-35/4, the 24-120/4 and (far more seldom) the 70-300VR then - none of these combinations weighs more than ~1.5kg.
And it was with the 70-300VR where I noticed the apparent problem with the center column first: I could clearly see the image in the viewfinder wobble for a brief moment after an exposure from the tripod after the mirror flapped down again.
Now, there's a lot of advice "out there" in the internet on how to avoid the mirror flap affecting sharpness, and amongst them would be
- using a remote release (cable or infrared) and/or the camera's self-timer (the latter being a crippled approach to getting one's hands off of the camera body when releasing the camera, it's better than nothing, but a remote is definitely better)
- using the built-in "exposure delay" mode, where the shutter opens not immediately after the mirror flaps up, but delayed, and
- using the manual mirror pre-release ("MUP" on the mode dial of Nikon cameras that offer this, more on that later)
So how did I find these times? I mounted D700 with the longest lens (70-300VR) on the tripod, taped a laser pointer to the lens hood and pointed the camera to a white wall (zoomed in all the way to 300mm). In manual exposure mode, I set an exposure time of something like 5 seconds so that the mirror flapping up and back down would be easily distinguishable. I then used the self-timer to release the camera, and closely watched the tiny red spot on the wall as the mirror flapped up and down.
And yes, it moved. :P The D700's mirror flap is hefty: the little dot moved at least 2mm up and down, with the camera being at a distance of ~4m from the wall. And as I already mentioned: with both segments of the center column extended, it would not stop for 3 seconds! That of course means: the camera's built-in Exposure Delay mode doesn't help! Nikon only added the option to actually program this built-in function for a delay of up to 3 seconds in the most recently announced D800 - with all other cameras, it is a maximum of 1 second only.
So in theory, when I have to extend the first section of our tripod's center column, using a remote and manual mirror pre-release, I should be fine if I wait for 2-3 seconds after locking up the mirror before I open the shutter. But what if it's windy? Here's a 30 second exposure at 120mm with wind:
Ooops! Of course, that is a rather challenging situation, even for the sturdiest of tripods, but it nicely illustrates my point (Mike Spinak has a useful tip for increasing tripod stability [YouTube video] in such situations).
And the remaining problem is: the smaller cameras like the D3x00 series, D5x00 series, Dx0 series don't even have the - IMHO, trivial but absolutely essential - manual mirror pre-release. You're completely out of luck - the feature is only available from the D7000 on upwards...
*) needless to say, the RRS TVC-34L has become the object of my desire. You can add some baby steps to getting closer to it by clicking on the ads around here... ;)