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Originally Posted by Bob Hastie
Thanks for the very detailed explanation, but for lesser mortals like myself could you please repeat this in laymans terms  |
I will try to answer some of the question raised in non-technical terms. I think that someone else on the forums has quoted the relevant law as Newton’s laws of motion which basically say
a) that a body will be remain at rest or in a state of uniform motion unless acted on by an external force
b) every force has an equal and opposite reaction.
We tend to think forces as being static, for instance if we put a cup of tea on the table, the cup doesn’t move, but it does exert a force on the table. This force is equal to the mass of the cup multiplied by the force of gravity so we can say that the force F=ma.
If someone slams a door the cup might jump and rattle. The door slamming has transmitted a force to the floor which is then transmitted through the table to the cup so it has been moved by an exetrnal force, thus obeying law a)
In this case the force from the door slamming is transient, acting only for a short time.
In the real world the reaction of a body to an external force depends on the three factors, these are mass stiffness and damping. The force associated with mass is equal to the mass multiplied by it’s acceleration. The force associated with the damping is proportional to its velocity and the force associated with stiffness is proportional to the displacement of the body, or the distance it moves.
All these forces together must equal the force that is applied to the body which will vary with time.
The relevance to all this on a camera on a tripod is that at any one time several forces are being applied. These include a rotary movement of the focussing motor, a longitudinal movement of the lens elements. A vertical (or horizontal) movement of the shutter curtains, and a rotational/translational movement of the mirror, All these forces can be regarded as transient, acting only for a short time.
Other forces can be regarded as long term and periodic, these include, movement of the operator, and wind induced vibration.
A classic case of wind induced vibration was the collapse of the Tacoma narrows bridge. Here we had a new bridge, built to a high standard and designed to withstand winds of more than 100mph and yet it failed in a 30mph wind, the reason was the shape, the wind blowing over the platform started shedding vortices. The frequency that the vortices shed was the same as the resonant frequency of the bridge, each shed vortex applied a minute force to the bridge’s structure, but the result was collapse. A similar effect was seen on the new Millenium bridge across the River Thames but in this case the frequency that caused the problem was the forces applied by peoples feet , lots of little forces all adding up.
The solution to all these problems are to increase the the structural damping, the easiest way of doing this is to use dynamic absorbers or more correctly auxilliary mass dampers.
In the case of wind induced vibration of a camera on a tripod there are two sources, a low frequency movement associated with vortex shedding from the heavy lens barrel, and a higher frequency associated with the lower mass , smaller diameter tripod legs.
Wind induced vortex shedding is the real killer, user vibrations can be minimised by correct technique, and some cameras are virtualy unusable at shutter speed between 1/30 and ¼ sec due to mirror bounce. Some tripods have leg sections that are assymetric and are largely unaffected by wind, but the majority are circular and are.
Most people buy a tripod by its ability to carry a certain weight. But the important parameter is torsional stiffness and damping. If you hold the top of the tripod when the legs are fully extended and twist the top, if the legs flex and there is a small rotational movement, forget it, you will never get sharp pictures with an extreme telephoto in anything but dead calm. Put the lens that you intend to use on the tripod and tap the end of the lens, if the resultant movement doesn’t die away almost instantly there is not enough damping and going back to the fact that force is equal to mass times acceleration , for a given force input, a light structure will move considerably more than a heavy one.
Hope this makes things a little clearer