Choosing Loupes and MagnifiersBy

Choosing
Loupes and Magnifiers

Perhaps no other piece of equipment can do collectors and dealers so much good for so little cost as pocketsized magnifying lenses. Surface textures that might indicate a repair may go undetected by the unaided eye but will show up clearly when enlarged. Decals can be distinguished from hand painted brush strokes, flaws in gemstones can be spotted and marks and signatures can be examined for authenticity.

In this article, we're going to discuss the different magnifiers available and which magnifiers to use on specific objects.

Lens Basics

Magnifying lenses create images that appear larger than the original object by bending light rays (see Fig. 3). The power of magnification depends on the curvature of the lens surface. Low powered lenses have surfaces with shallow curves; the surfaces of high powered lenses curve more sharply.

The distance from the lens that an object comes into correct focus is called the focal length (also called the "working distance"). The shorter the focal length, the closer the lens must be to the object. Most lenses have the focal length stamped or printed somewhere on the lens holder or housing (Figs. 4-A & 4-B).

The area in front and in back of the focal point that a lens can bring into focus at the same time is called depth of field. For example, if a lens has a 5" focal length and a 1" depth of field, the area in focus might be from 4 1/2" to 5 1/2" (Fig. 4-A & B). The field of view is the size of the area as seen through the magnifying lens (Fig. 5). An ordinary hand-held magnifying glass has a large field of view; a watchmakers loupe has a much smaller field of view.

Lens Quality

The quality of any magnifier is the result of how accurately the lens or series of lenses can bend and focus rays of light. Two of the more common problems in lenses are spherical aberration and chromatic aberration. Spherical aberration is a technical term for distortion. Spherical aberration causes straight lines of the original subject to appear as curved or blurred lines in the enlarged image (Fig. 6). Chromatic aberration means that the enlarged image a lens produces has different colors than the original object (Fig. 7).

This does not mean a blue object will turn green. Chromatic aberration generally produces a color shift that appears as a halo or slight fringe around the enlarged image especially at the outside edges of the lens (Fig. 7). Chromatic aberration is a serious problem in evaluating gemstones, examining color printing, inspecting repairs and any other observations where precise color is critical.

Note: all diagrams intentionally simplify complex principles; the diagrams are not intended as precise scientific drawings

When lenses are arranged, ground or treated to specifically reduce aberrations they are called corrected lenses. A lens corrected for both distortion and color is fully corrected; fully corrected lenses produce enlarged images that have the same flat lines and colors as the original object. A color corrected lens may also be called an achromatic lens.

Some limited corrections can be made to a single lens by applying coatings to the lens surface or grinding two different curves. The most accurate corrections, though, are made by making the lens thicker and by using two or more separate pieces of glass. The two lenses capable of the greatest corrections are the Coddington lens and the Hastings lens. The Coddington lens is a very thick lens with a deep groove around its outside edge. The Hastings is made up of three separate pieces of glass cemented or joined together to form a triplet. Each piece of glass in the triplet contributes special properties to assist in the correction. Hastings triplets are generally the most accurate of all lens styles followed by the Coddington, then coated single lenses. Unless a description of a lens specifically includes words such as "color corrected" or "Hastings triplet" or "corrected lens", it will usually be a simple, uncorrected lens. Corrected lenses cost more than uncorrected lenses because of the extra work; the more accurate the corrections, the higher the cost.

Magnification power

Increasing the power of magnification does not necessarily make an object easier to see. A 20X loupe for example, has virtually no depth of field. That means whatever you are examining will only be in focus at almost exactly the focal point and nowhere else. If there is a repair or flaw in front or in back of the focal point, you might not be able to see it (Figs. 9 & 10). It is also very difficult to hand-hold an object and a powerful lens steady enough to maintain focus during an examination. The slightest movement shifts the object out of the shallow depth of field and thus out of focus.

Most occasional users of magnifiers will get a more useful image with less magnification. Lower power lenses, 10X and below, are easier to use because they have longer focal lengths and greater depths of field which keep more of an object in correct focus. A typical focal distance of a typical 10X loupe, for example, is about 1" with a depth of field of about 3/8 to 1/2". That means you will be able to an object in focus from about 3/4" away to 1 1/4" away from the lens. Not everything may be in sharp focus, but at least you will see shadows or masses of color. This is why many professionals begin examinations with 5X -10X lenses which have greater depths of field. Once you find a suspected area (especially in repairs) you can always proceed to higher powers of magnification to investigate more closely. But if you begin with a high magnification, you may miss details which are out of focus due to the shallow depth of field.

