The 1377M is larger and heavier than the 1277M (2.0 lbs as opposed to 1.3 lbs). It will hold a bit more weight than the 1277M, and it allows a bit more range of motion (when not using the slot). The 1377M has a locking knob for the ball and socket joint, and a separate tensioning knob. The tensioning knob is slightly smaller and marked “Friction”. If you wish to put a small amount of tension on the ball and socket joint, the tensioning knob will provide slightly smoother friction than the locking knob.
Distance between the center of the ball and the plate-support surface of the clamp is 2.04”.
• Weight Capacity (Ball Head Alone):
It will hold 500mm f/4.0 lenses fine under most circumstances, but if you throw the entire rig over your shoulder and walk around, you have to tighten the ball forcefully to keep it from slipping, and even then it may slip a bit. If you are moving the tripod around a lot (e.g. quickly moving the tripod to get a different view of a subject) you must firmly tighten the ball before moving, or it may creep.
Lenses heavier than a 500mm f/4 can be held by the 1377M for static situations, but if you are moving the tripod around or walking around with your rig over your shoulder, there is a high probability that the ball will slip.
• Using the Sidekick with the 1377M:
The Sidekick works beautifully with the 1377M head, but the same weight concerns above apply when using the Sidekick. Larger lenses (300mm f/2.8 and larger) will feel weightless and fluid on the 1377M/Sidekick combo, but you must be sure to tighten the ball and socket joint very securely to make sure that the Sidekick does not creep and become misaligned when you move the tripod. If you are using the Sidekick with the ball head, tighten the ball-locking knob as tight as you can and leave the friction knob totally loose. This gives smooth panning with little play in the mechanism.
If you desire smooth tension control, use the friction control knob,
not the tightening knob.