Cinematic mode applies a depth-of-field effect that keeps the subject of your video sharp while creating a beautifully blurred foreground and background. iPhone automatically identifies the subject of the video and keeps it in focus throughout the recording; if a new subject is identified, iPhone automatically transitions the point of focus. You can also manually adjust the point of focus while you record, or change it later in the Photos app.
Cinematic mode is available on , then drag the slider left or right to adjust the depth-of-field effect.
On Change the camera’s video recording settings on iPhone