Arthropods, including plant pests and biocontrol agents, are often difficult to identify with the naked eye. A hand lens, also called a loupe, is therefore an important tool for a grower, scout, horticulturalist, and others who want to identify arthropods in the field and make informed decisions. However, it can be difficult to choose a proper hand lens as catalogue descriptions are often filled with undefined jargon that is difficult for people unfamiliar with it to interpret. This article examines hand lenses and the terminology associated with them in an effort to dispel this confusion. Finally, please note that while specific brands and makes of hand lenses are mentioned, their inclusion does not imply recommendation or endorsement by the author or Penn State University.
When comparing hand lenses, the terminology contained in the name (e.g., Hastings triplet magnifier, 10X, 19.8mm, Focus: 1″) can be confusing. However, each element of the name contains important information that describes different aspects of the hand lens and is easily understood once defined.
Lenses. Hand lenses typically contain one to three lenses (other imaging systems, such as microscopes and telescopes, can contain six or more lenses). The addition of multiple lenses corrects for various image aberrations but also increases the cost of a hand lens. A singlet hand lens contains a single lens; in many models the lens is convex at either end (Fig. 1). Uncorrected singlets have low magnification power (most up to 5×, high quality up to 10×) and suffer from high spherical aberration (an incompletely focused image) at higher magnifications; the common magnifying glass is an example of an uncorrected singlet lens. A Coddington lens is a type of single lens that is constructed from a thick piece of glass with a diaphragm grove around the equator that corrects for spherical aberration. A Coddington lens is capable of magnification up to 20× before significant aberration is encountered; this gain in magnification is offset by a reduction in the area seen through the lens due to the diaphragm groove (Figs. 4, 11). A doublet hand lens contains two lenses; some lens arrangements can correct spherical and chromatic aberrations. The lenses can be cemented together or spaced apart with air between them (Fig. 2). This type of construction is uncommon in hand lenses. When encountered, one of the lenses is often a Coddington lens. A triplet hand lens contains three lenses and has the best correction of spherical and chromatic (colored fringes around image) aberrations (Fig. 3). Triplet hand lenses are generally capable of magnification up to 20× before significant aberration is encountered. Hastings triplets are the most common type of triplet system in high-quality hand lenses. In Hastings lenses, the three individual lenses are cemented together with a clear epoxy. A quadruplet hand lens contains four lenses. Quadruplet hand lenses are not commonly encountered, although BelOMO offers a 20× model.
Lenses can be made from glass or plastic, which is typically acrylic. Glass lenses have the benefits of better clarity, higher potential magnification power, and are scratch resistant, but are heavier and generally more expensive. Plastic lenses are inexpensive, but are not capable of high magnification power, do not correct for various aberrations, and may scratch easily (although some forms are scratch resistant).
Magnification is the process of making an object appear larger and is expressed in terms of magnification power, which is the ratio between the apparent size (as viewed through the lens) and true size of the object. Hand lenses typically have magnification powers of 5×, 10×, 14×, and 20×. Some Hastings loupes can achieve magnification of 30×. Magnification power is directly tied to the size (diameter) of the lens, which is generally expressed in millimeters (“19.8mm” in the example given above). This relationship is due to the fact that magnification power is a result of the curvature of the lens. As lens diameter increases, lens curvature and magnification power decrease, and as lens diameter decreases, lens curvature and magnification power increase. Because of this relationship, hand lenses with higher magnification power have smaller fields of view, which may limit usefulness when viewing larger objects.
Other factors tied to lens size are working or focal distance (“Focus: 1”” in the example given above) and depth of field, both of which decrease as lens size decreases. Working distance is how far away from an object you must hold the hand lens so that the object is in focus. Working distance is not always included in the description, but is typically ¾ to 1” for 10× hand lenses and ½” or closer for 20× hand lenses. Depth of field is the distance between the closest and farthest part of an object that are in focus (Fig. 5). With a long depth of field, the entire object may be in focus and with a shallow depth of field only a thin slice of the object may be in focus. In addition to being affected by lens size, lens construction (e.g., singlet, triplet, Coddington, Hastings, etc) can affect depth of field.
When used for arthropod identification in the field, the following generalities can be made about the various magnification powers available (Fig. 6):
5× magnification loupes are not much better than using nothing at all.
10× magnification loupes will likely be the most used and useful as they provide enough magnification to see important details on small arthropods while having a large enough lens size to view of most of the arthropod.
14× magnification loupes may be useful for smaller arthropods and be used occasionally.
20× magnification loupes will rarely be used in the field as the field of view is often too small, especially when viewing fast-moving arthropods; the depth of field is short; and the short working distance makes it so that much of the light is blocked by the hand. Still, a 20× loupe does not take up much space and may be useful on those rare occasions it is needed.
30× magnification loupes are used almost exclusively by jewelers and gem graders. If magnification above 20× is needed to arthropod identification, it is best to bring the specimen in from the field and use a stereomicroscope.
Finally, caution should be used when selecting a hand lens to purchase, especially inexpensive models, as many generic or off-brand examples do not provide the magnification power or lens count they advertise (Figs. 7–9). Some ways to avoid being scammed include buying a hand lens made by a reputable company (e.g., Bauch & Lomb, BelOMO) and/or from a reputable supplier (e.g., BioQuip, US Geological Supply). If a hand lens is to be purchased through a website that allows sales by third parties (e.g., Amazon), be sure to read the product reviews as they will often indicate if a particular model does not offer the magnification power or quality advertised.