Hi there,
Most reasonably made light weight tripods can be made to be very rigid (like the old small Velbons, etc). The trick is to introduce some tension in the legs so that there is no room for vibration or movement. The simplest way of doing this is to take some nylon cord and attach in the following manner:
Set the tripod up with the legs fully extended and fully apart.
Just above the first release section (i.e the locking lever/ring/etc that releases the lowest section of the tripod leg) tie the nylon cord to the first leg, then stretch it taught and wrap around the second leg, then stretch it taught and wrap around the third leg and then repeat by stretching it taught and fixing firmly to the first leg.
Take another bit of cord and attach it to the same point on the first leg, then attach it to the second leg leaving enough slack cord so that it almost reaches the ground.
Repeat for leg three and the return to leg one.
You now have three cords hanging down nearly touching the ground.
Use another piece of cord to join the three loose pieces of cord that are hanging down just above the ground.
In use, you set up the tripod and when ready to take the shot, you tread on the three cords that have been joined together until they touch the ground. This makes for an incredibly stable platform. I have used this system for many years including photographing the North Devon coast in raging gales with 5" x 4" and 6cm x 6cm (with mirror locked up). All the results were sharp and shake/vibration free.
On my website there are six night images (2 - 4 minutes exposure) all using this technique on a small lightweight tripod.
Best,
David
www.dsallen.de