This is an ultra-premium six-element zoom eyepiece. It has a long eye relief that's particularly well-suited to those who must wear eyeglasses while observing (such as those with severe astigmatism). It is designed for use with Pentax 80mm and 100mm spotting scopes, but can also be used for astronomical observing in any telescope accepting 1.25” accessories. One of four grooves at the tip of its 1.25” chrome barrel engages a steel pin in the eyepiece port of the Pentax spotting scopes. This keeps the eyepiece from rotating when the knurled rubber power change collar on the eyepiece is turned to change magnifications. Be sure that the pin in the scope body is engaged properly with the eyepiece before tightening the locking collar on a Pentax spotting scope.
All of the lenses in the zoom are fully multicoated with proprietary Pentax SMC (Super MultiCoated) antireflection coatings for high light transmission. They are the same coatings as those used on the most costly Pentax camera lenses. For better contrast, in addition to the inside of the barrel being blackened, the edges of all lenses have also been blackened to eliminate any internal reflections within the lenses themselves. ED (extra low dispersion) glass lens elements are used to minimize chromatic aberrations and astigmatism. The result is very good contrast and center resolution, with very little ghosting. A rubber eyecup twists in and out of eyepiece body to optimize your eye position and shield your eye from ambient light. When the eyecup is fully retracted for eyeglass use, the eye relief ranges from 12mm to 18mm, depending on the focal length selected. This allows observers with eyeglasses to see the entire field of view at low powers, where the eye relief is 18mm, although some vignetting of the field occurs at higher powers. As you generally are looking only at smaller objects that do not fill the field at higher powers, the loss of some of the field edge due to vignetting is not usually a concern. The barrel of the eyepiece in marked in both magnifications with an 80mm Pentax spotting scope and in focal lengths for astronomical observing. An index mark shows the power in use.
As with any zoom eyepiece, the precise focal length or magnification of the Pentax zoom at any given point in time is not all that important. The experienced observer invariably adjusts the zoom magnification to match the seeing conditions and object size as he looks through the eyepiece, rather than first setting the zoom to an arbitrary focal length value according to the scale on the side of the eyepiece and using it at that power no matter what the seeing conditions. The flexibility of a zoom to instantly match the magnification to changing lighting or seeing conditions is one of the zoom eyepiece’s chief advantages. Match that flexibility to Pentax-grade optics and you have an exceptional tool for any observer – terrestrial or astronomical.
For astronomical use, the zoom is usable for medium to high power observing with telescopes of all optical designs – from long focal ratio refractors down to fast f/4.5 Dobsonian reflectors. The eyepiece is relatively heavy and physically large, so some care is needed in balancing free-moving astronomical telescopes (such as Dobsonians) when this eyepiece is in use. The eyepiece container listed below is recommended purely as a replacement should the standard plastic container supplied with the eyepiece be lost or damaged.
For terrestrial use with 80mm aperture, 520mm focal length Pentax spotting scopes . . .
The zoom at a barrel marking of 20x is actually 21x with a 93' field at 1000 yards and 18mm of usable eye relief.
At a barrel marking of 40x it is actually 43x with a 60’ field and 12mm of usable eye relief.
At a barrel marking of 60x it is actually 63x with a 49' field and 15mm of usable eye relief.
For terrestrial use with 100mm aperture, 630mm focal length Pentax spotting scopes . . .
The zoom at a barrel marking of 20x actually provides 26x, with a 78' field at 1000 yards and 18mm of usable eye relief.
At a barrel marking of 40x it actually provides 53x, with a 48' field and 12mm of usable eye relief.
At a barrel marking of 60x it actually provides 78x, with a 39.5' field and 15mm of usable eye relief.
For astronomical telescope use in a 1.25" focuser or star diagonal . . .
The zoom at a marking of 24mm has a 38 degree apparent field and 18mm of usable eye relief.
At 16mm (not marked on the barrel, but easily interpolated), it has a 44 degree apparent field and 15mm of usable eye relief.
At 12mm it has a 49 degree apparent field and 12mm of usable eye relief.
At 8mm it has a 60 degree apparent field and 15mm of usable eye relief.