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Optical Terms & Axioms

Exit Pupil: The objective (front lens) diameter size in millimeters divided by the power. Exit pupil describes the image that is projected to a point in space beyond the eyepiece. This is the point where your eye must be positioned in order to see the full, clearly focused image. The relationship between the dilation and contraction of your eyes and the size of the exit pupil determines light gathering potentials. The human eye pupil diameter ranges from about 2mm in bright light, to a maximum of about 7mm in total darkness.

 

 Eye Relief: Eye relief is the distance in millimeters between the glass surface of the eyepiece on which the exit pupil is reflected and that point in space beyond it where the user's eye must be positioned to see the full image.
 
De-alignment: De-alignment refers to a shift in the position of the lenses caused by excessive blows and jarring. A de-aligned optical system will perform poorly or not at all.
 
Dioptric Correction: Dioptric correction refers to the adjustment of the optical instrument to the varying visual acuity of a person's eyes to make images at varying distances sharply visible.
 
Focusing: Focusing refers to the adjustment of an optical instrument in order to make images at varying distances sharply visible.
 
Interocular Distance: Interocular distance refers to the distance between the center of the eyepieces. The distance between the center of the viewer's pupils should equal the Interocular Distance setting.
 
Magnification: A number followed by a x denotes the intensity of magnification. The number indicates how many times larger an object appears. For example, when observing a target at a distance of 1000 m through a 25x binocular, the object appears only 40 m away. When observing an object at a distance of 1000 m through a 40x binocular, the target appears only 25m away.
 
Mil: Units of angular measure equal to approximately 0.0572 of a degree.
 
Objectives: Objectives are the large lenses at the front end of a binocular. They comprise a system of two or more individual lenses of different types of glass.
 
Power, Eye Relief (ER) and Field of View (FOV) Relationship: To maintain resolution integrity, there is an optical axiom that needs to be understood. a) Increase power, ER + FOV specification levels are reduced. b) Increase ER, FOV decrease and vice versa, c) increase ER + FOV, power is lower. Only the costly increase in density and number of eyepiece lenses would allow wider latitude in these relationships, in order to maintain acceptable resolution. Average range of normal eye relief for full-size binoculars is 9-12mm. Long eye relief constitutes 15-18mm.
 
Real Angle of View or Field of View: It represents the segment of a 360 circle that the binocular is designed to view. The higher the number, the more of the subject you see from side to side.
 
Apparent Angle of View: With magnification as an added factor, the viewer appears to be (or "apparently" is) much closer to the scene than the actual distance, and the apparent angle of view is much wider at this close proximity. This apparent angle of view is used to determine wide-angle rating. It can be calculated by multiplying the magnification by the real angle of view.
 
Relative Brightness (RB) and Twilight Factor (TWF): RB and TWF are two important measurements of light gathering. They are a function of available light, combined with magnification/power ratings and objective lens sizes and the resultant exit pupil sizes.
RB (relative brightness) = E.P.2 (exit pupil size, in millimeters, multiplied by itself.) TWF (twilight factor) = The square root of the result obtained by multiplying power rating by the size (in millimeters) of the objective lens.
Relative brightness numbers from 4 to 16 are best suited for daylight; 16 to 25 for daylight, dawn and dusk; above 25 for all lighting conditions.
However, twilight factor is often considered to be a more meaningful criterion in your selection process. Twilight factor measures imaging capability under severely low-light and very low-contrast conditions, similar to those often experienced in wildlife observation at dawn and dusk.
 
Vernier: A small, movable, auxiliary scale for obtaining fractional parts of the subdivisions of a fixed scale.
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