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Pentax are probably better known for producing quality cameras than Astronomical eyepieces, perhaps that's why the XF range stayed under my radar for so long. Recently I purchased the XF 8.5mm for just under £150, a lot of money to spend on an eyepiece I was not familiar with but I had it on good authority that these 6 element design eyepieces were good performers so I took the plunge and waved goodbye to my hard earned cash.
On removing the lense from it's box I was initially saddened to see that instead of a nice comfy rubber eyecup the xf had a hard plastic raisable ring to shield the eye from local light pollution, while not the end of the world it was certainly something I'd have to get used to. The plastic ring did have 4 click stop positions which would be handy if you're observing with someone who wears glasses, a nice touch but I would still have just preferred a nice soft rubber eyecup instead.
Although we started off on the wrong foot I decided to allay my fears until I'd given the xf a fair trial so over the next few observing sessions I used the xf almost exclusively on first the solar system and then further into deep space to see how it would perform on those elusively faint wisps of cloud we know as galaxies.
After extensive testing I was relieved that I hadn't just ploughed £150 into a dud eyepiece, putting aside my initial disappointments regarding the plastic light pollution shield the xf is a fantastic performer and worth every penny of it's rather weighty price tag.
The xf uses 6 lense elements grouped into 4 and as such light scatter is kept very low (also due in part to the high quality Lanthanum glass pentax use in their eyepiece manufacture).
The SMC coatings are good and even, providing excellent contrast, You can see why Pentax refer to them as "Super" Multi Coatings. Even on really faint face on spirals the xf had no problems plucking subtle details out of the tenuous spiral arms.
Performance wise the Pentax xf is a triumph but sometimes eyepieces can be good visually whilst being incredibly uncomfortable to look through, thankfully the xf is comfortable enough so as not to detract one bit from its optical excellence. The xf has a generous 18mm of eye relief and the field stop is large enough that you don't feel like you're straining your eye through a pin prick.
The xf has a 60 degree field of view which is quite generous in an eyepiece with relatively few lense elements and it allows the observer to view their object for longer without having to nudge their scope to hold the object within the fov.
In conclusion the xf is a fantastic eyepiece despite it's plastic eyeshield, this is very much a matter of personal taste, I know a few guys who actually prefer the plastic shield, either way it's something that you can get used to and it's worth putting up with for the awe inspiring views I have had with this little eyepiece by Pentax.