Tripod Apex question

[basker]

(1) the diameter of the legs, assuming they are hollow, because larger diameter means more rigidity for the same weight
(2) in the joints between leg sections, because the joints are always much weaker in bending than in compression, so their ability to support a weight along the axis of the legs is excellent, their ability to resist a force applied to one side may be poor.

I am tall and well aged. Adapting to awkward viewfinder placement is extremely difficult. So a third factor, longer tripod legs, arises to push the limits of 1 & 2.

Earlier I said that extending the fourth section makes a difference. Now I wonder what causes that difference. Is it the smaller tube, the additional (smaller) joint or the increased total length? My first guess is all of those, so I have a lot more thinking to do.

[Seapy]

This mays seem like silly and going off at a tangent but I once had a strange experience which I have never explained.

Many years ago I leant a ladder against a stone built shed, the ladder was about 12 feet long and the shed about 9 feet high.  The ladder started to vibrate, like buzzing?  I don't remember the details but even if I moved it then put it back in the same place, it vibrated.  You could feel the vibrations with your hand and hear them, there was no wind to speak of.

This experience has led me to think that on occasions some things may vibrate in harmony with their surroundings. I don't pretend to understand it but I do believe there can be this phenomenon.  A ladder and a tripod are not dissimilar and although I realise we are talking stability, not vibrations, I just wonder if sometimes things have a natural harmony and can spontaneously vibrate without us realising, causing blurry images.

I have been surprised at the readings I get with the seismometer app on my iPhone when placed in apparently rock solid locations.  I realise calibration of these sorts of devices is important but the fact that it's picking up vibrations, apparently from nowhere, maybe they can be amplified by something like a metal or even a wooden tripod.

In my eyes, heavy is better because it needs more energy to cause vibrations in a heavier object.  If the energy is constant, increasing the mass should reduce the amplitude of the vibrations and probably the frequency too.

No, I haven't been near magic mushrooms! 

[Jack Dahlgren]

'Stability' as in the sense of being able to hold a camera/lens stable over time. Torsional rigidity means less proneness to being twisted during the exposure.

The "crutches" construction makes say a Sachtler quite light in weight compared to traditional tripods, yet it provides a very stable support. I have used these tripods for decades so am pretty familiar with their behaviour in the field. They also have the additional feature of working well immersed in mud, snow, or water.

With a properly balanced load, the single legs of a tripod have all forces in line with the leg, so no bending is encountered with a vertical load, but there are often rotational loads placed on tripod heads, especially with panning of a video head or similar.

The leg construction on Sachtlers resists bending due to rotation around the head and is a superior design to just adding fatter legs to an ordinary tripod. You see similar designs in surveying tripods where angular error is very much to be avoided.

[basker]

'Stability' as in the sense of being able to hold a camera/lens stable over time. Torsional rigidity means less proneness to being twisted during the exposure.

That was my intended meaning for stability in the original post, and that is exactly what I hoped torsional rigidity meant. Whenever I mount a camera via quick release plate and with the lens cantilevered off the body, I have to be wary of vibration in any plane. I must try to understand the process and learn to best use what I have. I want to get a true image, no matter how homely or mundane the subject may be.

The "crutches" construction makes say a Sachtler quite light in weight compared to traditional tripods, yet it provides a very stable support...

I can see that. Either the leg or the hinge will be a weaker link. Best case is when both are so strong it does not matter.
Clearly, the conventional tripod has limitations, but is it completely useless? Thinking that makes my wallet ache.

[Bjørn Rørslett]

If the substrate is soft, springy, or soggy, and the tripod doesn't make good contact to the ground, one can add a lot of weight to a tripod and still have it wobbling around

My setup for the first Venus Transit (2004) involved three Sachtler tripods, from front to rear these were Sachtler ENG 2 CF, DA-100L, and DA-75L, respectively. These weigh in around 2 kg without the head.

