I like Doremus approach (post No.6) and Bernice insights with provided link post No.5).
I was using various cheap loupes from x1.5 to x10 and to my understanding they do their job, but once I was lucky to acquire Schneider Aspheric x6, with price tag within two digits, cosmetical blemishes but very clean glass.
I highly reccomend it. It very bright, decision making where is the focus became an easy task even with older uncoated uncorrected lenses when CA, SA or coma is present. And there is a lot of periphery to look around focused point, its kind of wide angle.
This loupe is on the heavy side and skirt is quite big, otherwise outstanding tool.
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