Jewelers Loupe, tips on choosing the one you need.

i have sell Jewelers loupes on ebay(www.ebay.com). Hopefully this guide will help you to know the truth about the loupes offered through out ebay(www.ebay.com).
Loupes are generally listed by the strength of magnification or power first, ie: 10x 15x 20x 30x etc.
The next number in the description is the size of the lens, or diameter of the lens at the top of the loupe, ie: 12mm, 14mm, 18mm, 20.5mm or 21mm etc.
The types of loupes are generally referred to as "Doublets", "Triplets", "Quadruplets" or "Pentagonal". This refers to the number of lenses built into the loupe assembly, (more on this subject latter).
All loupes have a focal range, this indicates how far away from the object your viewing the lens will focus. This range generally decreases as the power of the loupe increases. ( if the object your inspecting is upside down, try moving the loupe closer to the object, this should correct the problem).
Expected focal ranges are from 1/8 of a inch to about 1&1/4 inches from the object your inspecting. A good average averge is 3/8 inch or greater. The strong 20x or 30x loupes with a shorter focal range may block a lot of the light needed to inspect the object.
Lenses sometimes have special coatings to correct the color.
Description of a "Doublet" lens loupe:
A doublet lense is constructed of a single piece of glass, with lenses ground on both ends of the glass.
Many lower cost loupes marked as triplets are actually doublets incorrectly marked as triplets, like the one pictured here on the left. The 10x 14mm loupe on the right is a Professional quality doublet.

Description of a "Triplet" lens Loupe:
A Triplet lense is constructed of three separate pieces of glass, mounted into one loupe assembly. The three lenses are often glued together, but on the higher powered loupes spacers are used to separate one of the lenses. These pictures are from a 30x 21mm triplet and a 10x 21mm triplet.

"Quadruplet" and "pentagonal" refer to four and five lenses built into the loupe, these are not as common but are available. Here is a picture of a Pentagonal set of Lenses, it has 2 sets of 2 lenses glued together and one single lens with spacers holding it in place in the center: