Review on SVBONY Barlow Lens 5X 1.25 inch: Fully Multi 🔭 Coated Metal Thread M42 - Ideal Telescope Accessory for Astronomical Observations by Nathan Ortiz

The mechanical body is in solid anodized aluminum and can be easily disassembled. The lens is in a cell that is removable from the bottom (inlet end) of the assembly. If you remove the lens cell, you have a simple 1.25" to T-mount mechanical adapter for front projection. The top (output end) receives 1.25" eyepieces, allowing you to insert the eyepiece to close the eyepiece project. All of this works well since you're not using the optics that come with the unit. What's disappointing is the negative lens that comes with the kit. It's a piece of plano-concave uncoated glass, 20mm diameter and -125mm focal length .Not cemented achromat and not even coated.So the technical specs ('full multilayer coverage', 'chromatic correction') are ridiculous, typical of those foreign friends telling us what we want to hear without any relation to it to have what the words mean. The -125mm element is located 22mm inside the 1.25" eyepiece arm of the telescope's focuser. The eyepiece socket is located 38mm from the focuser arm . Use Barlow's formulas to calculate the effective magnification factor of your instrument. The magnification is M = f/(f+d), where f = -125mm is the focal length of the lens and d is the distance of the lens within the focal plane of the lens. To get M = 2x as stated for this product, d must be -1/2 of f or 62.5mm within the plane of focus. I don't see how this is possible with this shorter tube, so the actual magnification would be much less than 2x.