Vortex does not state the eyepiece diameter of the Vortex Solo 8x36mm Monocular. I recommend using the measurement tool of the left of the product page below for the Butler Creek Lens Covers or contacting Vortex for the eyepiece diameter.
https://www.opticsplanet.com/butler-creek-eyepiece-flip-open-scope-covers.html
This is a monocular and there is no way to attach this to either a smart phone or a camera.
The focusing ring can be tight when brand new but should get a bit more forgiving with use. If it remains too tight for your liking, please feel free to contact us at info@vortexoptics.com and we can set up having you send it in for an adjustment.
The manufacturer doesn't state if the internal frame is metal or polymer. I recommend contacting Vortex in regards to that information.
This Vortex Solo 8x36mm Monocular is not a rangefinder. It's an 8X magnified monocular, so what you view through the Vortex Solo 8x36mm Monocular will look 8x bigger than how it would look with the naked eye.
No, this is a set magnification.
This is good for viewing with light but if your wanting night viewing then you are going to need night vision monocular. To see in the dark very well.
Miles,
The Vortex Solo 8x36 measures 2.1 inches at it's widest point.