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Please school me on the Pentax 65 (1 Viewer)


Well-known member
Very experienced with binoculars, but fairly inexperienced with spotting scopes. My personal scope up to now has been an old Bushnell Spacemaster with a fixed 20x eyepiece. It is remarkably good, which is why I haven't bought a new scope in 20+ years. Everything I've compared it to has left me wondering why I need to spend $400+ on a new scope.

However, I have a keen interest in the Pentax, not only because of the rave reviews it receives but also because of the universal 1.25" eyepieces it uses.

Here's where I need help. I know literally nothing about 1.25" eyepieces. I've read a bit, and know that you don't necessarily have to spend a ton on an eyepiece to get a great image, but I don't know how to convert mm focal length to the convetional "power" setting, etc.

I do know I'm a fan of fixed focal length eyepieces. My 20x eyepiece on my Spacemaster blows any variable eyepiece I've ever seen out of the water in clarity and FOV. So I guess my question is, what eyepiece should I be looking at for the Pentax 65 that will give me 20-25X and not break the bank?

Thanks in advance for your help!


It's just a flesh wound.
To calculate magnification, divide the focal length of the scope by the focal length of the eye piece.

Conversely, to solve for eye piece focal length, divide the scope focal length by the desired magnification.

The focal length of the PF-65ED is 390mm. So, for 20X take 390mm/20x = 19.5mm eye piece focal length.

The Pentax XW eyepieces are fantastic (and waterproof, most astro eye pieces aren't), and there is an XW-20 available. Not cheap, but good deals on used eye pieces can be found.


Well-known member

quite frankly, if you have a scope that is doing 20x well (which is not very difficult) and you only want/need 20x, there is not really a need to upgrade.

Regarding the EP question for the Pentax with a body focal length of 390mm we get 390mm / 20 = 19.5mm or 390mm / 25 = 15.6mm. My recommendation would be to get 30x though - 390mm / 30 =13mm. You will not find astro EPs with the exact focal length - get sth in the ballpark and divide body fl by EP fl for magnification. You want to get wide angle EPs from 60 deg apparent field of view onwards.

Also be prepared to find out that a 3rd party EP you get will not come to focus in your Pentax spotter - they have a lot less focus travel than real astro scopes after all. So either find some friends with astro scopes and EPs to try theirs (the local astro club is a good idea) or get them from a shop with a no questions asked return policy.

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