Refracting designs include achromatic and apochromatic types. The sub-telescope is a apochromatic Kepler refractor with a focal length of . The catadioptric camera lens' heyday was the 1960s and 1970s, before apochromatic refractive telephoto lenses. These are apochromatic telephoto lenses using a 5-element variation of the Tele-Xenar design. Most professional telephoto lenses since the early 1980s are apochromatic. Midland optical department was specialized in the research of retrofocus designs and apochromatic corrections. Apo-Artar HM lenses are an apochromatic symmetrical 6-element design, which is optimized for 1&colon;1 reproduction. Apo-Artar lenses are an apochromatic symmetrical 4-element design, which is optimized for 1&colon;1 reproduction. Classically designed apochromatic process camera lenses generally have a maximum aperture limited to about 9. These are Barrel lens for Enlarging & for a photoengraving, and perfect apochromatic lenses. In some cases, apochromatic designs without anomalous dispersion glasses are possible. The APO-Grandagon line is apochromatic, incorporating ED-glass elements. This occurs most commonly with apochromatic objectives. The APO-Sironar-N line is a 6-element, 4-group apochromatic design for general photography. Apochromatic refractors have objectives built with special, extra-low dispersion materials.