This mode is great for getting high quality stills of something you might see on your video, without having to take a messy frame grab from the video file. The quality images you'll capture more than makes up for the slight increase in card space required. You can even customise the photo capture interval (ie take a photo every 1, 2, 5 seconds etc).
GoPro 3 and 3+ users: select this mode from the "Capture Settings" menu.
GoPro 4 users: select this mode using the side settings button whilst in Video mode.
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