Occasionally unethical sellers may appear to be doing a buyer a favor by offering them a very strong lens. They know an inexperienced loupe user will not be able to keep the lens in focus and will probably have a more difficult time finding problems with a strong magnification than a weaker power.

The quality of the lens used in some applications is determined by law. All diamond clarity grading, for example, in the United States must be done with a color corrected Hastings triplet magnifier of 10X. This rule was established by the Federal Trade Commission (FTC) to guarantee uniform standards. Other specialized fields such as stamps and coins are not regulated by law but usually follow standards established by custom and tradition.

If your field of interest is not regulated by law or traditional standards, you can choose from a wide variety of magnifier styles and lenses. The list below describers the main types and advantages and disadvantages of each.

Styles of magnifiers

The case or frame that surrounds the lens should be non-reflective black. This prevents stray light which could affect the examination from being reflected up into the lens. Try to avoid highly polished finishes for the same reason.

WATCHMAKERS LOUPE-- A watchmakers loupe is usually a single lens mounted at one end of a cylindrical tube. As the name suggests, this loupe was originally used to pick up, inspect and work with small parts of clocks and watches. Watchmakers loupes are available in many powers usually ranging from 3-15X. Most frames are black plastic, some are metal. Better loupes have glass lenses, lower quality loupes may have plastic lenses. Watchmakers loupes are excellent for any general magnification such as examining signatures and marks, tool marks, repairs, some types of printing and general inspections. Watchmakers loupes are not suitable for inspecting gemstones or precise line work because most watchmakers loupes are not corrected.

LINEN PROVER (or Linen Tester) The Linen Prover is a specialized magnifier originally developed to count threads in textiles. Their bases are marked with several different scales of measurement allowing very accurate measurements. This is useful to verify some specific standard such as a certain number of threads per inch, dimensions of signatures and marks in art work and currency, certain types of printing and other applications where measurements are clues to authenticity.

Most have a single lens held at the correct focal length in an upright frame. Since it is always held in focus by its frame, it can be moved across flat surfaces rapidly and large areas can be inspected quickly. In most applications, a linen tester is set directly on the surface to be examined. Delicate surfaces can be protected by placing a clear piece of acetate on the subject and setting the magnifier on the acetate. Any reasonably thin sheet of optically clear acetate will not interfere with the examination. .

The comparator is a highly specialized type of Linen Tester. Rather than have markings on only the frame, the entire field of view of a comparator is filled with scales, test samples and information. A graphic arts comparator, for example, is marked to check type size, word spacing and other features with highly specialized scales used only in printing. Other comparators are available for many other specialized fields; some are adaptable to inspecting antiques, some not, depending on your interests.

The quality and power of lenses in linen provers and comparators varies. Lenses used for inspecting color printing and printed circuitry are fully corrected; lenses used for photo retouching and textiles may be distortion free but not color corrected. If your particular field of interest requires very precise measurement (say in hundredths of an inch) you may want to consider a comparator. If you can get by with accurate measurements to the nearest 1/16 or 1/32 of an inch, a good quality linen tester with markings on the frame should work fine. The price of linen testers is usually less than comparators. Prices of comparators depend on lens quality and how the lens is marked. Most linen provers fold flat for easy carrying; many comparators are mounted in rigid frames which can not be folded.

DIAMOND LOUPE (Hastings Triplet) "Diamond loupe" is the name commonly used for the fully corrected 10X loupes which meet the FTC's requirements for grading purposes. This style of magnifier uses three different pieces of glass to achieve the highest possible color and optical corrections. It is a necessity for working with diamonds and is the type of lens required in most professional evaluations of authenticity and condition for a variety of antiques and art.

Most diamond loupes are designed with a barrel shaped lens holder that swings in and out of a protective metal housing. With the lens swung out for use, the housing acts as a handle for your fingers to curl around and to help keep the lens in focus.

The advantages of this style magnifier are the accuracy of the enlarged image they provide and the high degree of magnification they offer. The disadvantages are a small field of view and a short depth of field. They also require practice to learn how to focus and use them correctly. (See "Using a loupe" which can be found at the end of this article.)

NON TRADITIONAL MAGNIFIERS

The lens in Fig. 14 is made of a sheet of plastic. It is non-breakable, flexible, light weight and available in many sizes up to 8 x 10 inches.

Deciding what to buy

An accurate, high quality magnifier should probably be considered "standard equipment" for antiques professionals and serious collectors. If you work with gemstones or prepare appraisals and statements of condition, you'll probably need some type of fully corrected lens such as the Hastings triplet. Decide what qualities you need in a lens, then shop for those features.

