optical systems for endoscopes , borescopes , dental scopes , and the like which are characterized by having three groups of lenses of positive optical power and an external entrance pupil . typically , all three groups of lenses are displaced from the pupil and focal planes . as a consequence , the displaced groups take part in the image transfer as well as in the pupil transfer . the optical power requirements can thus be shifted from one group to another , distributing as well as reducing the overall power requirement . moreover , the aberration correction can also be shared between these groups . the first group , which conventionally has the highest optical power , and consequently a large amount of aberrations to be corrected , can in this way transfer some of the optical aberration correction to the other groups . the sharing of the optical functions and aberration correction results in a fully integrated optical system . the reduction in the total amount of optical power is so large that a line - of - sight deviating prism can be readily accommodated between the entrance pupil and the first lens group . the resulting simplicity of the optical system makes it suitable as a disposable item .