Choosing and Using A Loupe

(Fig 1) Two typical loupes with quarter shown for size comparison. Style on left is a watchmakers loupe; folding style, right, is a jewelers loupe.
(Fig 1) Two typical loupes with quarter shown for size comparison. Style on left is a watchmakers loupe; folding style, right, is a jewelers loupe. | Source

Probably the least expensive and most helpful tool garage salers and antique collectors can carry is some type of pocket magnifier. For less than $5 you can buy a magnifier that will help you separate a laser print from a vintage photograph, tell you if a decoration is hand painted or an applied decal and make sure the mark you’re looking at is 14 karat gold and not 14 karat gold filled (which at $1500 an ounce is about an $800 mistake).

Mention “magnifier” to most people, though, and they immediately think of Sherlock Holmes’ trademark….a big round glass lens on a stick-like handle. While that may be a great look for a Victorian sleuth, the most practical magnifier to carry on present day junking and antiquing expeditions is a loupe (pronounced loop, rhymes with hoop).

The lens of a loupe generally ranges from about the size of a dime to no larger than the diameter of a quarter. Loupe lenses are generally mounted at the end of a cylindrical frame somewhat similar in shape to a miniature telescope. To conserve space, lenses of some loupes fold back into a protective handle to make themselves even smaller (Fig. 1).

The smaller loupes which fold into themselves are known as jewelers loupes and are primarily for hand held use. The slightly conical rigid frames with larger lenses are known as watchmakers loupes. Watchmakers loupes are so named because they were originally designed to be held in the eye by squinting—like a monocle—allowing hands free to hold tools or parts. For inspecting antiques and collectibles, though, both styles of loupes are almost always handheld by the average user.

There are several factors to consider when deciding which loupe is best for your particular area of interest. These include: field of view, focal length/depth of field, power of magnification and color accuracy.

(Fig 2) All styles of loupes should have a matte black finish inside the lens frame to reduce unwanted reflections. This watchmakers loupe, for example, has a shiny silver exterior with a matte black interior surface.
(Fig 2) All styles of loupes should have a matte black finish inside the lens frame to reduce unwanted reflections. This watchmakers loupe, for example, has a shiny silver exterior with a matte black interior surface. | Source

Attributes to Consider When Making Your Choice

Field of view is the size of the area as seen through the lens. If you want to examine the mounting prongs of jewelry, a small field of view would be fine. If you generally examine postage stamps, a larger field of view would allow you to see almost the entire stamp without moving the lens. The field of view is generally printed directly on the loupe. The field of view of the jewelers loupe in Fig. 1 is 18mm which is printed in gold lettering on the loupe frame.

Focal length/depth of field These are interdependent related features. Focal length is the distance from the lens an object comes into focus. Depth of field is the total area that will be in focus without moving the lens. Looking for damages in an oil painting wouldn’t require a great depth of field because painting are generally quite flat. A greater depth of field would be best for examining more three-dimensional items such as the hammer on a flintlock for example. The focal length is generally, but not always, printed on the loupe frame. The vast majority of both jewelers and watchmakers loupes are either 1 or 2 inches. If the focal length is not printed on the loupe frame, it will be included on the loupe packaging or instructions.

Power of magnification refers to the extent a given lens will bend light rays. A low power lens doesn’t bend rays as much as a high power lens and produces smaller increases in image size. However, more power is not always desirable. The higher the power of magnification, the shorter the focal length, the smaller the depth of field and the smaller the field of view. It is also much harder to keep a higher power magnfier in focus: the slightest movement throws the image out of focus due to the very small depth of field and very limited field of view. To look for damages, examine most marks and signatures, and to check for signs of authentic wear, a 5X power loupe is fine. If you routinely look at jewelry and gems, examine prints and photos to determine method of production and need to separate bone from ivory, then choose a 10X power loupe. The power is generally, but not always stamped on the frame. If the power is not printed on the loupe frame, it will be included on the loupe packaging or instructions.

Color accuracy refers to the degree to which a given lens will or will not distort the true color of an object. Since this is a very detailed subject involving the technical aspects of lens required by law to color grade gemstones, it is generally of concern only to professional gemologists and is too involved to cover in this short article. For casual users, it’s enough to just be aware that highly accurate advanced lens are available to correct for spatial distortions (spherical aberrations) and color distortions (chromatic aberrations).

Try to buy loupes with glass lenses; some loupes have plastic lenses which warp when heated and can produce distortion in the magnified image. Also make sure the interior of your loupe has a matt black finish to reduce unwanted reflections (Fig. 2). The aluminum frame watchmaker’s loupe with a glass lens like the one shown in Fig. 1 are widely available in 5X or 10X for around $5. The 10X jewelers loupe shown here with a color corrected glass lens in a steel frame sells for $20 to $25 and comes with a leather case. Loupes are widely available online, jewelry supply stores and hobby shops that cater to coin and stamp collectors.

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