There are several types of zoom eyepieces available, usually with focal lengths in the rage of 7 to 21 mm or 8 to 24 mm. They remove the necessity of changing eyepieces of various focal lengths in order to vary the magnification. The apparent field of view of a zoom eyepiece narrows from a generous 60° at 8 mm focal length to a somewhat small 40° at 24 mm, but if you can accept this restriction, a single zoom eyepiece can replace a drawer full of regular Plössl eyepieces. Zoom eyepieces also go well with the binocular viewer – a device that splits the single beam of light from the telescope’s objective and diverts in into two identical eyepieces. Also zoom lenses are very convenient in the field so that eyepieces do not have to be changed so often.ocality
Most eyepiece series are parfocal within the series. Parfocal eyepieces normally have to be bought as a set from a single manufacturer and will be more expensive than equivalent non-parfocal eyepieces bought separately. However, the extra expense is usually worthwhile because of the increased ease of use of the telescope.
To achieve the same thing, but much more cheaply, you can adjust your eyepieces to be parfocal by adding parfocalizing rings. These are available from various manufacturers, or you can build your own. To parfocalize your eyepiece collection, you first determine which of them requires the most in-focus. With that eyepiece focused sharply on a medium-brightness star, you lock the focuser.
You then add a parfocalizing ring to each of your other eyepieces and physically slide them in and out in the focuser until they reach focus. At that point, you slide the parfocalizing ring down to the top of the focuser and lock the set screw (or set screws).