[Bjørn Rørslett]

The small Sachtler for travels and close-to-ground work weighs just 1.2 kg. With a proper head on it, there is no problem with a 600/4 or similar. Properly set up it has extreme torsional rigidity and stabilty.

(I made it from an ordinary DA-75L by cutting-off the legs)

[David H. Hartman]

Think this is a fairly steady position to support the camera. 

I'd be concerned about resonance in the hanging center column. I would shorten the legs of the tripod and use less of the center column.

Dave

[Fons Baerken]

Deleted

[Bjørn Rørslett]

The various incarnations of DA-75L have been very popular amongst  Norwegian nature photographers for many years. There are 2- or 3-section models, material of aluminium (the "classic" model) or carbon fibre, and with or without a "quick release" mechanism. Cutting down the carbon fibre models require more skill and it's easier to break a leg as it were; however, this material makes the tripod even lighter and more pleasant to handle in the winter.

Personally, I now prefer the ENG-2 series as I mainly work out of a car.

[Les Olson]

'Stability' as in the sense of being able to hold a camera/lens stable over time. Torsional rigidity means less proneness to being twisted during the exposure.

What twisting force is acting during or near exposure?  The photographer's finger on the shutter - unless she uses a remote release - but to reduce the effect of that force what is needed is to reduce the length of the "lever" - the distance between the shutter release and the axis of support.  The 35mm design, with the shutter button high and far to one side, is a problem in this respect - the medium format design, with the shutter button low on the front of the camera is better - but what helps with a 35mm camera is a quick release plate and tripod head that extend as near as possible to the line of the shutter press (unfortunately, Nikons have the battery on that side so the plate cannot be as long as would be ideal).  A long lens also creates a twisting force - although not any more during exposure - but the remedy is the same: minimise the rotational force by shortening the distance between the axis of support and the weight.  That is why large diameter ballheads, and flatter ballheads, are better.  Small changes in the length of the lever make a big difference, because the bending of a beam supported at one end by a force applied to the other end is inversely proportional to the cube of its length. 

Sagging over time is a flaw of the ballhead concept that depends on friction between the ball and a flexible cup to prevent movement, because it won't.

As far as the crutch design is concerned, if it could provide the same rigidity with less weight it would be universally used where stiffness has to be maximised and weight minimised - bicycle and motorbike frames, eg - and it is not.  There is a good reason for that: the stiffness of a hollow tube is proportional to the fourth power of its diameter, while its weight is proportional to the square of its diameter, so, for the same weight of material, a single tube is much stiffer than two narrower tubes. 

[Bjørn Rørslett]

There are two possible approaches: theoretic or practical. As my experience shows the Sachtlers work much better than the Gitzos I used earlier, the choice is easy  They support my gear in a superior manner, are more light weight, far more versatile in how they can be set up in the field, and stand adverse conditions than would destroy a Gitzo.

If theory is contradicted by practical observations, the theory needs to be improved.

I once worked with this TV videographer on an assignment and she didn't bat an eyelid about submersing her Sachtler tripod to get the required camera position.

[Jack Dahlgren]

As far as the crutch design is concerned, if it could provide the same rigidity with less weight it would be universally used where stiffness has to be maximised and weight minimised - bicycle and motorbike frames, eg - and it is not.  There is a good reason for that: the stiffness of a hollow tube is proportional to the fourth power of its diameter, while its weight is proportional to the square of its diameter, so, for the same weight of material, a single tube is much stiffer than two narrower tubes.

Please look at the rear triangle of any bicycle and imagine downtube is the tripod head, axle is where the foot of the tripod leg is.

Likewise on many motorcycle frames at head tube.

The claim that something would be universally used makes no sense when not all use cases are the same.  Some are driven by cost, ease of construction, available materials etc.

The triangular leg, and its cousin the flat leg, are used in almost all cases for video and surveying due to their resistance to torsion about the vertical tripod axis. To achieve the same with a single tubular member would require making it bulkier than it needs to be. Beams are “universally” placed with their deepest section oriented in the plane they need to provide most resistance to bending.

