In this blog we illustrate how to set the zoom-in and zoom-out the focus of camera in windows 8.1 phone app.
Foe zooming a camera we use a buit-in ZoomControl class inside the MediaCapture which controls the zooming of the camera.

For the zooming the camera use the below code:

Code for .Xaml file:

<CaptureElement x:Name="capturePreview" Stretch="UniformToFill" />
<Slider x:Name="FocusValueSlider" Margin="12,0,15,0" Header="Adjust focus:" ValueChanged="FocusValueSlider_ValueChanged" Width="300" FlowDirection="LeftToRight" VerticalAlignment="Bottom" HorizontalAlignment="Center" Slider>

Code for .Xaml.cs file:

MediaCapture _mediaCapture;

// Initialization of the MediaCapture ...

if(this._mediaCapture.VideoDeviceController.ZoomControl.Supported) //Check whether the using device supports a zooming control or not.
{
    FocusValueSlider.Minimum = this.captureManager.VideoDeviceController.ZoomControl.Min;                    //Get the minimum zoom value supported by the capture device.
                    FocusValueSlider.Maximum = this.captureManager.VideoDeviceController.ZoomControl.Max;                    //Get the maximum zoom value supported by the capture device.
                    FocusValueSlider.StepFrequency = this.captureManager.VideoDeviceController.ZoomControl.Step;             //Get the smallest zoom value increment supported by the capture device.
                    FocusValueSlider.Value = ((FocusValueSlider.Minimum + FocusValueSlider.Maximum) / 2) / 2;
}

Code for slider when the its value is changes:

private async void FocusValueSlider_ValueChanged(object sender, Windows.UI.Xaml.Controls.Primitives.RangeBaseValueChangedEventArgs e)
  {
if (this.captureManager.VideoDeviceController.ZoomControl.Supported)           //Check whether zoom control is supported by the capture device.
            {
                    this.captureManager.VideoDeviceController.ZoomControl.Value = (float)e.NewValue;           //sets the zoom value to be used by the capture device.
                }
                else
                {
                    msgDialog = new MessageDialog("This device does not support zoom in/out feature", "Info");
                    msgDialog.Commands.Add(new UICommand("OK"));
                    await msgDialog.ShowAsync();
                    FocusValueSlider.Value = 0;
          }
     }