Image intensifiers are built to amplify existing ambient light provided by your chosen objective lens.
Image intensifiers have nothing to do with focal length or field of view.
Regardless of whether you are monitoring a target a mile or an inch away, the exit pupil of your objective lens provides the intensifier with a small amount of light in the form of an image. This light image is intensified and relayed to your CCD camera. The tube is comprised of four major parts. The photocathode, the microchannel plate, the screen, and the body that holds them all together. As photons strike the active area of the photocathode, electrons are produced on the interior of the tube.