Recently, Scott Bourne reviewed TriggerTrap (an app and cable combination that gives you a lot of custom control over your camera). I’ve since become huge fan and have made it a huge part of my time-lapse and HDR workflow.
A new update just dropped last night that adds support for using a motion sensor. You can now trigger your DSLR or DILC camera using the internal camera on your iOS device (Android version should be out real soon). This allows you to set up a shot, specify the number of faces that need to be detected, as well as ad any delays. Once you push the red button, your IOS camera stays on and can keep tracking the scene and firing shots. This mode joins other useful sensor modes like Sound Sensor and Vibration Sensor.
Triggertrap Mobile 2.2 for iOS is available in the App Store now.