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.
For reading signals at short ranges, say, up to 5 miles, these glasses are better than the telescope. Flag signals have frequently been read with glasses of this description at a distance of 10 miles.
_Light._--The illumination of an object when observed with the unaided eye is impressed upon the retina with a brightness in strict proportion to that of the object itself. If an object be viewed under equal illuminating conditions alternately with the naked eye and with a glass, the brightness of the image seen with the naked eye may be represented by 1, while that of the image in the glass will generally differ, being greater or less.
The light of the telescope or field glass is expressed by the number which shows how many times brighter the object appears through the instrument than to the naked eye. Light is a function of the dimensions of the object glass and of the power of the instrument, and is sometimes determined by dividing the square of the objective aperture (expressed in millimeters) by the square of the power.