ABSTRACT
A single element lens or single group consisting of two or more cemented elements has a single value of effective focal length (EFL) other than slight variation due to residual aberrations. When lens design after about 1890 had progressed to a stage where good-quality anastigmat lenses were available, some designs were deliberately made convertible and could be purchased in sets of elements. Telephoto lenses of long EFL for large format cameras are bulky and heavy. Some lenses or optical systems may be converted for alternative use by physical reversal in the lens mount. To provide extended versatility from a camcorder zoom lens, it may have convertible features. The idea of using a common group with two or more others was used in contemporary asymmetrical telephoto designs by Nikon for large-format cameras. High-quality symmetrical design concept was carried on in symmetrical lenses for large-format cameras into the 1970s.
