While cleaning the garage a couple days ago I found my long lost hand lens. It was a vital piece of equipment for me as a geology student and as a geologist, and it has similar useful applications for people involved with biology and tree research. I am not talking about the old fashioned magnifying glass, but a hand lens.
The photo above shows a typical hand lens. This is an example is a hastings triplet 10x magnifier. It is one of several types of similar design. According to Ben Meadows:
Hastings Triplet - Highly corrected magnifiers feature three separate optical glass lenses bonded together to form a compound lens. Provide a sharp, distinct image especially around the edges of viewing area. Also give high color correction. Plastic mount swing-away, nickel-plated case protects lenses and serves as a handle. $41.90 http://www.benmeadows.com/search/magnifier/20693/
Coddington Magnifiers - Single-thick lens of optical glass with a central groove diaphragm provides high correction and produces a sharp image http://www.benmeadows.com/store/Laboratory_Products/Laboratory_Equipment/Microscopes_and_Magnifiers/20695/ $23.55
Doublet Loupe Magnifier - Glass lens mounted in nickel-plated case which slides into nickel-plated brass body for safe storage. $9.30 http://www.benmeadows.com/search/hand+lens/20694/103184/?isredirect=true
My model is a Hastings Triplet 10x made by another company many years ago. It is the most expensive of these types, but you pay for the quality you get. For looking at details on leaves or tree rings any of these models should be adequate and would be a useful tool to have.
If you look at the descriptions of many tree species the text describes the presence or absence of hairs on part of the leaf, glands on the leaves, tiny details of buds, various features of leaves, or flowers, most of which are hard to see unless you have perfect vision. Tree rings when tightly packed are difficult to distinguish. These simple hand lenses will let you see these fine details easily in the field.
They are small and can be carried on a loop or lanyard around your neck so that they are always ready for use, or stuffed into a pocket. Most are smaller than a nickel in diameter and perhaps a half an inch thick.
Kooter's Geology Tools page describes "How to Use A Hand Lens" and pretty much has it right on the money:
A hand lens is optically related to a microscope. So just like a microscope, your eye should be close to one side of the lens and the object will be close to the other side of the lens. With a little practice you will get the hang of manipulating the object and hand lens to get things into focus. We have seen people use a hand lens like a reading glass magnifier, with the hand lens held away from the body. It works up to a point, but you won't see the same thing as you would by using it properly. The main problem with using a hand lens is getting the right light on the object. In bright sunlight you will have to maneuver around to find a position where the object is not in your own shade or the shade of your hat, but the bright sun is not shining into your eyes and making it difficult to see.
As for what magnification to get - 10x is a good all around option. It gives a good magnification but still has a workable depth of field. Those with stronger magnification have shallower depth of field that is in focus and require more light to see your subject well. For many applications 7x would also be acceptable, it has less magnification, but greater depth of field. So it is also a viable option.
Since you are looking through the lens when close to your eye, you need one big enough to easily see through, but it does not really need to be any bigger than that. The extra size doesn't gain you anything. For most uses a diameter of 13 mm to 18 mm is about right. If you look at the photo above the hand lens has two parts. The first is a lens mount of plastic, brass, or steel that holds the lens in a a short tube for viewing. This is mounted on a pivot that lets it spin around into a metal casing that protects it from damage and scratching when not in use. This is the style that tens of years of field work by innumerable people have found to be the best for field use.
I would encourage each of you to purchase one for use in the field. Remember that you can also use it to look at other things besides leaves while in the woods.