Is there a rule of thumb for balancing a boom on light stand?

Question

After a shoot I noticed that one of my booms developed dents on the stem. With chinese quality I am not surprised- however I own a second chinese boom that handled the same weight.

Therefore, are there any recommended physics behind boom loading and length?

eg. 2/3rd of total length ahead of the fulcrum 1/3rd of the total length behind the fulcrum with weights?

Obviously this can be calculated accurately with some physics, however this question has to do with photographers and any existing rules of thumb that would make this quicker and safer work.

Answer 1

Any rule of thumb would be dependent on the physical build of the boom, and I doubt that is standardized enough to give a generic guideline. However, as an R&D engineer who also does photography, I can comment on the mechanics, which might help ...

It comes down to the length of boom you want (from stand to lighting head). As you know, the longer this lever arm is, the heavier your counter weight has to be, by the equation L1·W1=L2·W2. (But the boom sees the force of the two masses, W1 + W2.)

If you seldom or never care much about the boom length of your lighting kit, and you're worried about the materials bending too easily, then a good choice to lighten the load on your boom will come from placing the boom at it's lengthwise midpoint on the stand, and using a counter weight equal in mass to the weight of the lighting head.

An even shorter boom (like 1/3 stand to lights, and 2/3 stand to counter weight) would allow an even lighter counterweight, but you would surely be into diminishing returns -- that is, the boom had better hold up just fine under twice the weight of the lighting head, so going any lighter-still gives you no further structural benefit. Conversely, a longer boom (e.g., 2/3 stand to lights, 1/3 stand to counter-weights) would require a heavier counterweight (twice as heavy in this example), increasing the net load working toward bending the boom.

Still, unless the boom pole is getting close to 4 mil aluminum (like a pop can), it should be okay at a 2:1 position, though. Even a 4:1 position should not be too gross a load (at five times the mass of the lighting head: 1 for the lighting head + 4 for the counterweight).

Trust your instincts. I bet you have gut feel of what's too extreme a boom reach. Say your lighting head weighs 5 lbs. With the boom at a reach of 9:1 (90% of the boom on the light side, and 10% on the counterweight side), the counterweights will weigh 45 lbs, and the net force at the stand, on the boom pole is 50 lbs ... which probably would strike anyone as excessive.

Answer 2

Is it possible that somebody knocked your light stand over or that the dent was caused by impact, not weight? In any case, there obviously cannot be any "rule of thumb". We don't know the materials, size, mass of counterweights or any of the particulars and strength and weight of materials and build quality is highly variable. I'm sure the weight of your lights and other factors also come into play. Anyway, you could calculate the forces acting on the joints and try to minimize them. If the quality is cheap, maybe never move past the mid-point and use a counter-weight equal to the weight of your lighting.

Anyway, the kind of damage it sounds like you're describing would be more likely caused by impact, I'd think. Otherwise it sounds like you are grossly overloading a very cheaply made boom and you should cut way back on your boom extension if using that grade of equipment.

Answer 3

With me, the idea is to use the minimum necessary force to do the job.

I adjust the counter balance to compensate for the weight of the luminary. The light stand is placed at the fulcrum of the light pole (boom) and the friction lock is used to hold the position of the assembly.

Putting more or less puts the stress on the friction head of the light stand with little or no benefit.

Occasionally, the light stand boom is too short for an extension so there is some leeway available; but, there is no set ratio for a "correct" weight distribution, per se.

If you need more extension for the boom you have, get a longer boom and adjust the counterweight accordingly.

There was a "ride" in the playground called a teeter-totter (a see-saw is some places) where a heavier person could sit closer to the fulcrum to balance the load of a lighter person. Use the same principle for your lighting stands, booms, and counterweights.