Abstract: There is disclosed an afocal zoom lens attachment of variable magnification which is intended for use with a prime imaging lens of fixed focal length and which is particularly suited for systems using radiation having a wavelength in the infrared region of the spectrum. The attachable zoom lens system is comprised of a fixed focus front objective element and a fixed focus eyepiece with a zooming doublet and a zooming singlet therebetween and is configured to form a mechanically compensated, non-Galilean afocal zoom attachment. The system includes means for continuously varying the magnification by simultaneously moving the zooming singlet and doublet along linear and non-linear paths respectively. The doublet contains a color correcting element. A real intermediate aerial image is formed between the zooming singlet and the eyepiece at low magnifications and that image is recollimated by the eyepiece.