Stop the spin!
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I have had a few people send emails and Facebook messages asking about ways to prevent a lamp head from rotating on a boom arm when a heavier modifier is on the lamp head. I have a variety of boom arms in my studio, but mostly use the Avenger D600CB arm**, sometimes called the Avenger Mini Boom (if you are in the Seattle area you can also get it at Glazer's Camera). It is very sturdy and has large, easy to use locking knobs. But I have found that some of my lamp heads, especially the Einstein strobe, don't lock down quite tight enough (more likely an issue with the mounting hardware in the lamp than an issue with the boom) and start rotating on the boom when used with heavier modifiers (large softbox or octa, etc.)
The solution that I have found (and which is shared by my friend Joel Grimes, as I noticed the same modifications on his boom arm during his CreativeLive classes** that I assisted him in) is to grind down a flat spot on the spud at the end of the boom arm. I did my modification using a Dremel Moto-Tool** with a heavy-duty cut-off wheel**. Remember to wear proper eye protection when using power tools! If you are not comfortable with power tools, please ask a friend who is to help you out or take the boom arm to a local machine shop.
Here you can see a short video of the procedure and a photo of what the spud looks like after being filed down. I extended the filing down the stud to indicate where the flat spot is when mounting a lamp head to the boom arm. When you mount your lamp head onto the boom arm, simply align the mounting screw with the flat spot on the spud and tighten down normally (no need to use pliers or risk breaking the screw).
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