Poor optical quality (huge sharpness destortion at every focal lenght). And equally poor mechanical quality (especially threading). It was actually impossible to me to mount/screw it properly even into their own RISE(UK) 105mm (82-105 in my case) adaptet ring. And it was entirely impossible to mount it to the front of other (Haida NDs in my case)105mm filters.
Fully Coated HD 1.25 Inch 4mm Plossl Eyepiece Lens For Telescope Accessory