Video has this requirement because the camera often pans horizontally against resistance. Surveying equipment has the same requirement because it needs to stay stable in environmental conditions such as wind.

Still cameras can benefit from this rigidity, but it is dependent on the use case and what is considered “good enough”. For some people a monopod or even human tripod is sufficient. So most camera tripods are designed to be light and portable with just enough stiffness for the job at hand.  Not all people need a Sactler ENG series tripod, many can’t afford it, and many don’t want to carry it. For long exposures, the resistance to wind caused torsion is a good thing.

As always, suitability for the circumstances  at hand should govern choice.

[Les Olson]

[quote author=Seapy link=topic=7235.msg116771#msg116771 date=1521317032
This experience has led me to think that on occasions some things may vibrate in harmony with their surroundings.

In my eyes, heavy is better because it needs more energy to cause vibrations in a heavier object.  If the energy is constant, increasing the mass should reduce the amplitude of the vibrations and probably the frequency too.
[/quote]

The phenomenon of small vibrations acting on a structure to produce larger amplitude vibrations is resonance.  It requires the driving vibration to be in a narrow range near the resonant frequency of the second structure.  Any structure can resonate, including tripods.  The mass of a structure affects its resonant frequency, but not the amplitude of the resonant vibrations when it is subjected to vibration at its resonant frequency.  That is why quite small inciting vibrations can cause catastrophic structural failure of large structures.

If you make a noise - create vibrations - by hitting an object the amplitude of the vibrations (= the loudness of the noise) is less if the stiffness of the material is greater but is not affected by the mass of the object.  The frequency of the vibrations (= the pitch of the noise) is reduced by both more stiffness and more mass. 

[Les Olson]

The triangular leg, and its cousin the flat leg, are used in almost all cases for video and surveying due to their resistance to torsion about the vertical tripod axis.

For long exposures, the resistance to wind caused torsion is a good thing.

Yes, resisting rotational forces requires a different design, but that has nothing to do with stability during still photography.

Wind exerts a force on an object equal to 1/2 x density of air x wind velocity x surface area of the object.  A 300mm f/2.8 lens has a surface area of roughly 300 square cm, so if the lens was a flat surface you would need about 50kph of wind for the wind-loading to exceed the 2.9kg weight of the 300/2.8 (plus the lens is not flat, which reduces the wind loading).  That sort of wind makes walking difficult, so if your tripod is not inconvenienced by the weight of the lens, wind is not an issue unless you are a specialist cyclone photographer. 

Wind can also cause problems because of flutter, but that will be no better with a crutch design than with a single tube design.

[Seapy]

The phenomenon of small vibrations acting on a structure to produce larger amplitude vibrations is resonance.  It requires the driving vibration to be in a narrow range near the resonant frequency of the second structure.  Any structure can resonate, including tripods.  The mass of a structure affects its resonant frequency, but not the amplitude of the resonant vibrations when it is subjected to vibration at its resonant frequency.  That is why quite small inciting vibrations can cause catastrophic structural failure of large structures.

If you make a noise - create vibrations - by hitting an object the amplitude of the vibrations (= the loudness of the noise) is less if the stiffness of the material is greater but is not affected by the mass of the object.  The frequency of the vibrations (= the pitch of the noise) is reduced by both more stiffness and more mass.

Thank you kindly Les,  that is very interesting, I had hoped someone might reply, I realise it's a bit OT, but it is a bit related and also related to the issue Michael had photographing flowers a while back with a resonating floor.  My answer to most thing is more mass, but a damping mechanism would probably be more effective in absorbing the energy perhaps?

I suppose a practical example of frequency is a bell, small bells go 'ting-a-ling', large bells go 'BOING'.

If the mirror lifting shakes the tripod that energy has to dissipate before the camera becomes still again.  A heavier tripod/camera would still vibrate with the same amplitude, but at a lower frequency, slower. Interesting.  How does the mass affects the decay of the energy?  More mass longer decay?  Or visa versa...