A Beginner's Microguide to the Hand Lens
Serious study of natural history (especially botany and geology) requires a hand lens for use in the field. The art is always having it with you as you never know when you will need it. Often the hand lens is strung on a shoelace and worn around the neck, but there is a lot to be said for attaching it to your key-ring as a fob - that way you will always have it with you, and it gives your hand lens the same importance as your house- or car-keys.
The most useful magnification for the beginner is x10. Anything less than x10 is hardly worth it. For specialist applications (particularly the identification of mosses) x20 is useful. A compromise is x15. Above x20 you should use a microscope, as at high magnifications the depth of field, field of view and working distance are reduced and the demand for good light is increased. Hand lenses are sold up to x30 but this magnification can only be used under optimal conditions.
The wider the hand lens the wider the field of view. However there is a trade-off between magnification and size: high-curvature/high power lenses are more easily made small whereas low curvature/low power lenses can be wide. Combining high power with width is difficult and such lenses tend to be expensive. A typical x10 hand lens ranges from 12mm to 20mm. On the other hand x20 lenses are rarely wider than 12mm and may be as small as 7mm. Power comes at the expense of field of view.
This is the main determinant of cost and quality. There are four main types of magnifier: single lens, doublet, achromatic, triplet. A single lens (i.e. a simple magnifying glass) is only suitable for low power work (x2 to x6) as at higher power (higher lens curvature) spherical and chromatic aberrations become problematic. Cheap hand lenses therefore use a doublet arrangement - two lenses in juxtaposition. For the beginner a simple doublet lens may be perfectly adequate. A better arrangement is the achromatic lens, in which two lenses of different types are cemented together in order to provide very good correction of chromatic aberration. Better still is the triplet lens in which three lenses are cemented together for excellent all-round optical performance. If you want the best, get a triplet hand lens made by a famous optics company like Bausch & Lomb or Zeiss.
Hand lenses can cost anywhere from $2 for a simple plastic lens to $200 for an optically perfect triplet lens with special coatings. The student and beginner may be able to get a perfectly serviceable basic hand lens for around $10 (and certainly under $20). The serious user will wish to choose a high quality hand lens that is optimal for their particular use and may wish to spend around $50. Triplet lenses usually start around $50 but a good quality achromatic lens may be preferred.
For students at UBC the Beaty Biodiversity Museum shop sells a basic doublet hand lens for $11.95 plus tax (information correct as of July 2013). A greater range of choice can be found at Deakin Equipment in Vancouver (1361 Powell Street - just East of Clarke Drive).
Use of a hand lens
The hand lens should be held close to the eye and kept still. Focus the specimen by bringing it up towards the lens until it is in clear view. It is common to see beginners trying to focus by waving the hand lens back and forth. This is very inefficient: the performance of a hand lens is best when it is close to the eye. Keep it clean and dry and free from abrasion. Above all, keep it on you.