Sony SLT-A77VM - Manual (Page 166)

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Deleting images (Delete)
You can delete all still images or movies by the same view mode currently
displayed.
Deleting all the images or movies in the same view mode
1
MENU button t 1 t [Delete] t [All in Folder] or [All
AVCHD view files]
2
Select [Delete] with v on the multi-selector, then press the
center of the multi-selector.