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. All night vision devices need some available light to work. It is possible to see a bright image in low light or no light conditions with the use of an Infrared Illuminator. An Infrared Illuminator is standard on all Bushnell Night vision devices.