How to focus at point when i programly take a photo in windows phone 7.1

#region Assembly Microsoft.Phone.dll, v2.0.50727
// C:\Program Files\Reference Assemblies\Microsoft\Framework\Silverlight\v4.0\Profile\WindowsPhone71\Microsoft.Phone.dll
#endregion

using System;
using System.Collections.Generic;
using System.Security;

namespace Microsoft.Devices
{
    // Summary:
    //     Class that represents the basic camera functionality for a Windows Phone
    //     still camera application. Includes members for enabling and configuring functionality
    //     such as image capture, focus and flash mode. Also, it contains the primary
    //     focus events, image acquired event, and image saved event.
    [SecuritySafeCritical]
    public sealed class PhotoCamera : Camera
    {
        // Summary:
        //     Initializes a new instance of the Microsoft.Devices.PhotoCamera class.
        public PhotoCamera();

        // Summary:
        //     Returns a collection of the available preview buffer resolutions.
        //
        // Returns:
        //     Returns System.Collections.Generic.IEnumerable<t>. Collection of available
        //     buffer preview resolutions.
        public IEnumerable<captureresolution> AvailablePreviewBufferResolutions { get; internal set; }
        //
        // Summary:
        //     Gets or sets the flash mode. Unsupported flash modes silently default to
        //     "off".
        //
        // Returns:
        //     Returns Microsoft.Devices.FlashMode. The flash mode.
        public FlashMode FlashMode { get; set; }
        //
        // Summary:
        //     Gets or sets the preview buffer resolution.
        //
        // Returns:
        //     Returns Microsoft.Devices.CaptureResolution. The resolution of the camera
        //     image capture.
        public CaptureResolution PreviewBufferResolution { get; set; }
        //
        // Summary:
        //     Represents the current YCrCb plane layout for GetPreviewBuffer.
        //
        // Returns:
        //     Returns Microsoft.Devices.YCrCbPixelLayout. Pixel layout.
        public YCrCbPixelLayout YCrCbPixelLayout { get; }

        // Summary:
        //     Provides data for the Microsoft.Devices.PhotoCamera.AutoFocusCompleted event.
        public event EventHandler<cameraoperationcompletedeventargs> AutoFocusCompleted;
        //
        // Summary:
        //     This event occurs when the capture sequence is complete.
        public event EventHandler<cameraoperationcompletedeventargs> CaptureCompleted;
        //
        // Summary:
        //     This event is occurs when the capture sequence is complete and an image is
        //     available.
        public event EventHandler<contentreadyeventargs> CaptureImageAvailable;
        //
        // Summary:
        //     This event is occurs when the capture sequence has started.
        public event EventHandler CaptureStarted;
        //
        // Summary:
        //     This event occurs when the capture sequence is complete and a thumbnail image
        //     is available.
        public event EventHandler<contentreadyeventargs> CaptureThumbnailAvailable;

        // Summary:
        //     Cancels the current focus operation.
        public void CancelFocus();
        //
        // Summary:
        //     Captures the current image displayed in the viewfinder.
        public void CaptureImage();
        //
        // Summary:
        //     Starts a camera auto focus operation.
        public void Focus();
        //
        // Summary:
        //     Copies the current viewfinder ARGB frame into a buffer for further manipulation.
        //
        // Parameters:
        //   pixelData:
        //     The ARGB pixel data.
        public void GetPreviewBufferArgb32(int[] pixelData);
        //
        // Summary:
        //     Copies the luminance data for the current viewfinder frame into a buffer
        //     for further manipulation.
        //
        // Parameters:
        //   pixelData:
        //     The YUV pixel data.
        public void GetPreviewBufferY(byte[] pixelData);
        //
        // Summary:
        //     Copies the current viewfinder frame into a buffer for further manipulation.
        //
        // Parameters:
        //   pixelData:
        //     Pixel data.
        public void GetPreviewBufferYCrCb(byte[] pixelData);
        //
        // Summary:
        //     Determines whether a particular flash mode is supported on the device.
        //
        // Parameters:
        //   mode:
        //     The flash mode.
        //
        // Returns:
        //     Returns System.Boolean. True or False on whether the mode is supported.
        public bool IsFlashModeSupported(FlashMode mode);
    }
}
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