17 mm in diameter The nominal focal length of the eye is around 17 mm, however it changes with accommodation. The nature of human binocular vision, which employs two lenses rather of a single one, and post-processing by the cortex, differs greatly from the process of creating and producing an image, video, or film. In these cases, magnification is used instead, with the images being projected onto the retina via the lens. This is because humans can only focus on a small area of the visual field at any given time. If we tried to focus on something far away, the image would be too big for the limited space on the retina. Therefore, to see something close up, we need to bend the light from that object using our own optics. The easiest way for us to do this is by adding more glass pieces with different shapes and sizes as required for each task. For example, if you need to read a map closely, you will need to add a magnifying lens to your eye. This could be a regular lens or an optical instrument designed for this purpose. In either case, it works by bending the light from far away into a thin beam, which allows you to see details you couldn't otherwise. A microscope uses further magnification to give an even closer view of objects too small to see with the naked eye.