One of the drawbacks to standard eyepiece designs is that at short focal lengths (high power on most telescopes), the eye relief is very short. Eye relief is the distance the observer’s eye must be from the eyepiece lens (see diagram below). Short eye relief makes viewing quite difficult, especially for eyeglass-wearers.
Above: Eye relief is the distance from the eyepiece to the observer’s eye
Some eyepieces are specially designed to have long eye relief (LER), even at very short focal lengths. This makes high-magnification viewing much more comfortable. These eyepieces use more lens elements (usually 6-7) than a standard eyepiece. However, modern optical coatingsmake this possible with no noticeable loss of image quality. In fact, the better long eye relief eyepieces provide extremely good image quality with excellent sharpness and very high contrast, making them ideal for planetary observing.
Above: The inner workings of a typical long-eye-relief eyepiece
Most wide-field eyepieces do not have long eye relief at short focal lengths. However, wide-field eyepieces are intended primarily for low power viewing, so this is not normally a problem. Some wide-field eyepieces are designed with long eye relief, making them ideal for short focal length scopes where they would still be used as medium-power eyepieces. These long-eye-relief wide-field lenses usually do not have quite as wide anapparent field as the widest eyepieces, but are wider than a standard eyepiece (usually 60-70°).
Long Eye Relief Eyepiece Prices
You will pay a bit extra for the added convenience of long eye relief, but it is almost always worth it. Very short-focal-length eyepieces with short eye relief can be all but impossible to use. Inexpensive LER designs start around $70. Higher quality designs run $100-200, with the best costing around $250. LER wide-field eyepieces start around $200 and can cost up to $500 or so.