The Tripod Adapter for iPhone 4 from Gary Fong is a bracket that slides on to an iPhone and mounts directly on to a tripod. It must be noted that the fit it is based on tension, so Gary Fong does not recommend aggressively shaking or subjecting the stand to sustained movement. However, if you're using it in conjunction with their optional Flip-Cage to stabilize vertical video, it's becomes a perfect combination.
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Gary Fong Tripod Adapter for iPhone 4