Magnifiers, whether pocket, handheld, loupe or hands free all follow some basic rules of optics. When ordering, keep these rules in mind.
As magnification goes up in a magnifier, lens size goes down. A 2x magnifier, such as the Bausch & Lomb 2" x 4" Rectangular 2x has a lens size of 4 inches. This is suitable for reading a book or text or just taking a look at things in general. On the other hand, a 10x magnifier, such as the Nikon 10x Jewelry Loupe Jeweler Pocket Round Magnifier - 6947 has a lens size of about a 1/2 inch or less and will therefore offer a much smaller "window" to see things. It is great for high magnification and a super close look at many objects, but it will not perform as well if reading a book. Unfortunately, you can't have it both ways, so you need to decide if magnification or lens size is more important for your application.
As magnification goes up, focal distance goes down. Think of focal distance as the distance the magnifier needs to be from the subject to be in focus. A 10x magnifier, such as the Nikon Magnifier above, will have a very short focal distance, usually about one inch or less. In other words, the magnifier must be held one inch from the subject to properly focus and at such a short focal distance, there will not be much room for error. A fraction of an inch too close or too far will cause the magnifier to go out of focus.
On the other hand, a large, 2x magnifier will have a much greater focal distance, often as much as 5 inches and it will be much more forgiving as to the exact distance. You can be off a couple of inches either side of the listed focal length and still be in focus, though you may not get maximum magnification or clarity.
Unique Optical Instruments
By the way, magnifiers are not designed to be used to see distant objects. If you are trying to see objects more than a foot or two away, you need to look at a monocular or binocular, some of which can also focus within two feet.
In one respect, magnifiers are unique optical instruments in that the magnification (how many times the image is enlarged over normal) can change as you hold the magnifier closer or farther from the subject, though you are much more likely to notice this phenomenon with low power magnifiers. Since magnification can change with the distance the magnifier is held from the object, the lens in a magnifier is sometimes given a diopter rating. Diopter refers to the strength of the lens, not the magnification and it rates all lenses as if they had the same focal length. Overall, though, no need to worry about diopter unless you have a specific technical need to do so. Just check the stated magnification and focal distance if listed and go from there.
Text magnifiers are used to enlarge text. Some, such as the Bausch & Lomb Magna-Page Magnifier 81-90-07 are page size, but very low power (2x) because of their size, but most, such as the Bausch & Lomb Magna-Bar Magnifier 2x 81-26-17 are much smaller, covering 4 lines of text at a time. All are designed to be laid flat on a page.
Hands Free Magnifiers
Hands free magnifiers are widely used by hobbyists to work on projects that require the use of both hands. They can be as simple as a magnifier on a stand, or they can be visor mounted units such as the Bausch & Lomb Magna Visor 81-42-00.
Wallet and billfold magnifiers are the size of a credit card and some, like some Carson Magnifiers are even flexible, so no cracking if you sit on it.
Choosing a Magnifier
Choosing a magnifier can be daunting at first, but they make great gifts and are incredibly useful to have around the house. Everyone needs a magnifier now and then.
A loupe is a magnifier designed to be held close to the eye, though the term is often used for magnifiers in general. You can hold a loupe at arms length over a subject and it will still work, but the field of view (amount of the subject you can see) will be very narrow. If you want the entire field of view, hold the loupe and the subject up close to your eye. The older traditional loupe is placed and held with the socket of your eye as in an old fashioned monocle, but these days most loupes are handheld and typically have an attached, folding case which doubles as a handle.
Uses for Loupes
Loupes have many uses. Jeweler's loupes, such as the excellent Nikon 10x Jewelry Loupe Jeweler Pocket Round Magnifier - 6947 offer the highest optical correction since subtle differences in gem color greatly influence gem value. Film loupes are also very high quality and are used to evaluate photo negatives and slides. Botanists and naturalists also use loupes to study flower and insects and a wide range of natural wonders. A good magnifier choice here would be the less expensive Bausch & Lomb Coddington Magnifiers which holds your insect captive long enough for a good look. Measuring loupes, such as Bausch & Lomb 10x Lenscope Magnifier B&L 10x have scales of varying types mounted in the magnifier to measure very small items (scales sold separately at Bausch & Lomb Scales). Since most loupes are fairly high magnification, they are also quite small and fit easily in a pocket.