For short duration inspection (few minutes) you don't need to spend a lot of money. But if you are using a loupe all day, every day (like me), or even just for a few hours, then you need to purchase something of quality to reduce eye fatigue. I use 4X for general assembly work where not a lot of magnification is required - I have a good quality flip down single loupe (Behr) that mounts to a pair of reading glasses, like you see sitting on my bench here:
For more detailed tasks I use a 10X loupe on a wire that goes around my head, and that is an aplanatic loupe - this type reduces spherical aberration in the field of view, so the view is clear and undistorted out to the very edges - this reduces eye strain a great deal compared to a regular loupe.
Then for even more detailed work (oiling co-axial escapements for example) I use this:
52X magnification, very clear optics, illumination above and below that can be adjusted independently, and lots of working distance. This is the same scope I used at Omega to train on the co-axial, so I knew it would work well for those, but of course I use if for many other tasks as well.
Cheers, Al
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