The purpose of a photography sandbag is to weigh your light stand down to prevent it from being knocked over by your clients or a gust of wind, for example. The sandbags are also critical when you use a boom arm and need a counterweight to balance out your flash light.
A typical sandbag has pockets that you can fill with sand. Generally, it works well, but there are a few issues with that. First, you either need to have access to sand or buy it separately. Second, you cannot adjust the weight of the sandbag. (Technically you can dump some of the sand to make the sandbag lighter, but it's hard to imagine doing that during a photo session, unless you are shooting at a beach.) Lastly, you have to carry the sandbag around, so it can significantly contribute to the total weight of your gear. The Karamy KAC-LB1 Sandbag addresses all of the issues mentioned above in an interesting way.
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