SIGMA DP3 Merrill - Manual (Page 97)

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CAUTION !!

Locking does not prevent files from being erased when the card is
formatted. Review card contents carefully before formatting.
TIP

Locked files have a “read-only” status when viewed on a computer.
TO LOCK A SINGLE FILE
1
Display the file that you wish to lock.
2
Press the button and open [ Playback Menu] (P.28) to
select [Lock]. ( [Lock] in the sub-menu is already selected.)
3
Press the button or button.
 A key icon will appear in the file
indicating that the file is locked.
TIP

To unlock a locked file, select the locked file and follow the same steps
as above.