Magnifiers and Loupes


How to Select Magnifiers
Diamonds are graded for clarity at 10X. If a flaw in a diamond cannot be seen by a trained eye at 10X, the flaw does not exist for the purpose of grading clarity.
A 10X triplet loupe is the conventional jewellers' loupe. A triplet has three lenses which greatly reduce or entirely remove distortion and colour aberrations (depending on the quality of the loupe), compared to single lense or doublet loupes. 
Magnifiers are also classified by working distance (focal length). A 10X loupe has a working distance of 1" which means the magnifier will  be in clear focus when held 1" from an object.
The higher the power, the shorter the working distance,and the shorter the depth of field
Precision work routinely requires magnification to increase the visual size of objects. As the eye comes closer to an object, the apparent size and detail in increased; however, the ability of the eye to focus correctly diminishes at distances closer than 10" (254mm). Use of a magnifier provides increased focusing power, resulting in the visual effect of a clearer, larger image of the object.

Four Characteristics of Magnifiers
Power of Magnification  The ability of the lens to increase the visual size of an object. The symbol X (times) is used to identify this factor. A magnifying lens bends light rays resulting in an image that looks larger than the actual object. For example, a 3X magnifier triples the size of an image.
Working Distance ( Focal Length )  In addition to power, a magnifier will also be classified for working distance. The higher the power of a lens, the shorter the working distance (the distance between the object and the magnifier). For example, a 2X lens has a working distance of 5" (127mm), which means that the magnifier will provide double magnification and be in clear focus when held at a distance of 5" from the object.
Field of View The size of the area that is seen through the magnifier. The field of view decreases as the power increases.
Depth of Field The distance that a magnifier can be moved from an object and still have the object in focus. The higher the power, the shorter the depth of field.

Corrective Magnifiers
Certain magnifiers have lens systems which correct optical distortion or aberration that might otherwise be present as follows:
Distortion  Uncorrected lens will curve "straight lines" of light toward the centre.
Chromatic and Spherical Aberration  Size and shape differences will occur in the image of an uncorrected lens. These aberrations are clearly noticable when viewing a matrix of dots of the same size.

Definition of Common Terms

Achromatic Lens: Lens that eliminates chromatic aberration.
Aplanatic Lens: Lens that corrects for spherical aberration and coma.
Chromatic Aberration: Inability of a lens to focus light of different colours at a point.
Coma: Blurry appearance around an object viewed through an uncorrected lens.
Corrected Lens: Lens system that corrects for distortions and aberrations to produce a sharp image.
Spherical Aberration: Lens defect that produces an image that lacks contrast.