Nikon Z8 User Manual page 275

The option currently selected is shown by an icon in the display.
If [ON] is chosen for [Image area] > [DX crop alert] in the video
recording menu, an image-area icon will flash in the shooting
display when the DX or 2.3× crop is selected.
The sizes of the different crops are shown below.
FX-based video format
DX-based video format
DX-based video format is selected automatically when a DX lens is attached.
When 7680 × 4320 is selected for frame size, image area is fixed at [FX]. The frame size changes to
3840 × 2160 when a DX lens is attached.
Selecting [ON] for [Electronic VR] in the video recording menu reduces the size of the crop.
Format
2.3×
Approx. 35.9 × 20.2 mm
Approx. 23.5 × 13.2 mm
Approx. 16.7 × 9.4 mm
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Size
Video Image Area Options