1. In an interactive virtual reality imaging system, a process for determining length of focus of an eye of a system user for adapting focus of an image generated by the imaging system, wherein said eye has a lens, a cornea and a retina, the process comprising: identifying an optical path to the retina of said eye; projecting a first image along said optical path to the retina of said eye, said first image being focused at a first focal point along said optical path beyond the retina of said eye; detecting said first image from said optical path reflected from the retina of said eye and generating a first signal proportional to an intensity of said first image; projecting a second image along said optical path to the retina of said eye, said second image being focused at a second focal point along said optical path in front of the retina of said eye; detecting said second image from said optical path reflected from the retina of the eye and generating a second signal proportional to an intensity of said second image; comparing said first and second signals for indicating a relative intensity of said first and second images to determine an accommodation of said eye to provide the system user's length of focus; and adjusting the focus of the image to correspond to the user's length of focus.