Wide field eyepieces (with apparent fields greater than 50°) were originally invented for military applications during World War I. The Erfle design consists of 5 or 6 lens elements and give an apparent field of 60-70°. They are relatively inexpensive but suffer from distortion and astigmatism at the edge of the field of view. They also do not perform well at high magnifications, but are intended primarily for low powers. Many of the inexpensive wide-field eyepieces available today are similar in design to Erfles and have the same drawbacks, although some are quite good.
Above: Optical diagram of a 5-element Erfle eyepiece
Modern wide-field eyepieces have extremely large apparent fields, up to over 80°. The best eyepieces have excellent off-axis star images, and are simply stunning to look through. Most observers comment that adding a good wide-field eyepiece to their collection is like getting a whole new telescope! The diagram below is of the original TeleVue Nagler design. Currently, Naglers come in even more advanced designs such as the Type 4, Type 5, and Type 6, which incorporate 6 to 7 lens elements. Compare the diagram below to that of a simple Plössl design and you can see why there is such a difference is price and performance between standard and wide-field eyepieces.
Above: Optical diagram of the original Nagler eyepiece design
Wide Field Eyepiece Prices
Wide-field eyepieces cover the largest range in prices of any eyepiece style. You tend to get what you pay for, so the performance of a less expensive eyepiece will not equal that of the most expensive wide-field eyepieces, which are typically the best you can buy. Inexpensive wide-field eyepieces start under $100, but the better quality models will start in the $150-200 range. The best eyepieces start around $250 and can cost over $600, but the views are well worth it!