Tripod Apex question

[Jack Dahlgren]

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.

I don’t think that leg design is a significant factor in keeping the tripod from blowing over. Not sure why you would be considering that as something different. Also note that the wind force usually doesn’t have to be as large as the gravitational force on the camera to make the whole thing fall over. As long as horizontal force due to wind * height of center of mass of camera and tripod is greater than force of gravity * distance from leg to center of mass of tripod, the overturning moment is enough to topple your tripod. (Positing symmetry as a simplifying assumption)

Fluttering in the wind occurs at lower speeds, and is dependent on leg design as it affect stiffness.

Whether those effects are important in your work depends on the nature of your work. For long exposures outdoors in blustery weather they may be. One can certainly get great photos without it.

[basker]

...My answer to most thing is more mass, but a damping mechanism would probably be more effective in absorbing the energy perhaps?... 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...

I bought the four section legs to satisfy a height requirement. One rationalization was that an additional lock would give additional damping. Still do not know if that was just fantasy. Imagined uses vs. actual application is a sneaky relative of theory vs. practice.

There are several ways I can shorten the legs, and all of them seem to reduce flexing. Maybe the answer is a folding stool.    An earlier post shows a youngster kneeling on a pad. A brilliant solution but I might require assistance getting up.   

[Bjørn Rørslett]

...  An earlier post shows a youngster kneeling on a pad. A brilliant solution but I might require assistance getting up.   

That was in fact me although before the hair went grey and other things changed as well. Knee pads are still a great invention whether used on their own or embedded in the trousers workman-style.

[basker]

...related to the issue Michael had photographing flowers a while back with a resonating floor...

I wonder if that one got resolved. 

[Bjørn Rørslett]

Shooting with really long lenses is the true acid test for tripod support. Here my 1200/11 ED-IF on the Sachtler ENG 2 CF HD with Video 20 Fluid Head. This combination supports the long lens with ease so the user just can concentrate on locating the subject (not always easy, thus having levers to move the head rapidly is essential) and shoot.

[basker]

Here my 1200/11 ED-IF on the Sachtler ENG 2 CF HD with Video 20 Fluid Head...

Awe inspiring! Reminds me of a dry old IBM witticism, "there's probably a two week course in Poughkeepsie for that one."

[basker]

That was in fact me although before the hair went grey and other things changed as well...

Time does fly, but "Modern Excalibur" alone buys you a lifetime pass to my good opinion.

[Les Olson]

I don’t think that leg design is a significant factor in keeping the tripod from blowing over. Not sure why you would be considering that as something different. Also note that the wind force usually doesn’t have to be as large as the gravitational force on the camera to make the whole thing fall over. As long as horizontal force due to wind * height of center of mass of camera and tripod is greater than force of gravity * distance from leg to center of mass of tripod, the overturning moment is enough to topple your tripod. (Positing symmetry as a simplifying assumption)

There are two separate issues, with both weight and wind. 

One issue is the tripod pivoting around one of its feet and toppling over.  For the tripod to pivot around one of its feet the weight of the camera etc has to be lifted against gravity.  For the usual arrangement, with the camera etc near the apex of the tripod, and one of the legs in the line of the wind, the wind loading x sine of the angle between the tripod leg and the ground has to exceed the weight of the camera x cosine of the angle between the tripod leg and the ground.  What you want to resist toppling is widely splayed tripod legs and more weight.  Weight suspended from the tripod works because it lowers the centre of mass and therefore the angle with the ground, and because its own wind loading does not create any lifting force, so the wind loading on the camera alone has to lift the camera plus the suspended weight.  Even more effective is attaching the tripod to the ground with tent pegs.

Leg design could affect the likelihood of the tripod blowing over in two ways.  One is if the legs bend, but unless you have a very flimsy tripod that would require hurricane force winds.  Perhaps more likely is the feet being driven into soft earth or snow, or slipping because you have carbide tips on rock.  Where leg design will make a difference is flutter and buffeting, and a multiple tube design will be worse - for the same reason as you get vibration when you partially open a car window.

The second issue is the camera pivoting on the ballhead - ie, pitch in the case of weight and wind from the front or behind, and roll and yaw in the case of wind from the side - more yaw with long lenses.  Because the ballhead is symmetrical its ability to resist all those movements is the same, so weight is the same as wind loading as far as the ballhead is concerned. 

Another issue, that Bjorn has alluded to, is .

[Jack Dahlgren]

Les,

You are sort of correct, but also not completely correct. When the moment created by horizontal forces of wind (includes wind against camera, legs, sandbag etc.) is higher than the moment created by gravity around that same point - the downwind leg - the rig will overturn. This is simple physics, but doesn’t really make any difference in leg design choice.

[Seapy]

This is my answer to wind...

