The modern three objective (that turret changer is ubiquitous) microscope so common in school rooms, medical offices, and amateur or professional labs around the world may easily contain several dozen individual optical lenses. A common 4mm objective might have as few as three or four optical components. It is not until an effort is made to correct aberration that the number of components starts to climb. A single objective corrected for some chromic aberration and flatness of field (a plan achromat to be technical) may be composed of ten or more lenses. Top of the line super corrected objectives (say one of the infinity corrected apochromatic immersion variety) might contain over a dozen at the low end and rapidly approach twenty optical components or more.
When one works with a device composed of so many precision optical lenses it’s easy for something as humble as the hand lens to seem unimportant, if it is considered at all. Why spare a moment to even think about such a simple and inexpensive magnifier when objectives formerly costing thousands of dollars are becoming more common and accessibly priced every day? The simple answer is of course that one can not always take their optical stand into the field (nor should one). With the recent proliferation of intensely portable field microscopes one might be tempted to forgo the purchase and use of a hand lens entirely, but in the long run it is often a better decision to ignore the flashy field microscopes instead.
A good hand lens can be as essential a component of a microscopists kit as anything. When identifying a specimen or searching for the best candidate for mounting, the few diameters of magnification provided by a hand lens are often indispensable. Able to be taken to the field with no more specialized equipment than a pocket or a lanyard, a hand lens is eminently portable. With an improvised stand, a hand lens may be pressed into service as a dissecting microscope or a bullseye condenser. It’s easy to see that with a low cost, small size, and myriad of uses, no microscopist should be without one.
Unfortunately for the beginner, it may initially seem as if there are as many configurations and names for the hand lens as their are subjects to view. Should one consider a linen tester, a burning glass, a loupe? What about these other things, I thought Coddington and Hastings were towns in England?
While the appearance of a hand lens may be nearly unlimited, it is absolutely not the case optically. There are only so many ways a lens or series that will fit in a small package can be arranged. Once composed that optical lens (or series) may be fitted into nearly any handle, and that is where the overwhelming variety is rooted. The case or fitting of the lens is determined more by convenience or the intended use of the hand lens than its optics. The heavy brass and glass Sherlock Holmes style reading glass so often pictured when one thinks of a hand lens is no different optically than any of a dozen plastic bodied magnifiers one might pick up from the corner druggists. Well, there is certainly one significant difference, and it will be the first considered.
Glass or Plastic
When considering the choice between a glass or plastic hand lens many people will assume glass to be superior optically and plastic more economical. Eyeglass wearers will immediately see things differently. Few are the optometrists who will not sing the praises of expensive polycarbonate or optical plastic lenses. Throwing around terms like impact resistant, shatter proof, feather-weight, or scratch-free, as if they mean anything is roughly as helpful as describing ones pick-up truck as “Like a Rock.” There will be endless talk about the fragility and weight of glass, how prone it is to scratching, and whatever else can be said to sell an expensive plastic lens.
The simple fact is that glass is superior optically, period. Naturally, glass can shatter, but that is only because it is so incredibly hard. High quality optical glass may easily rate a seven on the Mohs scale of hardness. Only substances as hard or harder are able to damage the surface of a glass lens, so it’s easy to see why rating a seven on a scale which only goes up to ten is desirable. Glass does tend to be rather heavy though so if you simply must have a very large detective style hand lens with you on every trip to the backyard or pond plastic may be the way to go. However, one must be cautioned that any plastic lens with handled roughly or wiped with a dusty shirt tail will rapidly become a cloudy, scratched mess.
Some high quality plastic hand lenses are available with various special coatings and treatments exactly like the ones they provide for eyeglasses. Such coatings often cause the lenses to cost more than a similar glass lens. A small price tag being in my opinion the chief point in plastics favor, I have never been willing to give it up for an ounce or two of diminished weight. I very much recommend going with glass optics for any hand lens.
The Optics
Now the best part! The optical quality of the various configurations of hand lenses is surprisingly varied, but tend to be of three basic types only; the single biconvex lens, the Coddington magnifier, and the Hastings triplet. Each has points in its favor and drawbacks, times when it shines and times when it’s use is infuriating.
The vast majority are composed of a single simple biconvex lens, generally with a field of view spanning an inch or more. A positive lens (also called a convergent lens) such as this almost never provides a level of magnification of over seven diameters. High levels of magnification require a shorter focal distance and must be held close to the eye. In general anything providing over five diameters will be called a loupe and will be fitted into a housing convenient for placement very close to ones eye. Nevertheless it is not uncommon to see simple magnifiers marketed as providing ten diameters of magnification, and they may but it is of dubious value.
Single lens magnifiers naturally provide the worst quality image (the most optical aberration) but are often the largest when it comes to field of view. When less than five diameters of magnification is needed a simple biconvex hand lens can serve nicely for most purposes. Lower levels of magnification suffer from less aberration and many botanists and etymologists find that a two or three diameter lens with large field of view (providing for a long focal distance) can be just the thing for field work. There’s an enormous variety out there but the majority are simply a bare lens with some sort of handle so care must be taken that the lens is not damaged.
