Magnifiers for Insect Monitoring
Courtesy of Dr. Alan T. Eaton, Extension Specialist, Entomology
Entomology Hand lenses are small inexpensive optical devices that are easy to carry on a lanyard or folded up in a pocket. Most of them magnify 5 to 20 times. The correct way to use one is to hold it so that the lens is an inch or so in front of your eye. This is an important point. Holding the lens far from your eye barely works…just barely. Also, the field of view is severely restricted if you hold the lens far from your eye.
I rest the hand holding the lens against my cheek bone, to ensure it stays close, and at a constant distance. Then, with my other hand holding the specimen, I bring the subject up to the lens. I usually touch my two hands together once I’ve found the correct distance between lens and subject. That keeps it there, at the correct distance. In most cases, the specimen will be in focus if it is one to three inches from the lens. Hold it parallel to the surface you’re examining. If you hold your head down to do these things, you’ll be shading the subject, so it is usually better to hold the subject up, and turn so the light illuminates the subject. A hat or long hair can also shade things, so you may wish to remove or reverse your cap and tie up loose hair.
I usually recommend buying a lens with no more than 10x magnification. The reason is that higher magnification creates a shorter focal distance, making it much harder to use the lens. Also, high magnification means a very small field of view. The correct way to use one is to hold it so that the lens is an inch or so in front of your eye. This is an important point. Holding the lens far from your eye barely works…just barely. Hand lenses with a lens diameter that are relatively large… ¾ inch or so is about the biggest hand lens you’ll find. Tiny ones are really difficult to use. Most hand lenses are built so that they fold up when not in use, to protect the lens surface from scratches. I use 10X lenses to look at small things… look at buds on trees, identify mites and aphids and look for small details on insects.
Magnifiers are usually larger than hand lenses. They usually offer magnification in the 2x to 8x range. The larger lens size means you have a much larger field of view, compared to using a hand lens. I especially like the large rectangular ones…with lenses about 4inches long. That gives you the opportunity to use both eyes if you wish. I find it much more useful to hold the magnifier relatively close to my eye, compared to holding it far away. If you hold it far away from your eyes, and have the subject only an inch or two below the magnifier, you’ll get almost zero magnification! [In other words… don’t do that!]
Some magnifiers have a long handle, and do not fold up. That means it can be convenient to put one in your pocket, with the handle sticking out. That’s quick and convenient, but be sure nothing else hard is in your pocket, or you’ll scratch the lens surface. That’s a common way to ruin one, especially if it has a plastic lens. A few magnifiers offer battery powered lights attached to them. Some folding pocket magnifiers have two or three large diameter (1.5 inches) lenses. By stacking one lens over the other, you increase magnification, but you also increase the possibility that dirt or scratches will mar the view. I like them, even though they are a bit expensive. I use 2x two by four inch rectangular magnifiers (with a small 4x insert lens built in) to count insects on sticky cards, look at apple maggot traps, examine blueberry fruit fly traps and other tasks. When not in use, mine usually stay in their cases. The folding pocket magnifiers get folded up and placed in a pocket. They’re so expensive, I usually place mine in a pocket that has a button or zipper to close it. That way, it won’t fall out of my shirt pocket when I lean over. A lanyard is useful.