d ue89e ue89e h 8 where x is the coordinate in the direction of the optical axis and h is the coordinate in the direction perpendicular to the optical axis the direction in which light advances being taken as positive r is the radius of the osculating sphere and k b c and d are the aspheric coefficients also the notation d u2212 0x means 10 31 x fig6 shows the total zooming movement of each of the lens units in the paraxial zone of the numerical example 1 the optical system of the numerical example 1 is of the 2 unit type with the minus plus refractive power arrangement the negative first lens unit as the compensator and the positive second lens unit as the variator are moved in differential relation to vary the focal length as is apparent from fig6 in the numerical example 1 the overall lens length a distance between a lens surface closest to the object side and a lens surface closest to the image side of the zoom lens is shortest in the telephoto end in application of the invention to the optical system of the numerical example 1 it is therefore desirable that the retraction.