Why do you focus both the front (Objective lens) and Eye-piece (Ocular)?
The short answer is that you don't see through the night vision device, rather you see the phosphor screen on the back of the tube. The objective lens focuses the image on to the front of the intensifier tube. That image is then converted in to electron energy. It paints that picture on the back of the tube. You need to have a sharp image to start with. Then your eye needs to be focused onto the back of the tube where the image resides. Every eye is different and therefore the ocular focus is very important. Focus the ocular first and then move to the objective lens. You may have to adjust each lens several times before getting a sharp image.