QML ImageCapture preview property does nothing?

import QtQuick
import QtQuick.Window
import QtMultimedia
import QtQuick.Controls

Window {
    width: 640
    height: 720
    visible: true
    CaptureSession {
        imageCapture : ImageCapture {
            id: imageCapture
            onImageCaptured: function( id, preview ){
                console.log( "Image captured: >" + preview + "<" )
            }
        }
        camera: Camera {
            id: camera
        }

        videoOutput: videoOutput
    }
    VideoOutput {
        id: videoOutput
        anchors.top: parent.top
        anchors.left: parent.left
        anchors.right: parent.right
        height: parent.height / 2
    }
    Image {
        id: photoPreview
        anchors.top: videoOutput.bottom
        anchors.bottom: parent.bottom
        anchors.left: parent.left
        anchors.right: parent.right
        source: imageCapture.preview // always shows the last captured image
    }
    Button{
        text: "Capture"
        anchors.left: parent.left
        anchors.bottom: parent.bottom
        onClicked: imageCapture.capture()
    }
    Button{
        text: "Check whats in imageCapture.preview"
        anchors.right: parent.right
        anchors.bottom: parent.bottom
        onClicked: console.log( "imageCapture.preview >" + imageCapture.preview + "<" )
    }

    Component.onCompleted: camera.start()
}