Nikon D5 - Manual (Page 91)

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The Movie Crop
Movies have an aspect ratio of 16 : 9 and are recorded using a crop
that varies with the frame size.
Going from a frame size of 1920 × 1080 or 1280 × 720 to 3840 ×
2160 increases the apparent focal length by approximately 1.5×,
while going from 1920 × 1080 or 1280 × 720 to 1920 × 1080 crop
increases the apparent focal length by approximately 3Ч.
3840 Ч 2160 1920 Ч 1080 and
1280 Ч 720
1920 Ч 1080 crop