Nikon D5 - Manual (Page 9)

9
The Movie Crop
The movie crop varies with frame size and, at frame sizes of
1920 × 1080 and 1280 × 720, the option selected for Image
area > Choose image area in the movie shooting menu.
3840 × 2160
(4K UHD)
1920 × 1080 or
1280 × 720,
FX image area 1920 × 1080 crop
1920 × 1080 or
1280 × 720, DX
image area
Footage with a frame size of 3840 × 2160 (4K UHD) can be
filmed with DX lenses.