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A 50mm Plossl is useful in long focal length telescopes with slow focal ratios. At f10 you get 5mm exit pupil for brighter images. The Tele Vue 55mm Plossl has a 46mm field stop diameter which is the same as the 41mm Panoptic. The Panoptic will give 4.1mm exit pupil at f10 but has a wider 68 deg apparent field and the 55mm Plossl will give 5.5mm exit pupil at f10 with 50 deg apparent field.
Note the Clave Pic Du Midi observatory Plossl series had 30, 35, 40, 45mm all with 48 deg apparent field in 50mm or 50.8mm barrels. Also in same barrels but with smaller true fields (deg) were 55mm (42), 65mm (37) and 75mm (32).
A good quality Plossl is not just for planetary also for deep sky to tease our the faintest detail or even just pick up the faintest galaxy. Such contrast boost possible from the superb polish on all glass surfaces that make up a good Plossl eyepiece not just the reduced number of elements in its makeup.
On the subject of eye relief with short Plossls, you will find it considerably more comfortable if you are tracking the target with an equatorial mount. You can turn away to rest your eye and return to the view without losing the target.
I did a test with Jupiter one night using a 6mm Clave Plossl, 6mm Brandon and 6mm Tele Vue Radian on my 10.1” f6.4 Newtonian on an equatorial platform. Each time I changed the eyepiece I thought I saw some new detail I didn’t see before. The view I enjoyed the most was with the Radian simply because of the comfortable 20mm eye relief. If seeing was better that night perhaps one eyepiece would show more than another more consistently but not that night.
I still enjoy my Plossls and one absolute gem is the Tele Vue 8mm Plossl which gave me superb lunar views one night through my Newtonian. I do also enjoy the Nagler Zooms (4-2mm and 6-3mm) for their ability to change magnification at a reasonably generous 10mm of eye relief. My all time favourite combination is the Clave 10mm Plossl with Clave 2x Barlow for an absolutely exquisite 5mm eyepiece.
Note to see the difference a quality Plossl gives requires the best of seeing and most nights simply wont offer that opportunity. Note too there are many modern alternatives such as Tele Vue Delos or DeLite but not having used any I cannot comment on their performance.
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