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. During zooming to higher magnifications the singlet may move through the plane of the real image thereby moving its plane to contribute to aberration correction. In preferred embodiments the lens system is used as an attachment to a prime thermal imaging device and each of the lenses has a useful spectral bandpass for energy in the 2-20 micron region of the infrared spectrum.
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