swift 4 if let statement

Error message says like this. in func prepare Camera's if let part. Initializer for conditional binding must have Optional type, not '[AVCaptureDevice]'

how can I solve this using if let statement? or Is there another solution?

func prepareCamera() {
    captureSession.sessionPreset = AVCaptureSession.Preset.photo

    if let availableDevices = AVCaptureDevice.DiscoverySession(deviceTypes: [ .builtInWideAngleCamera], mediaType: .video, position: .back).devices {
        captureDevice = availableDevices.first
        beginSession()
        }

}

func beginSession () {
    do {
        let captureDeviceInput = try AVCaptureDeviceInput(device: captureDevice)

        captureSession.addInput(captureDeviceInput)

    }catch {
        print(error.localizedDescription)
    }