These lens units vary the power obtained with any given eyepiece, while maintaining excellent image quality. Their precise corrections for spherical and chromatic aberrations at all photo visual wavelengths result in sharp, high resolution performance on telescopes of any optical design. Barlow Lens units are multi coated for maximum image contrast and light transmission. The #127 includes a precision sliding lens assembly, permitting continuously variable power factors between 2x and 3x. Positive stops are provided at both ends of the lens travel, and an engraved scale indicates the power in use. Clear aperture is 25mm. This popular, high performance Barlow is recommended for use on telescopes with focal ratios of f/6 and longer. Not recommended for ETX90, ETX105 and ETX125 telescopes.