Working Monocular (Need Lens!)

Here is a monocular that I made as a first pass of an idea that I had to try to make my own monocular spyglass. I bought some lens from AliExpress to practice (30mm and 50mm diameter link below). I got the set for 5 euros, so I designed this monocular to fit the lenses and to test them. It has 3 components: the shaft and eyepiece and the end lens piece. The eyepiece and end piece has a place where the lens can be inserted and is screwed close to keep the lens in place. These two pieces can then be screwed to the shaft to complete the monocular. It has a screw at the top and bottom so you can move the lenses a bit to focus if needed. You can make the components any color of your choice to make it cooler or more vibrant, I just made this spyglass/monocular as a test case in black and it worked perfectly so i just kept it. It’s supposed to magnify x4 according to physics, but I'm not sure how to test the exact magnification number. I just know it magnifies.

Any tips on improving the design are appreciated. 

Lens Specs: 

1. Double concave lens diameter 30mm, focal length-75mm (eyepiece lens)
2. Plano convex lens diameter 50mm, focal length 300mm (lens )

Mine are a 0.2mm smaller than the specified diameter (probably manufacturing defect)

Link for the lens: 

https://fr.aliexpress.com/item/1005004111564361.html?spm=a2g0o.order_list.order_list_main.76.642c5e5bm96ZZI&gatewayAdapt=glo2fra

The assemble is simple, everything just screws in. Fit the Lens in the Eye piece and end piece and then fit both on the shaft (the long body of the monocular)

You look through the smaller end