Why Center Columns Reduce Tripod Stability

The center column has long been a ubiquitous feature on tripods.  It provides a number of major advantages in terms of versatility and ergonomics over tripods that lack one, but, has also been known to reduces the tripod’s stability.  Unfortunately, there has been little in depth discussion on why this is or in what scenarios a center column should be used or avoided.  I have seen claims across the spectrum from center columns from brand X are so good that they don’t reduce stability at all, to assertions that the mere presence of a center column will ruin a tripod’s ability to competently hold camera equipment.  The truth of course lies somewhere in between.  In this post we are going to break down exactly why center columns reduce stability hopefully help inform users on when they might want to avoid using one.

A center column reduces the stability of a tripod in four principal ways.  1) The lock mechanism that secures the center column isn’t as stiff as solid metal, which is what the apex would be sans center column.  2) The center column itself has some flex to it.  3)  Raising the center column moves the camera away from the effective rotation point, increasing its moment of inertia.  4)  The center column acts as a lever arm, increasing the torques that the tripod must resist from forces on the camera.  We will discuss each of these in depth below.  First though, I want to quickly remind ourselves of the benefits that a center column provides to head off the crucifixion I would otherwise receive in the comments if I didn’t.

Center columns are a common feature on tripods, and for good reason.  The center column provides additional height to a tripod while adding relatively little weight, and it allows the user to quickly and accurately adjust the height of the camera.  To reach a particular height with the tripod legs alone, you need a total length of tubing that is roughly three times height plus ~10% to account for the legs being at an angle to vertical.  The center column by contrast adds height in direct proportion to its length with only a single tube.  From a height to weight perspective, the center column is clearly more efficient.  The center column can also nest between the tripod legs, adding very little (or in some cases, nothing) to the overall folded length of the tripod.  Combined, these factors allow for a tripod that has a compact, lightweight form factor and a reasonable maximum working height.  The popularity of a center column on travel tripods is rather understandable in this context.

For users of larger studio tripods, it is the fast height adjustment of center columns that provides the most compelling benefit.  When precision speed and precision are required in setting the height of the camera, a center column is invaluable.  If you have ever tried to get a tripod to a specific height, and somewhat level, you know how tedious the experience of adjusting each individual leg can be.  It isn’t particularly difficult, but can take up valuable time that you would rather spend focusing on actual photography.  For many users, this isn’t just a convenience, but a critical aspect of their workflow.  As I present the reasons why center columns reduce stability below, keep in mind that they do not invalidate these ergonomic benefits.  Like basically everything else in photography, there is just a tradeoff.

Center Column Lock Mechanism

The first, simplest, and least significant reduction in stiffness comes from the locking mechanism that secures the center column to the apex of the tripod.  These mechanisms come in a variety of designs, with the most common being the twist style compression clamp ubiquitous on tripods.  These are very secure, and operate by forcing a wedge shaped piece of plastic between the center column and the rest of the apex.  This piece of plastic has some small amount of flex to it that is greater than the solid aluminum or magnesium alloy of the apex.  The apex -> plastic wedge -> center column interfaces also create the opportunity for some small amount of slip that would not exist if there were no center column.

Most tripods do not allow for the swapping of a center column and its locking mechanism into an otherwise identical apex.  Systematic style tripods are an exception to this, where the top plate can be replaced with a variety of accessories, such as a center column.  The only one these tripods and replaceable center columns that I have on hand is the Really Right Stuff TVC-23.  The table below shows the stiffness of the tripod with and without the center column installed:

No Center ColumnWith Center Column
Yaw Stiffness1287 Nm/rad1133 Nm/rad
Pitch Stiffness3943 Nm/rad3145 Nm/rad
Mean Stiffness1941 Nm/rad1666 Nm/rad

So we see a roughly 15% loss in stiffness just from adding the center column.  This is not a particularly significant difference in real world use.  If you take advantage of the aforementioned ergonomic benefits of a center column, this will be completely negligible.  If however, you are trying to maximize the stiffness to weight ratio of your tripod, it is significant.  While the loss in stiffness and increase in weight (225g for the RRS 2-series center column) are both modest, together they make for a significant difference in the tripods ‘score‘.  You can see why tripods with a center column are at an inherent disadvantage in the simple quantitative rankings on this site.

Center Column Stiffness

The center column itself has some flex to it.  Any time another component is placed into your stability system (the full stack from ground to camera), it reduces the stiffness of the system.  The flex in each component adds together (the stiffnesses add in reciprocal) to contribute to the overall flex, and nothing is infinitely stiff.  So even though the center column is typically stiffer than the tripod legs, its addition can only reduce overall stiffness.  How much loss in stiffness you get varies among tripods, but in general I have found that 40% loss in stiffness is typical when the

RRS TVC-23 stiffness vs. center column extension
Ascend-14 Stiffness vs. Height of the center column.

