To choose the correct magnifier for the job, first determine what tools are to be used on the job; then determine the size and the character of the subject; and finally, analyze the object's surface character. Then review the following aspects of magnifiers:
Number of Elements
A single lens is satisfactory for low powers. Higher power magnifiers require two or more lens elements for improved resolution and correction of chromatic or other aberrations.
The distance from the magnifier to the object viewed is the working distance. This distance is an important consideration with regard to the type of work that must be done under the magnifier. If your work requires the use of tools, a magnifier with a long working distance will allow enough space to both use the tools and comfortably view the object. Small working distance magnifiers with higher powers are preferred for close-up inspection work.
Field of View
The field of view is the area seen through the magnifier. As power increases, lens diameter and field of view decrease. At 5 power (5X), field of view is about 1.5". At 10 power (10X), it is about 0.5". Usually, it is best to use low power for scanning larger surfaces and high power for small areas.
The maximum distance the eye can be from the magnifier and still provide a full field of view. Longer eye reliefs generally provide more comfortable viewing.
Depth of Field
The distance between the closest and furthest points at which a magnifier in a fixed position stays in focus. The depth of field decreases as power increases.
10" is assumed to be the closest distance the human eye can focus for comfortable vision. An object only 1" from your eye would be 10 times larger, but out of focus. A magnifier's function is to assist your eye in focusing closer. Since a 1" focal length lens brings clear vision down to 1" from the eye, an object at this distance is clearly seen and appears to be 10 times closer than it does when viewed from 10" away. Such a magnifier is commonly called a 10X or 10 power. Using this definition, the magnifying power of a lens can be approximated as follows: MP = 10/FL if the focal length is specified in inches. If the focal length is specified in mm, the formula will be MP=250/FL.
Actual magnifying power will vary slightly, depending upon working distance, eye relief distance and the characteristics of the observer's eye.
Size VS. Power In Magnifiers
A perfect magnifier would be lightweight, have a large diameter, provides a wide viewing area, and offer high, distortion-free magnification. However, incorporating all of these features into one unit is optically impossible. The magnifying power of a lens depends on its focal length (fl). The focal length, in turn, depends on the lens curvature; the greater the curvature, the shorter the focal length and the greater the power. In the design of a simple, inexpensive magnifier, the lens diameter will typically decrease as the curvature increases to provide higher power. Conversely, as the curvature is decreased to lower the power, the diameter generally increases with a resulting increase in viewing area. In addition, distortion generally increases with an increase in curvature. Thus, a magnifier with a large diameter typically offers more viewing area and less power. So, both wide field of view and high magnifying power cannot be incorporated into a single design without elaborate, weighty, high-cost lenses.
Quality of Magnifier Systems
How much you spend on a magnifier should be determined by the application for which it is being used. Buying the least expensive magnifier could lead to unsatisfactory and frustrating results. Any magnifier you buy should be a tool to fit the rigors of the environment in which it will be used. Additionally, it may be unwise to expect the same magnifier to satisfy the requirements of several functions. Below are several factors affecting the quality of the magnifier, as well as the function for which it is best suited.
Flatness of Field
Due to physical laws, the outer part of the image formed by a simple lens may appear out of focus. This is caused by the curvature of the lens. The greater the magnification - and the greater curvature of the lens - the greater the problem. This can be easily overcome by designing a magnifier that has more than one lens. A triplet has a "flat field" which means the entire area of view is in focus and undistorted.
Because of physical laws, the lens may produce a prism effect giving the image false color fringes known as chromatic aberration. Simple lenses focus various colors at different points. Achromats with two simple lenses cemented together correct this by causing many colors to focus at the same point.