HTC One M8 User Manual page 118

118 Camera
Post-processing effects for Duo Camera may not work if your
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subject and background are too close or when you're taking a
close-up shot. So, mind the distance between your subject and your
background, as well as the distance between the camera and your
subject.
1 m ~ 2 m
Still life photography looks great with Dimension Plus
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processing. If you'll use Dimension Plus, however, you can arrange
your subject and background to be closer together and the
distance between your camera and subject can be at least 20
centimeters.
You can't apply Duo Effects to burst shots or zoomed-in photos.
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