Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 386)

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You can either select and erase unnecessary images one by one or
erase them in one batch. Protected images (p.358) will not be erased.
1
Select the image to be erased.
Press the < x > button to play back
images.
Turn the < 5 > dial to select the
image to be erased.
2
Press the < L > button.
X The Erase menu will appear.
3
Erase the image.
Select [ Erase ], then press < 0 >. The
image displayed will be erased.
L
Erasing Images
Erasing a Single Image
Once an image is erased, it cannot be recovered. Make sure you no
longer need the image before erasing it. To prevent important images
from being erased accidentally, protect them. Erasing a RAW+JPEG
image will erase both the RAW and JPEG images.
Setting [ 8 7: Default Erase option ] to [ [Erase] selected ] makes it faster to
erase images (p.443).