The power is also defined as the ratio of the focal distance of the object glass to that of the eyepiece.
The power of a field glass can be roughly determined by focusing the instrument on a wall or a range rod, by looking at the object through the instrument with one eye and at the same object directly with the unaided eye. A comparison of the diameter of the two images gives the ratio.
The power of a telescope or a field glass can more accurately be measured by means of a dynameter, which is a microscope which can be fitted over the eyepiece end of the instrument, and which magnifies the image. The end of the dynameter next to the eyepiece of the instrument is ruled with a series of lines one-hundredth of an inch apart. On focusing the dynameter, the image of the emerging pencil appears as a sharply defined ring of light with the magnified scale of the dynameter across it.
The number of subdivisions covered by the diameter of the ring of light is noted. The diameter of the object glass is similarly measured by means of a pair of dividers and read to the hundredth part of an inch.
The ratio of the diameter of the object glass to that of the image as seen in the dynameter gives the power of the instrument. This method is not applicable in the case of the Galilean telescope or the field glass consisting of two Galilean telescopes, due to the fact that the rays from the eyepiece of the Galilean telescope are divergent.
Field glasses in which the image appears magnified from one to six diameters are known as "low-power" glasses. Field glasses which produce an image magnified over six diameters are termed "high power."
For the mounted man a glass of but 4, or at most 6, powers, can be used with advantage; on foot, with free hand, instruments of not to exceed 10 powers can be used. If more than 10 powers are desired, a holder becomes necessary, and if the holder is intended to be portable a greater power than 50 is not practicable, as the movement of the air or the slightest touch of the hand sets up vibrations that render clear vision impossible.
Field glasses with low magnifying power, which are usually preferred by ordinary observers, have their chief value in the comparatively extensive field of view; they should be used to observe extensive movements, where large tracts of country must be taken in one field of view or in sweeping the landscape to find the tents of the enemy, their wagons, etc., or other objects, to be afterwards more closely examined with the telescope.
They may be used on shipboard or in boats, where the rolling motion interferes with the use of the telescope; also on horseback or in hasty examination made on foot or in trees, and generally for all observations not critical or those to be made under circumstances where the telescope can not be conveniently handled. The field glass ought to be held by both hands when in use, and to steady it the arms should be kept close to the body.