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Apple added a number of new features to the camera in iOS 5, as more and more users drop their point-and-shoot digital cameras in favor of the increasingly powerful cameras built-in to smartphones.
For users looking to take pictures quickly, when the phone is locked, simply press the home button twice -- then press the camera icon that appears to be taken directly to the camera. This works even when the phone is locked with a passcode.
Apple's engineers also added a feature that caused Camera+, a popular camera app, to be banned from the App Store for four months. Users of Apple's standard camera application can press the volume-up button to take a snapshot, rather than pressing the shutter button on the touchscreen. This makes the iPhone more like a standard camera, with the shutter release on the top.
Finally, as a corollary to the volume up shutter release, users can also plug in headphones with headphone controls -- like the headphones that are included with the iPhone -- and use the volume up on the headphones as a remote trigger for the camera.
Top Rated Comments
As opposed to an iPhone propped up without headphones recording 1080p video?
So that's how they got so close to the squirrel.
The feature isn't exclusive to Apple's Camera app. It's part of the API now. If the developer uses the standard UIImagePickerController, the volume button takes a photo.
Some apps like "ShakeItPhoto" now also have volume button support under iOS 5, but I'd guess it just got the feature "automatically" since it's using an unmodified UIImagePickerController.
I'm not sure how Instagram was built, but it may not be using the standard UIImagePickerController.