Tripod Selection for Bird Photography

[robski]

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

That's the trouble with us engineers we get carried away with the techno-speak. It made perfect sense to me - LOL

In a nutshell thought the design and use of materials it killed the vibration faster on one tripod than the other. Therefore giving less movement while the shutter was open.

[yelvertoft]

This is all very informative and highly analytical. Being an engineer myself, and having ridden many bicycles over the years fitted with both aluminium forks and carbon fibre forks, I fully agree that CF has far superior vibration damping properties. But can someone explain to me why my CF tripod is far more prone to picking up wind induced vibration than my (very cheap) aluminium one?

If ever I want to take pictures during an earthquake I'll use the CF one, if I want to take pictures on a windy day, I'll use the ally one.

Duncan

[John]

A very interesting post, but 1/2 s, that seems far to long when you are shooting at say 1/15 s. It would be even more interesting if you could tell us the time for the amplitude to reduce to about 10% of its initial value. This might be much less than 1/2 s.

Originally Posted by windyridge50

I wouldn't agree. I have tried several large tripods incluing the Benbo Mk1, an old monster Kodak tripod that weighed 15 lbs, the manfrotto 501 and the Manfrotto 075B which is about the sturdiest non Studio tripod they do (12lb weight with hydrostatic ball head and rated at over 27 llb support weight and with well centre braced legs as well). Non were perfect with a 300mm f2.8 with 2x conv. then I bought the 055B with the large magnesium hydrostatic ball head (rated at 35lb) which with the adjustable frcition lock is superb for 3D panning and things got a lot better, I can now get shots that out-resolve the CCD at 1/5th sec (better than 54llpm).

I couldn't understand why at first so I carried out some measurements using a B&K miniature accelerometer and an Analogic FFT analyser. the answer is in the higher intrinsic damping of the spirally wound CF tripod legs, the unique top leg locating expansion clamps which impart further damping and the associated lower leg locking clamps. For the same shock input derived from a solenoid driven hammer the vibration with the 055 died away in less tha 0.5 sec with all the metal tripods the decay time was over 2 seconds, also Manfrotto have made the best use of the unique properties of composite media by moving away from tubular legs to a pentagonal section which places a much larger proportion of the applied stess in tension which of course is where composites really score.

[windyridge50]

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

[Bob Hastie]

Thanks for that, I'll read it a couple more times but I think I've just about got it.
One quick question though.
The legs on my tripod are assymetrical in that they are circular with a flat on one side. Do I assume these would be considered symetrical as any wind would hit a rounded surface at some point whilst passing over the legs?

[windyridge50]

Originally Posted by Bob Hastie

Thanks for that, I'll read it a couple more times but I think I've just about got it.
One quick question though.
The legs on my tripod are assymetrical in that they are circular with a flat on one side. Do I assume these would be considered symetrical as any wind would hit a rounded surface at some point whilst passing over the legs?

it is likely that only one leg would be excited at any one time when the flat surface was at right angles to, and on the reverse surface to the airflow, in this case the other two legs would not be. A search on google for "Von karman", "Vortex street" , "Vortex shedding"+vibration (logical AND), "flow induced vibration"will throw up several references "Google Scholar" is probably best as this directly accesses university sites.
I had a quick look and this gives a concise picture of the vortex shedding phenomena.

http://www.stoneman.co.uk/flowinduced.html.

[Dr.Manjeet Singh]

Originally Posted by windyridge50

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

I think i understood some of what you are saying but i thought that that damping it is reduced if the tripod is heavy as the kit and if it has stable legs(the base of the legs is circler and heavy).Please tell me if i am wrong(i was a idiot in Physics.(p.s. oh as for tapping the lens i did -THE LENS FELL OF(AH). Just joking-windyridge50 your explaination i did understand -and one of the best for slow learns like me but my question still remains-(above).Thanks and sorry for the bother.

[windyridge50]

Originally Posted by Dr.Manjeet Singh

I think i understood some of what you are saying but i thought that that damping it is reduced if the tripod is heavy as the kit and if it has stable legs(the base of the legs is circler and heavy).Please tell me if i am wrong(i was a idiot in Physics.(p.s. oh as for tapping the lens i did -THE LENS FELL OF(AH). Just joking-windyridge50 your explaination i did understand -and one of the best for slow learns like me but my question still remains-(above).Thanks and sorry for the bother.

I think you have to think of mass, stiffness and damping as three independent variables. Obviously a heavy tripod is preferable, as for any given force input the resultant acceleration will be less, also a stiffer tripod is good for the same reason that for a fixed force input the displacement will be less. So I agree with you on this.

However once the tripod is in motion the vibration is controlled by damping. the normal way of expressing damping is by the use of a term called tan d (d is really delta but I can't get the symbol here) where tan d is the ratio of the imaginary to real component of the complex Young's modulus (E) where E*= E'+jE" and tan d=E"/E'. A structure with a tan d of 2 is called "dead beat" and for any given energy input will come to rest in less than one cycle of vibration, typical welded steel structures will have a tan d around 0.001.

Damping may be of the coulomb friction type (two bits of metal rubbing together, say in a bolted joint,) or may be tuned to a specific application by the use of a viscoelastic polymer treatment.

The general solution for any vibrating system, treated as a "lumped parameter" system is covered by a second order differential equation, but for higher frequencies, where "wave effects" can throw up a number of nasties in a "distributed system" a one-dimensional solution of the three dimensional wave equation provides a much more precise answer.

Hope this helps :-)

[Jon Sharp]

It's easy to spend considerable sums of money on cameras and lenses and then cut back on other bits of kit. I've done it often enough myself and then later regretted a hasty purchase which later proved in practical or poor quality.

My advice would be save a bit more and by a good platfrom for the type of gear your using and the pictures you intend to take, this should then last you a lifetime. It'll be more cost effective in the long run.

[Leif]

Originally Posted by windyridge50

I think you have to think of mass, stiffness and damping as three independent variables. Obviously a heavy tripod is preferable, as for any given force input the resultant acceleration will be less, also a stiffer tripod is good for the same reason that for a fixed force input the displacement will be less. So I agree with you on this.

However once the tripod is in motion the vibration is controlled by damping. the normal way of expressing damping is by the use of a term called tan d (d is really delta but I can't get the symbol here) where tan d is the ratio of the imaginary to real component of the complex Young's modulus (E) where E*= E'+jE" and tan d=E"/E'. A structure with a tan d of 2 is called "dead beat" and for any given energy input will come to rest in less than one cycle of vibration, typical welded steel structures will have a tan d around 0.001.

Damping may be of the coulomb friction type (two bits of metal rubbing together, say in a bolted joint,) or may be tuned to a specific application by the use of a viscoelastic polymer treatment.

The general solution for any vibrating system, treated as a "lumped parameter" system is covered by a second order differential equation, but for higher frequencies, where "wave effects" can throw up a number of nasties in a "distributed system" a one-dimensional solution of the three dimensional wave equation provides a much more precise answer.

Hope this helps :-)

Fascinating stuff. Would it make sense to fill the legs of a triopd with a foam substance? Obviously this could only be done for the lower sections, but for a two section tripod it would be feasible.

Also bracing seems not to be used these days. But would some form of bracing between the legs help? I'm thinking about something light such as elastic bungee straps linked to each leg via a hook and linked together via a central ring. Obviously this would be slightly fiddly to set up, but would not add much weight.

Leif