This 21mm triplet loupe from Hobby Tool Supply uses 3 lenses to provide a sharp image with little to no spherical aberration/distortion. Thanks to the case's eyelet, the loupe can be attached to a chain or string.
magnification with three 21 mm lenses
3 lenses drastically reduce spherical aberration
Eyelet allows the loupe to be attached to a chain or string
Folding housing protects the lens when not in use
Leather storage pouch