Nikon D3400 Triple Lens Parent's Kit Owner's Manual

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This mode is recommended for snapshots or whenever you 
want to leave the camera in charge of shutter speed and 
aperture. The camera automatically adjusts shutter speed and 
aperture for optimal exposure in most situations.

In shutter-priority auto, you choose the 
shutter speed while the camera 
automatically selects the aperture that 
will produce the optimal exposure. 
Rotate the command dial to choose the 
desired shutter speed: rotate right for 
faster speeds, left for slower speeds.