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Fig. 1 Background, traditional magnifying glass. In front are two loupes: a loupe with multiple lenses, left; a watchmakers loupe with a single lens on the right.

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Fig. 2 A Linen Tester or Linen Prover on the right and a Comparator on the left have built in scales and grids for making precise measurements.

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Fig. 3

Lenses magnify objects by bending light rays. The illustration on the top shows how an original image appears in the eye without a magnifying lens. On the bottom is an illustration of how the same object appears when viewed through a magnifying lens. By bending the rays of light, a lens makes the original image appear to be larger than it really is. The image created with a magnifying lens is called a virtual image.

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Fig. 4 Focal length and depth of field

Focal Length is the distance from the lens that an object comes into correct focus.

Depth of field is the area of the object that will be in focus.

EXAMPLE (A): You are looking along the teeth of a comb with a lens that has a 5" focal length. The tooth 5" away from the lens will be in focus (5" focal length). Also in focus will be an area in front and in back of that tooth (depth of field). The higher the power of magnification, the shorter the focal length and the shorter the depth of field. EXAMPLE B: The more powerful lens, only two of the comb teeth are in focus in the short depth of field. The lower power lens A has a greater depth of field which keeps 5 comb teeth in focus.

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Fig. 4-C The focal length of this loupe is 2".

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Fig. 5 Field of view: The field of view is the size of the area seen through the magnifying lens. As the power of magnification increases, the field of view decreases. The illustration above shows the approximate fields of view for a typical ring. Each increase of magnification reduces the area that is visible at the same time.

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Fig. 6 Spherical Aberration

Fig. 6 The illustration above shows a grid of straight lines as viewed through an uncorrected lens (A) and a corrected lens (B). In view A the straight lines bend in toward the center. All the lines in view B are squared.

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Fig. 7 Chromatic Aberration

Fig. 7 An uncorrected lens can add false colors or mask true colors. Contrast can also be affected. Illustration A above shows how dots used in full color printed images might appear through an uncorrected lens. The uncorrected lens A has added a halo of color around each dot and the exact outlines and intensity of the dots are unclear. Image B is the same set of dots viewed through a corrected lens. The colored halo is removed and the size and intensity of individual dots is clear and precise.

Types of lenses

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Fig. 8

Fig. 8 A-D These are the shapes of basic lenses and lens systems. (A) single lens, can achieve limited corrections with surface coatings; (B) double lens or doublet, uses a second separate lens for limited correction; (C) Coddington lens, a one piece lens whose thickness aids correction; (D) Hastings lens, made of three different pieces of glass cemented or joined together to form a single lens called a triplet. This lens, a Hastings triplet, provides the most accurate corrections.

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Fig. 9

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Fig. 10

Figs. 9-10 The illustrations above are side views of a gemstone with three flaws. In Fig. 9, at 10X magnification, there is enough depth of field to bring all three flaws in focus at the same time. At 20X magnification, in Fig. 10, though, there is virtually no depth of field. All three flaws will never be in focus at the same time.

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Fig. 11

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Fig. 12

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Fig. 13

Figs. 11-13 Close up view of scales of measurement on the base of a Linen Tester on left. These are 1/8", 1/16", 1/32", and 1/64". Most Linen Testers fold flat for easy carrying. Above, circular views show the field of view through two specialized comparators.

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Fig. 14 Magnifiers made of plastic sheets come in large sizes like the 8x10" piece shown here.

How to use a jeweler's loupe

A loupe (pronounced LOOP) is a useful tool to examine everything from diamonds to cast iron toys. With a little practice, almost anyone can learn how to use it properly.

For occasional use, it is more practical to hand-hold a loupe rather than learn to hold it in your eye like a monocle. Hand holding a loop also means you do not have to remove eyeglasses if you wear them.

You hold a watchmakers loupe between your thumb and forefinger (Fig. 1). Hold a diamond loupe the same way but with the extra step of wrapping your fingers around the lens housing to help support the loupe.

If you are examining a small object, hold the object in your free hand. Now brace your elbows against the sides of your body and bring both your hands up towards your face (you may brace your elbows on a desk or table top if you are seated).

As you raise your hands, bring the fleshy parts of your palms, the heels, together (Fig. 2). This creates a movable "hinge". Keeping the loupe close to your eye, pivot the hand with the object in and out until you get a sharp focus (Fig. 3). It is very important to brace yourself to insure a steady focus. If you cannot maintain focus, you cannot make accurate observations.

You will avoid eye strain by keeping your free eye open during an examination. This may seem awkward at first but comes easily with very little practice.

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Fig. 1

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Fig. 2

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Fig. 3