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Eye relief (ER) is the distance your eye must be from the telescope eyepiece lens to achieve focus. Long eye relief is of importance to eyeglass wearers, who will likely prefer longer lengths to be comfortable, such as something over 15mm. This is a matter of comfort, however, and that comfort level is different for everyone. There are many who like long focal lengths who do not wear glasses while observing. If you are a first timer or are not very familiar with this factor, it is better to err on the side of long than short.