The RRS TVC-23 tripod loses very little of its overall stiffness as the center column is raised.  This is thanks to a rather fat aluminum center column which performs better than carbon fiber in this application.   With can conclude that even at full extension, the center column is significantly stiffer than the legs, which provide most of the flex in the system.  This shouldn’t be a great surprise as the legs are much longer than the column, and intuitively we know that bending a long stick is much easier than bending a short one.  The loss in stiffness observed here, alone, isn’t enough to discourage one from using a center column.

Looking at the RRS Ascend-14, and we find a somewhat different story.  The Ascend loses about half of its stiffness as the column is extended to full height indicating that the column is about as stiff as the legs (Two equally stiff pieces in series result in the whole being half as stiff as the individuals).  The drop off in yaw stiffness is more dramatic than we saw in the TVC-23 and results from the use of a ‘Y’ shaped center column as opposed to the traditional round ‘O’ cross section on the larger tripod.  It turns out that the torsional stiffness of a beam is more closely related to its cross sectional area than its diameter.  The ‘I’ beams used in construction for example have good bending stiffness, but poor torsional stiffness.  A tube (‘O’ cross section) will have both good torsional and bending stiffness and is used in applications such as a driveshaft, where torsional stability is paramount.

Overall though, the stiffness seen even on the center column of the Ascend isn’t bad and wouldn’t present an issue in most everyday shooting.  Generally we would be very happy to have a tripod with the option for more height when conditions permit, and a more stable lower configuration for when they do not.  Unfortunately, this is not the end of our discussion and we will find that even an infinitely stiff column would still present stability problems as it alters the geometry used to keep the camera stable.

Camera Moment of Inertia

Moment of Inertia (MOI) is the rotational equivalent of mass (the weight of the camera) and the metric that quantifies the effective load on the tripod.  MOI depends not only on the mass, but also the distance that mass is from the center of rotation.  For tripods, the center of rotation occurs at the apex and the entire purpose of the tripod is to minimize any rotational vibrations that would result in a soft image.  Previously when we were looking at the moment of inertia for cameras with an attached lens, we only looked at the MOI in the yaw direction.  Unlike mass, the moment of inertia differs based on the direction the camera is rotating.  When the camera is placed at the center of rotation, the pitch and yaw MOIs can be treated as approximately the same as they will be dominated by the lens that sticks out in front of the camera.  Lenses are basically cylindrical and thus exhibit symmetry with respect to yaw and pitch rotations.  As the weight is centered about the axis of the lens, the roll MOI will be negligible compared to the other two.

The center of rotation for a tripod is the point in space in which a set of imaginary lines through the center of each leg intersect.  Most of the time, the camera will be placed close to this point so the above discussion applies.  If however, the camera is raised above this point via a center column, we will have to calculate the effective MOI of the camera that the tripod needs to stabilize in a different way.  Fortunately, a simple solution is available in what is known as the parallel axis theorem.   When the camera is raised a distance z above the center of rotation of the tripod, the MOI (I_{total}) is given by

I_{total} = I_{CM} + Mz^{2}

where I_{CM} is the MOI of the camera about the center of mass and M is the mass of the camera (and ballhead).  As the center column is raised, the second term very quickly begins to dominate I_{total} and the MOI increases as the square of the height of the column.  We thus find the that effective load on the tripod increases dramatically as well.  This is the non-intuitive aspect of dealing with rotational motion.  Even thought the weight of the camera hasn’t changed, we alter the dynamics of the system simply by changing the placement of that weight.

To demonstrate this effect, I placed my Fuji GFX50S and 32-64mm (~ 24-70 f/2.8 FF equivalent in size and weight) atop a Benro TMA27A tripod.  This tripod has a very strong aluminum center column paired with aluminum legs that are relatively weak, especially compared to the RRS TVC-23 tested above.  The flex of the center column is therefore negligible.  I then measured the decay of vibrations with the center column in three positions:  fully retracted, extended halfway, and fully extended.  The results are below.

We see a dramatic difference in how long it takes vibrations to die out between the three column positions.  When the column is fully extended, vibrations decay four times slower than they do in the fully retracted position.  The vibration decay time is given by I/C, where C is the damping constant.  Since the MOI I increases with center column height, it then makes sense that we see the decay time correspondingly increase.  Note that the frequency of the oscillation also declines dramatically as the center column is raised, much more so than could be explained by the small loss in stiffness alone.  The frequency is also affected by the change in MOI (\omega = \sqrt{\kappa/I}).

