When Apple introduced 60 fps video recording on the iPhone 6 and iPhone 6 Plus, it helpfully added a setting in iOS 8 to toggle it on and off. With the higher resolutions now offered by the iPhone 6s and iPhone 6s Plus, iOS 9 expands these settings to provide even further control over both the recording resolution and frame rates.
You’ll find these in the Photos & Camera section of your iOS Settings app, with separate options for standard video and slo-mo video. What you can do with these will depend on your iPhone model — iPhone 6s and/6s Plus users will get resolutions and frame rates up to 4K at 30fps or 1080p at 60fps for normal video and 1080p at 120 fps for slo-mo video, while the iPhone 6 and 6 Plus will cap out at 1080p at 60fps for normal and 720p at either 120fps or 240fps for slo-mo.
These settings are particularly handy if you’re using a lower-capacity iPhone and looking to save on storage space for your videos — you’ll even see a helpful summary below the settings as to how much storage each setting will consume for a minute of video, and Apple’s choice to label the lowest-quality option as “space saver” clearly suggests that’s the main purpose of the setting.