It is also rather common to find multi-lens units which feature a number of simple biconvex ( or rarely plano-convex) lenses which may be slid out of the handle for use, and are protected by the handle otherwise. In units of this type each lens generally provides a different degree of magnification. Multi-lens models of this sort often claim to provide compoundable levels of magnification, each lens singly and then in conjunction. Although multiple biconvex or plano-convex lenses used in conjunction can provided compoundable magnification, I have never seen a hand lens of this sort which provided for the necessary space between the lenses to do so effectively. Do not buy a double lens unit with a two diameter lens and a five diameter lens thinking it will provide ten diameters when using both in conjunction.
When performing non-critical work which requires five diameters of magnification or less single lens unit will do nicely. In some cases the spherical and chromatic aberration of such a lens is unacceptable. Other times, five diameters is simply not enough magnification. A step up in quality (and complexity) from the single biconvex lens is a funny little outfit known as a Coddington magnifier. It’s another single lens outfit but a trick of optics lets it provide a higher level of magnification while stopping back much of the aberration that limits other single lens units to about five diameters of magnification.
In a Coddington magnifier one finds a single biconvex lens of significant thickness. Unlike the simple biconvex lens style discussed above, a Coddington magnifier is often asymmetrical, having one optical surface that is significantly more convex than the other. This asymmetry can be traced back to another type of hand lens known as a Stanhope which is almost never seen today. In any case a Coddington magnifier is able to provide significantly more magnification than other simple biconvex lenses, up to about twenty diameters, because its optical surface is highly convex. As described to this point a Coddington would still provide a high level of aberration, to eliminate this aberration it is removed rather than corrected. A deep grove is cut perpendicular to the optical axis and this equator effectively blocks the more distorted rays of light (those farther from the optical axis) which would have passed through the lens into the eye of the user. Although this feature effectively limits much of the aberration present it also drastically reduces the field of view so that a one inch Coddington magnifier providing five diameters of magnification may only have a field of view of a few sixteenths of an inch. A Coddington will often cost a bit more than a simple two or three lens biconvex unit, but if more than five and no more than fifteen diameters of magnification is needed can be worth it. It’s not uncommon to see Coddington magnifiers boasting magnification of twenty or even thirty diameters but due to the nature of the lens it’s really not worth exceeding ten diameters when purchasing a Coddington.
The other common option is the Hastings magnifier, also called the Hastings triplet. As the name suggests this is a series of three lenses (two convexo-concave lenses and one biconvex lens) cemented into a single optical unit. Rather than stop out aberration as in a Coddington, a Hastings magnifier corrects for most spherical and some chromatic aberration, producing a very flat, bright, field or view. This means that a Hastings magnifier is able to provide a much higher level of magnification without losing either image quality (as in a single biconvex magnifier) or drastically reducing the field of view (as in a Coddington magnifier). A Hastings triplet of ten or twenty diameters magnification is the style one will see slung around the necks geologists or bryologists tromping about in the field, and is what I would recommend for anyone who simply must have a high level of magnification.
Hastings triplets are often available which provide from five to thirty diameters of magnification, but there is never a reason to purchase one in excess of twenty diameters magnification. I’m told a Hastings triplet of ten diameters is often used for the grading of gems, and I can’t help but think this has led to the increasing demand for higher and higher levels of magnification (the buyers being amateur prospectors I suspect). The Hastings magnifier is the finest and most complex optically of those mentioned and it is also the most expensive. Bargain hunters may seek out and find models of Hastings magnifiers that are sold at a price drastically lower than those that can be found elsewhere. On occasion these may prove to be wonderful finds, but more often they are poor quality and will be found to be drastically over-corrected when put into use.
The End (Finally)
Well this got rather long, so I’ll try to wrap things up quickly. The best hand lens is the one you’ll carry with you. It’s not worth spending a tidy sum on a fancy decorator piece if you’ll be unwilling to risk it in the field and it’s delusional to think you’ll never want to. If that means using a cheap plastic magnifier (nearly all seem to be lighted as well these days) and buying a new one when it gets scratched to death, thats just fine! They’re cheap, easy to use, and work just fine. I’ve had at least a dozen over the years and they are a great place to start, especially for children. Don’t rush out an buy a fifty dollar Hastings (or Coddington) for junior because he said he likes bugs. Coddington and Hastings magnifiers have very short focal lengths (higher magnification makes it even shorter) and must be held very close to the eye to work their best, most children (and many adults!) find this both hard to remember and uncomfortable.
If you’re ready for something more than a cheap plastic magnifier or absolutely need to have more than five diameters of magnification, go ahead and get a Hastings. The larger field of view compared to a Coddington makes them easier to work with and although Coddingtons fill a demand for less expensive higher magnification lenses, a good Hastings is worth the added expense. Always be sure to deal with a reputable dealer who will provide a refund if the optics prove to be poor quality. When in doubt pay a little more and buy from a known manufacturer like Edmund Optics or Bausch & Lomb.
Well, I hope I’ve cleared up a few things about hand lenses for those whose interest in microscopes is just starting, or even old hands who have simply never considered a hand lens as worthy of attention before.
vade mecum microscope 