BINOCULARS
Binoculars can be very useful in scanning. The wider their field of view, the more effective they are likely to be. Binoculars chosen for use in the field should be of high quality for many reasons. One of these is that they will be given very rough treatment, and misaligned binoculars cause rapid eye fatigue. General optical quality, including good resolution, will help ward off early eye fatigue. Binoculars with individually focused eyepieces, usually made for marine use, are sealed better against dust and water than are most center focus types (there are exceptions), but are less convenient to focus.
Lightweight binoculars and those with power no greater than about 7 or 8 are easiest to hold and thus can be used for longer times without fatigue. The binoculars should be steadied as much as possible by supporting the arms firmly. This can be done, for example, by sitting on the ground with the arms against the knees, or, if standing, by holding the arms tightly against the chest or by using a natural support such as a tree. Simply steadying the binoculars by placing the thumbs on the cheekbones is nearly as effective; this is a particularly useful trick for observers wearing eyeglasses when the binoculars cannot be steadied against the eyebrows. To avoid early eye fatigue, binoculars should not be used steadily aboard aircraft because of vibration and motion; they are useless aboard helicopters.
A high powered spotting scope is sometimes of great use, but should be supported very securely, and an observer should stare through one for a limited time, say less than an hour.
Binoculars may be used to great advantage at night. One measure of the effectiveness for night use is the “exit pupil,” which is the diameter of the bundle of light rays exiting towards the eye from the eyepiece of the binoculars. This number is simply found by dividing the objective lens diameter by the power, e.g., for 7X35, 35/7 or 5 mm exit pupil, and for 7X50, about 7 mm. Since the pupil of the eye is about 7 mm diameter when fully dilated, the eye is not being fully used if the exit pupil of the binoculars is less than 7 mm. Thus 7X50 binoculars are a good choice for night use. Note that for daytime use a small exit pupil is satisfactory since the eye pupil will be only about 2 mm in diameter; however, the large exit pupil is advantageous because of the ease of centering the eye behind the eyepiece. Note also that the larger the objective diameter, the more light is gathered. Another measure of night usefulness is the “twilight factor” which is the square root of the product of the objective diameter and power. Thus the effective binocular brightness at night depends on both the exit pupil and power, and also on the overall optical quality of design and manufacture.
In use at night, the binoculars should be held aimed straight forward, and the eyes turned off center, to avoid using the insensitive center of the retina. This technique requires practice, but is quite effective.