Saturday, 12 May 2007

How a tripod works

Orkhon River, Mongolia, September 2006

Be warned: this is a long post.

I just acquired myself a new tripod, a Gitzo 3540LS, about which I'm going to write a fairly sizeable review in another post. Before that, though, I though it important to get down some basics of tripods that underlie the way I'll assess its capabilities. There is a lot of pseudo-science, old wives' tales and plain error in the discussions on tripods out in web-land. I though I'd lend my take, applying a little more engineering science to the whole issue.

A camera tripod serves 2 functions: firstly to support the camera's weight above the ground and secondly to stop it moving. The first is easy to achieve, any reasonably strong support of sufficient height will support the weight and any number of materials can be used. Even the cheapest tripods meet this requirement - the only challenge is to ensure sufficient strength when using large cameras such that it doesn't collapse. It is the second part, the movement issue that is harder to solve and thus forms the rest of this discussion (and probably occupies the minds of tripod designers the most).

There are 3 principle modes of movement of the tripod that have to be considered, each with its own challenges.
1. Lateral movement of the vertical axis - sway, if you like.
2. Torsional movement (twisting) around the vertical axis.
3. Resonant vibration of the entire set-up - a sort of "ringing".

1. Lateral movements

These are, in fact, relatively simple to overcome. A tripod is an inherently stiff and stable structure, this is the key reason that tripods are chosen as the primary support structure. It is also the reason why supporting the weight is quite straightforward. Any reasonably stiff material will be sufficient to prevent the system swaying. If telescoping legs are used (as is mostly the case) then the joints also need to be sufficiently rigid to avoid movement.

Using an extending centre column increases movements because suddenly the top of the set-up is an inverted pendulum. One is relying on the stiffness of the column itself to prevent the movement, rather than the structural stiffness of the tripod shape.

To get any appreciable deflection you'd need a large force. By that time, either the thing has fallen over or you've buried it into the ground it stands on.

A similar effect can occur from the bowing of the legs. Leg(s) bow in on one side and out on the other. Again, a large force is required to produce this effect, even with relatively flexible legs.

Of the three movement mechanisms, I would suggest this is the least important in that it is easiest to solve and least likely to actually occur.

2. Torsional movements

This is the twisting of the tripod around a vertical axis. The effect is to create a panning motion in the camera. It is an effect due to the fact that a tripod is not a perfect tetrahedron (pyramid with triangular base) which is effectively perfectly rigid. The degree of movement is a function of leg stiffness (shape & material), leg length and leg angle. Flexible (flexible material, small diameter, thin wall) legs, long length and high angle (narrow leg spread) make things worse. Flexibility of joints is also an issue.

Again, this shouldn't be a major issue, although a tripod that is stiff enough laterally is not necessarily stiff enough torsionally. In this case, also, a large force is require to produce appreciable deflection.

It's mainly going to be an issue if there is a size offset of the camera, i.e. a larger part extending one direction relatively to the other. A large lens (think 500mm here), even perfectly balanced, has a larger volume one side of balance to the other. This can become a bit of a sail in high winds. Large format with large extensions and dark cloths also have issues. This is needed to produce the rotational force through the lever principle.

Both of these first sets of movement tend to result in a deflection then rapid return to the centre position. Therefore the effect is short lived.

3. Resonant vibration

This is almost certainly going to be the biggest problem. Sharp images, especially with smaller formats, are very sensitive to very small movements of the capture plane - think microns here. Any movement of the order of the circle of confusion diameter or larger will cause blur. This is why SLR photographers get so hung-up on mirror slap (me included). Resonant vibrations can last a long time and be very difficult to observe and eliminate in the field.

Vibrations in the tripod need to transmit through to the camera to cause problems. This can come from the wind, or banging the tripod or the possible amplification of mirror slap. These are the causes, the motive force for the vibration.

For resonance there needs to be a corresponding tendency of the system to vibrate at the same frequency as the applied force. This is called a harmonic frequency. Hitting a harmonic will cause the vibration to be worse and potentially last a long time. Resonance in this case is largely a function of the material and the leg design (both in terms of profile and length). even very structurally stiff systems can resonate, rendering the stiffness ineffective. This will be a bigger problem for metal tripods as they are linearly elastic - there is limited tendency for vibrations to fade away once started. Wood and carbon fibre are good in this respect in that they have natural damping that reduces the effect of resonance - the composite materials aren't linearly elastic.

Damping is added, deliberately or not, through the way the tripod connects to the ground, through the way the joints are constructed and in any looseness on connections & joints. The last of these, however, contradicts the aims of points 1 and 2.

Using different materials in different parts also helps as each section will then have a different harmonic and reduce the tendency for the vibration to transmit through the whole system.

So that's the tripod parts. Note that at no point have I mentioned tripod weight, and only material to a limited degree. From the main structural point of view, these are of marginal impact. it is perfectly possible to build something structurally stiff enough (points 1 and 2) out of virtually any material with careful selection of leg size & design. Want stiff aluminium legs? Make the diameter of the tubes larger. A large diameter tube is stiffer, weight-for-weight, than a small diameter. This is a fact that bicycle designers exploit a lot, to enable lighter, stiffer bikes by altering the tube dimensions. Fundamentally the stiffness comes from the basic shape.

The idea of a heavy tripod I believe really comes from the wooden days when extra weight was the means of gaining extra stiffness and reduced vibration. That is important when you're using solid section legs and a material like that. Modern design and materials has moved us past that. Weight will really only be useful if you use the tripod at odd angles with the camera hanging off the side - in effect it becomes counter-balance, nothing to do with the other problems of movement. An interesting comparison is with bicycles. With modern designs and materials frames are now lighter, stiffer, stronger and transmit less vibration than ever. The basic strength comes from the triangular shapes (just like a tripod), lateral stiffness and vibration are down to the tube design and material. No one would ever propose that a bicycle needs to have extra weight to provide those properties, so should it be with a simple camera tripod (which after all sees far less deflection or force than any bicycle).

One has to remember, however, that similar factors affect the camera and mount (whatever device you use to connect camera to tripod). The whole system will only be as effective as its worst part. If there is a flexible connection to the tripod or a loose camera plate in its mount then the best tripod in the world will not stop the movement.

Of course there are other compromises in tripod design: packability, carrying weight, maximum extension for tall people etc but in principle none should fundamentally affect the ability to provide a sufficiently rigid platform for a camera.

1 comment:

  1. Very interesting, thank you.

    I ride lightweight, folding bicycles, & am looking for a good light tripod, as "normal" ones approach the bike weight!

    I'm considering building such from aluminium tubing, using the bike as part of the tripod, so I'll bear your notes in mind.



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