Such binoculars also usually have changeable eyepieces to vary magnification. Binocular eyepieces usually consist of three or more lens elements in two or more groups. Amateur astronomers tend to refer to telescope eyepieces by their focal length in millimetres. Also, total magnification in both fixed and zoom systems can be varied by changing eyepieces. Wide-field binoculars are possible, but require bulkier, heavier, and more complex eyepieces. The name was later shortened to Tasco as inventory expanded to include binoculars and eyepieces. Some high performance objective lenses may require matched eyepieces to deliver the best optical performance. They have objective lenses that are approximately in a line with the eyepieces. The first eyepieces had only a single lens element, which delivered highly distorted images. Long focal-length eyepieces usually have ample eye relief, but short focal-length eyepieces are more problematic. Even today these eyepieces are considered good eyepieces for planetary and lunar viewing. The individual eyepieces have their focal lengths written on them as well. Hence these eyepieces are regarded by many amateur astronomers as a luxury. This design has been widely copied in other wide field or long eye relief eyepieces. The eye relief is better than the Huygenian and worse than the Ramsden eyepieces.