Unfortunately the increase in MOI is both the most important and least intuitive reason for the loss in stability when a center column is raised.  It is not that the tripod itself lacks rigidity, it’s that the effective load the tripod needs to stabilize increases despite the weight of the camera remaining constant.  The heavier the gear used, the more this will become a problem.  We conclude then that center columns work best when paired with smaller and lighter gear.  This is an interesting result as previously we had argued that the camera’s MOI is the important metric for load on a tripod.  While this is true for gear placed at the apex, at the top of a center column, the mass becomes the more important figure.  This may create an avenue for a simple weight rating for tripods based quantitatively on their performance, at least for the ones with center columns, but that is beyond the scope of this post.

Increased Torque

Raising the camera above the center of rotation at the apex not only increases the MOI, it causes the center column to act as a lever arm, magnifying the effect of any forces on the camera.  Tripods are rotational systems and so are moved by torques \tau about their centers of rotation.  Torque is the rotational equivalent of force given by \tau = F \times r, where r is the distance from the center of rotation.  Here, r is the amount the center column has been raised so even assuming constant forces from wind or a finger pressing a button on the back of the camera, the torque will be increased as the column is raised.  This is a slight simplification of the problem, but illustrates the basic idea which really isn’t more complicated than using a lever for mechanical advantage.  For a system we are trying to keep as still as possible though, it becomes a big disadvantage.

If you are shooting in environment with very few forces acting on the camera, say tethered in a studio, this effect won’t be important to you.  If you are shooting in the high winds of Iceland or Patagonia, the camera is going to act like a sail on top of a center column and it will be impossible to get a sharp image.  The effect of this increased torque will stack with the increased MOI of the camera.  Say you are pushing a button on the back of the camera.  This torque will cause a greater amplitude of vibration which will die off at a slower rate as the column is raised.  It won’t take a whole lot of column extension when using a heavy camera to go from feeling pretty stable to completely unusable.  If using a small lightweight camera though, the effects of the increased torque may be manageable as the damping rate will still be fast enough for a reasonable workflow.  As demonstrated in the vibration decays above though, even the modest Fuji GFX50S & 32-64 takes about five seconds to come to rest with the center column fully extended on the Benro, too long for most photographers to consider acceptable.

Summary

The most important reasons the a center column reduces the effective stability of a tripod are the increased torques and MOI about the center of rotation as the camera is moved away from that center.  These have nothing to do with the rigidity of the column itself but instead the changing geometry of the system.  Extra weight will affect the damping rate through its greater MOI while the forces such wind cause greater displacement of the camera through the mechanical advantage of a raised column.  If one is working with either a heavy camera or in a windy environment, a center column can typically still be tolerated.  The combination of both though will make even the stiffest center column unusable.

The reduction in stiffness from adding the column is still important though.  The arguments we made regarding torque and MOI apply primarily to pitch vibrations, but yaw vibrations will still be magnified from the loss in stiffness.  Anyone looking to extract the maximum stability from their tripod should best avoid a center column.

We can now better understand when a center column is a useful tool, and when it is not.  In the studio where there are few external forces on the camera and one is using heavy and stable set of legs, a center column can be invaluable for its fast and precise height adjustments.  On the opposite end of the spectrum, a photographer hiking in the mountains will likely experience adverse conditions and will try to maximize the stiffness/weight ratio of their tripod.  For them, a center column wouldn’t be usable even if the tripod had one, so there is no point in hauling around the extra weight.  In between lay an infinite array of different scenarios and use cases.  A traveler might want a compact tripod for selfies with a small camera making a center column very useful.  Another might be photographing wildlife with a long telephoto lens, in which case it wouldn’t be.  Most photographers have already figured out if a center column works for them or not already, so I hope this was at least helpful for understanding the underlying physics.  If not, I hope it made your decision easier, albeit at the cost reading a rather dense blog post.  As for exactly how much weight constitutes ‘heavy’ and how much wind constitutes ‘adverse’, that depends on the stiffness of the underlying tripod legs and will have to be the subject of a future inquiry.

 

7 thoughts on “Why Center Columns Reduce Tripod Stability

  1. Using a remote shutter release or self-timer shutter release should somewhat diminish camera motion, but does
    it help significantly?

    1. Yes, it will certainly help if the forces on the camera are from pushing the shutter button. If they are from anything external, wind, etc, then no.

  2. Really nice, especially with the real-world data! You’ve
    got any tips on how to acquire similar data for
    yourself? E.g. to compare the extension of the legs,
    how widely the legs are set, …
    Because this could be especially in Astro-
    photography where the height usually doesn’t matter
    but stability is key.

    1. See the methodology section on this site, as well as the articles on stiffness vs height, and stiffness vs leg angle. Easiest way at home would be to attach an accelerometer with a continuous and recordable readout to your setup.

    1. Yeah, there are a number of tripods out there with such a design from RRS, Feisol, Leofoto, to name few. Most larger tripods have this feature as well.

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