Nikon Df - Manual (Page 308)

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Retouching Copies
Most options can be applied to copies created using other retouch
options, although (with the exception of Image overlay) each option
can be applied only once (note that multiple edits may result in loss of
detail). Options that can not be applied to the current image are grayed
out and unavailable.
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Image Quality
Except in the case of copies created with Trim, Image overlay, NEF
(RAW) processing, and Resize, copies created from JPEG images are
the same size and quality as the original, while copies created from NEF
(RAW) photos are saved as large fine-quality JPEG images, and copies
created from TIFF (RGB) photos are saved as fine-quality JPEG images of
the same size as the original. Size-priority compression is used when
copies are saved in JPEG format.