The spikes are fitted with foot kickers, so one can stamp them into the ground in soft conditions, the ground here was hard under the grass.

Wastwater, English Lake District.  A brief lull, on a VERY windy day.



This was one of the photographs I took.  The gusts were breathtaking.

[Bjørn Rørslett]

Surveyor's tripods were all the rage in my  neck of the woods until nature photographers switched to Sachtlers instead. A lot less to care in the field and properly set up in the same stability league as well.

Nice picture on the Wastwater - I had the impression all water bodies in he Lake District were calm and serene, but obviously I was mistaken

[basker]

This is my answer to wind...The gusts were breathtaking.

Steely determination, ingenious solution and impressive results, as usual from Robert.
Very cool stuff 

[basker]

The edited statements below are from a tripod vendor's website.

"The longer the focal length is, the more apparent vibration becomes, and the demand for eliminating or preventing that vibration is increased."

"Even though a 300mm/f4 lens might only weigh as much as a 70-200mm/f2.8, you still deal with the same magnification factor in terms of "revealing" the vibration within a system. This may be emphasized by lower mass of the lens, since a lighter setup has less inertial resistance to outside vibration."

"Even when a 400mm magnification factor can be handled by a particular tripod, the weight and bulk of a lens that size may be easier to handle and safer on a broader and more rigid platform system."

Heavier and longer lenses are more demanding, but in my case that does not matter. I think that what I have is adequate if used properly, even though I also want improvement.

[Les Olson]

Les,
When the moment created by horizontal forces of wind (includes wind against camera, legs, sandbag etc.) is higher than the moment created by gravity around that same point - the downwind leg - the rig will overturn.

Yes: the conclusions are the same whether you calculate moments or treat gravity and the wind force as vectors normal to the tripod leg and along the tripod leg, whose sizes relate to the sin and cos of the angle the leg makes with the ground. 
 
The reason I would rather think about it in terms of the leg angle is that I think it makes it easier to understand two aspects of using a tripod in windy conditions. 

The first is that once a wind gust starts the tipping, and the angle the leg makes with the ground increases, the tipping will continue despite the gust abating, which is why your expletive deleted tripod (or tent) appears to be stable but then tips over suddenly and without warning.   

The second is that splaying the tripod legs increases resistance to being tipped over by the wind, but the relationship between how much you splay the legs and increasing stability is not obvious.  My Gitzo tripod legs make an angle of about 70 degrees to the ground in their default position, and sin 70 = 0.94 and cos 70 = 0.34.  If high wind is the problem, 70 degrees is way too steep. But sin 60 = 0.87 and cos 60 = 0.5, sin 45 = 0.7 = cos 45, sin 30 = 0.5 and cos 30 = 0.87, and sin 15 = 0.26 and cos 15 = 0.96.  So going from 70 degrees to 60 degrees does not help all that much, and to make a big difference to stability in high winds you need to get the tripod much lower to the ground.  The other side of that coin is that getting much lower to the ground makes it much harder for the legs to support the weight of the camera plus lens.  So if you are going to have to work in high winds, you need a tripod that can get low, and you need a much stronger tripod than if you could leave the angle of the legs at 70 degrees.

If the sandbag is suspended to form a pendulum its wind loading displaces the mass away from the vertical.  The weight is supported by the tension in the string, that has a vertical component and a horizontal component, and their relative sizes are related to the angle between the string and the vertical.  The horizontal component, like the wind loading, has two components, one along the tripod leg and one normal to the tripod leg, and their relative sizes are related to the angle between the tripod leg and the ground. Sin 45 = cos 45  = 0.7, so if the tripod legs form an angle of 45 degrees with the ground and the sandbag is being blown so the string is parallel to the tripod leg (some wind), only 70% x 70% = 049% of the wind-loading on the sandbag is tipping the tripod over.  In a more realistic scenario, where the sandbag is only displaced a few degrees, a much smaller proportion of the wind-loading on the sandbag makes a tipping force; eg, if the string is displaced 5 degrees and the tripod leg is at 45 degrees to the ground, the tipping force is 9% x 70% = 6% of the wind loading on the sandbag.     

[Erik Lund]

Thinking out of the box,,, This is quite a stable setup 


26728876092_1e78dcbfe4_o by Erik Gunst Lund, on Flickr


L1043808 by Erik Gunst Lund, on Flickr


_EGL2762 by Erik Gunst Lund, on Flickr


Be aware of how and where you attach all of the elements of a 'tripod'


Camera and or lens
Lens Plate and or rail
Clamp
Head
Tripod top base
Legs, upper and lower
Feet
Ground


All of these parts and interfaces between these are more or less equally important for stiffness and vibrations.


Consider if it is possible to leave out parts. For convenience a dovetail system is nice but not always cleaver or absolutely needed.


How 'well' is the interface between the leg sections designed.