Nikon Z 8 Reference Manual page 230

Tip: "Side-by-Side Comparison"
Choose [Side-by-side comparison] to compare retouched copies with the unretouched originals.
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Options used to create copy
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Source picture
The source picture is displayed on the left, the retouched copy on the right.
The options used to create the copy listed at the top of the display.
Press 4 or 2 to switch between the source picture and the retouched copy.
If the copy is an overlay created from multiple source pictures, press 1 or 3 to view the other
pictures.
If the source has been copied multiple times, press 1 or 3 to view the other copies.
To view the highlighted picture full screen, press and hold the X button.
Press J to return to playback with the highlighted picture displayed full frame.
To exit to playback, press the K button.
The source picture will not be displayed if the copy was created from a photograph that is now
protected.
The source picture will not be displayed if the copy was created from a photograph that has since
been deleted.
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Retouched copy
The i Button (Playback Mode)