Binoculars Glossary

1. Basic Binocular Anatomy

Binoculars – Two matched telescopes aligned for both eyes.
Objective Lenses – The large front lenses that gather light.
Eyepieces (Oculars) – The lenses you look through.
Barrel – Each tube containing lenses/prisms.
Bridge – Connects the two barrels.
Hinge – Allows adjustment for eye spacing.
Focus Wheel – Central wheel that sharpens the image.
Diopter Adjustment – Fine focus for one eye to match vision differences.
Eyecups – Rest against your eyes; twist-up or fold-down.
Neck Strap – Keeps binoculars secure.

2. Binocular Size Numbers (Critical)

Magnification (X) – How much closer objects appear.
Example: 8× means 8 times closer.

Objective Diameter (mm) – Size of front lenses.
Example: 42mm = good brightness.

8×42 – 8× magnification, 42mm objectives.

Compact – Small objectives (≤25mm).

Full-Size – Large objectives (42–56mm).

3. Optical Performance Terms

Exit Pupil – Light beam reaching your eye.
Formula: objective ÷ magnification

Bigger = brighter & easier to use.

Field of View (FOV) – How wide the image is.
Measured in degrees or feet at 1000 yards / meters.

Apparent Field of View (AFOV) – Perceived width when looking through binoculars.

Eye Relief – Distance your eye can be from eyepiece.

Critical for eyeglass wearers (15mm+ recommended).

Close Focus Distance – Nearest distance you can focus.

Depth of Field – How much stays in focus without refocusing.

Light Transmission – How much light reaches your eyes.

4. Prism Types (Very Important)

Prisms – Flip the image right-side-up.

Porro Prism – Offset barrels
✔ Brighter, better depth
❌ Bulkier

Roof Prism – Straight barrels
✔ Compact, rugged
❌ Needs better coatings to match brightness

BK-7 Glass – Lower-quality prism glass.

BaK-4 Glass – Higher-quality prism glass
✔ Sharper edges, brighter image

5. Lens & Prism Coatings

Coated – One surface has coating.

Fully Coated – All air-to-glass surfaces coated.

Multi-Coated – Multiple layers on some surfaces.

Fully Multi-Coated (FMC) – Best; all surfaces have multiple layers.

Phase Coating – Corrects light phase in roof prisms.

Dielectric Coating – Boosts brightness in roof prisms.

6. Image Quality Terms

Resolution – Ability to show fine detail.

Contrast – Difference between light and dark areas.

Sharpness – Clarity of fine details.

Edge Sharpness – Image clarity near edges.

Chromatic Aberration (CA) – Color fringing around objects.

Distortion – Straight lines appearing curved.

Coma – Edge blur, noticeable at night.

7. Mechanical & Build Terms

Chassis – Internal frame (often magnesium or aluminum).

Armoring – Rubber coating for grip and protection.

Waterproof – Sealed against water ingress.

Fogproof – Filled with nitrogen or argon gas.

O-Ring Sealed – Prevents moisture/dust entry.

Shockproof – Resistant to drops (varies widely).

8. Comfort & Usability

Interpupillary Distance (IPD) – Distance between your eyes.

Twist-Up Eyecups – Adjustable eyecups for comfort.

Blackouts – Shadows caused by poor eye positioning.

Kidney Beaning – Crescent-shaped dark areas in view.

Balance – Weight distribution in hands.

Hand-Shake – Visible movement at higher magnification.

9. Binoculars for Astronomy

Astronomy Binoculars – Large aperture, lower magnification.

Wide-Field Viewing – Seeing large sky areas.

Star Fields – Dense clusters of stars.

Milky Way Scanning – Sweeping large sky areas.

Tripod Adapter – Mounts binoculars on tripod.

Parallelogram Mount – Adjustable mount for long sessions.

10. Common Binocular Sizes & Uses

7×50 – Wide view, relaxed viewing.

8×42 – Best all-around size.

10×42 – More detail, harder to hold steady.

10×50 – Brighter, good for astronomy.

12×50 – Needs steady hands or tripod.

15×70 – Astronomy-focused, tripod recommended.

11. Common Abbreviations

FOV – Field of View
AFOV – Apparent Field of View
IPD – Interpupillary Distance
ER – Eye Relief
FMC – Fully Multi-Coated
CA – Chromatic Aberration