How to Record 4K Video on the iPhone 6s
With each iteration of the iPhone the camera just gets better and better. With the new iPhone 6S you can record in 4K video despite the Apple TV doesn't support it yet. Although the ability is there it is not enabled by default so you have to go hunting around in the settings to enable 4K video recording. Be warned, this isn't for those of you who have the 16 GB model. In its default mode the iPhone 6S records video at 1080p resolution at 30 frames per second. This is a good D4 option as it gives you the best quality and frames per second without file sizes going completely crazy.
To enable 4k recording you have to drop into the settings app rather than the iOS camera app itself.
To change your video recording mode, head to Settings > Photos & Camera > Record Video
there are currently four video recording modes available for the iPhone 6s and iPhone 6s Plus in iOS 9: 720p at 30fps 1080p at 30fps 1080p at 60fps 4K at 30fps
Apple offers some approximate file size guidance for each format at the bottom of the list, with one minute of recorded video requiring about 60MB on the low side (720p at 30fps) all the way up to 375MB for 4K.
As with any recording it's always best to record at the highest possible, best quality you can achieve. when it comes to editing and making any sort of colour correction you will certainly appreciate the fact you recorded in the best resolution possible on a handheld device.