Since the 400mm and 800mm (new version) lenses advertise ED glass, here is an explanation about it. ED is Pentax's proprietary name for low dipersion glass. Its largest benefit is in long focal length lenses where color separation (longitudinal chromatic aberration) is a serious problem. Low dispersion glass does not eliminate color separation, but it does lessen it considerably. Using ED glass can improve off axis (margin areas of the image)performance so that telephotos can be shot wide open with only a slight degradation of image quality. Can telephotos be made using non-ED glass that perform as well as the ED lenses?? Yes. There have been lenses that were corrected for four colors (superachromatic) and even five colors at the film plane. When designed correctly, these lenses will be similar to the low dispesion types(ED,SLD,SD etc.) Hassy has two superachromtic telephotos in their line. (250mm & 350mm) A glass is considered low dispersion if its dispersion is significantly lower than its refractive index would indicate.In general, as the refractive index increases in glass, so does the dispersion. Lithium and Calcium Fluoride are the well known examples of this type of glass but there are many more, with many being secret/proprietary. SR

-- Steve Rasmussen (srasmuss@flash.net), July 12, 1998