Abstract: A single focal length lens system has a first negative lens element and a last negative lens element. The first negative lens element is disposed at an object side end of the lens system and the object side surface thereof is concave. The last negative lens element is disposed at an image side end of the lens system. The lens system fulfills the following condition: ##EQU1## wherein .phi.f represents the refractive power of the first negative lens element; and .phi.1 represents the refractive power of the last negative lens element.

Abstract: The present invention relates to a retrofocus type of objective lens system for endoscopes which has a wide field angle, a short total length and a short outer diameter, comprises a limited number of lenses and is well corrected for various aberrations. This objective lens for endscopes comprises a first lens component L.sub.1 having a negative refractive power, a second lens component L.sub.2 having a positive refractive power, an aperture stop S positioned just in the rear of the second lens component L.sub.2 and a third lens component that is a cemented lens obtained by cementing together a positive lens and a negative lens.

Abstract: A vari-focal lens system including, in the order from the object side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, an aperture stop, a third lens unit having a positive refractive power and a fourth lens unit having a negative refractive power; and using at least one radial GRIN lens element on each side of the aperture stop. This vari-focal lens system has only a very small number of lens elements and had favorably corrected aberrations.