for users of an eyepiece to want to calculate the actual field of view, because it indicates how much of the sky will be visible when the eyepiece is used with their telescope. The most convenient method of calculating the actual field of view depends on whether the apparent field of view is known. If the apparent field of view is known, the actual field of view can be calculated from the following approximate formula: or where: is the actual field of view, calculated in the unit of angular measurement in which is provided. is the apparent field of view. is the magnification. is the focal length of the telescope. is the focal length of the eyepiece, expressed in the same units of measurement as . The focal length of the telescope objective is the diameter of the objective times the focal ratio. It represents the distance at which the mirror or objective lens will cause light to converge on a single point. The formula is accurate to 4% or better up to 40° apparent field of view, and has a 10% error for 60°. If the apparent field of view is unknown, the actual field of view can be approximately found using: where: is the actual field of view, calculated in degrees. is the diameter of the eyepiece field stop in mm. is the focal length of the telescope, in mm. The second formula is actually more accurate, but field stop size is not usually specified by most manufacturers. The first formula will not be accurate if the field is not flat, or is higher than 60° which is common for most ultra-wide eyepiece design. The above formulae are approximations. The ISO 14132-1:2002 standard determines how the exact apparent angle of view (AAOV) is calculated from the real angle of view (AOV). If a diagonal or Barlow lens is used before the eyepiece, the eyepiece's field of view may be slightly restricted. This occurs when the preceding lens has a narrower field stop than the eyepiece's, causing the obstruction in the front to act as a smaller field stop in front of the eyepiece. The precise relationship is given by This formula also indicates that, for an eyepiece design with a given apparent field of view, the barrel diameter will determine the maximum focal length possible for that eyepiece, as no field stop can be larger than the barrel