Introduction to Optics

[CharlesHarris]

The diameter of a fully dark-adapted, dilated pupil in a young adult of military age is about 7mm. The exit pupil of an optical system is determined by the unobstructed objective lens diameter, divided by its magnification, for example, for a standard "night glass" providing a exit pupil diameter of 7mm, a 4X rifle sight must have an unobstructed objective lens diameter of 4x7=28mm, a 6x sight 6x7=42mm, a standard Naval binocular 7x7=49mm, a night hunting scope 7x8=56mm.

If exit pupil is less than the fully dilated eye, the glass is still useful in daylight conditions where the iris of the eye is constricted, but a smaller exit pupil is less easily centered over the eyes, and is more apt to induce fatigue if the observer is standing on an unsteady support, such as the pitching deck of a ship, a moving vehicle or an aircraft in flight. For this reason, military optics are usually designed to provide an exit pupil of 7-8mm.

Brightness is determined not only by exit pupil diameter, but is also affected by light transmission losses through reflection. Anti-reflection coatings enhance brightness, but to be effective, ALL optical surfaces must be coated, not just the exterior surfsces you can see. Using Bundeswehr Zeiss 8x56 roof prism binoculars, I could readily identify feiend or foe symbols and read side numbers on tank turrets, which I was unable to do using an inferior 10x80mm gunsight, through smoke in evening nautical twilight. There is no substitute for quality. Check any prospective purchase for hoop or barrel distortion by counting bricks on a building across the street. Are the lines between masonry courses parallel? Are the blocks sharp, with parallel sides all the way to the extreme edges of the field of view?

Water tightness and resistance to fogging is tested in military optics by immersing completely under water, in bell jar, and pulling vacuum on the system, while watching for bubbles. Anti-fog properties are conveyed by proper sealing of the system. Nothing more than dry air in the clean room where te optics are assembled is needed. Evacuating systems and filling them with dry nitrogen is mostly advertising hype, for if the system is not sealed the nitrogen will leak out and be displaced by ambient air.

More important than magnification is resolution. Resolving power is determined by counting line pairs on a standard NATO resolution test target. I was able no