title: object dimensions in image according to camera distance faraway
body: <p>I have a camera placed 10 meters faraway from a portrait (rectangle) having width = 50cm and height = 15cm, I want to get the dimensions of this portrait inside the image captured. The image captured has width=800 px and height=600 px. </p>
  
  <p>How can I calculate the dimensions of the portrait inside the image? Any help please?</p>