The field stop is the metal ring inside the eyepiece barrel that limits the field size. It's projected by the eyepiece so that it appears as a circle out in space when you look through the eyepiece. The following key spec summary for all Tele Vue eyepieces will help you calculate the true field size you will achieve with your telescope in combination with Tele Vue eyepieces. Please note that eyepieces are listed according to their field stop diameter, from largest to smallest.
Eye relief is given for reference only.
For example, factors of 1.4 or 2.0. When choosing higher power eyepieces, use ratios of magnification. . Then add eyepieces covering uniform increments in smaller field stops. To further fill in with incremental steps, add eyepieces with approximate field stop diameters of 28mm and 14mm. Of course, avoid duplicating focal lengths, when filling in.