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From Manufacturer
The Vello Battery Grip for Sony a7 III Series can be installed directly into the camera's battery chamber, just remove the door and attach it to the side of the grip's stem. Align the grip and camera contacts inside the chamber to ensure power is being supplied to the camera, and tighten the grip against the body via the thumbscrew in the 1/4"-20 tripod socket.
Once the grip is installed, it accepts two NP-FZ100 rechargeable batteries which doubles the camera's battery life and shooting time. The grip offers a comfortable, extended, rubberized, nonslip grip, and a shutter release button for vertical use. It also provides front and rear dials, C1 and C2 function buttons, AF-ON and AEL buttons, and a Multi-Selector Joystick, so you still maintain full functions of the camera while shooting vertically.
The grip maintains tripod support with the threaded 1/4"-20 socket on the bottom, and a strap mount on the side, so a hand or shoulder strap can be securely attached. Vello recommends aligning the mounting stud to the camera's socket, to prevent the camera's threads from stripping when mounting the grip.