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Owner's Manual For BIG EYES® 25/40x100mm Binocular


Your Big Eyes® 25/45x100mm binocular is a precision optical instrument with variable magnification, 25x and 40x, excellent for both long-range terrestrial and celestial observation. Big Eyes® enormous 100 mm objective lenses maximize light collection resulting in extraordinarily bright and sharp images.

Whether used for work or pleasure, enjoyment of your Big Eyes® will be a result of silky smooth controls, crystal clear images, extraordinary light gathering ability, excellent depth perception, stereoscopic effect, individual focusing for each eye, superior quality, and the knowledge that you possess the ultimate spyglass.

Now it's time to set up your binoculars and see the world through Big Eyes®.

Setting Up Your Big Eyes®

To avoid damage due to extreme and rapid changes in temperature, wait at least one hour before opening the STORAGE/ TRANSPORT CASE (Fig. 1.13) when moving your binoculars from a cold to a heated environment. This will allow the temperature inside the container to equalize with the outside temperature at a moderate rate. Conversely, allow 15 minutes when moving from a warm to a cold area.

Do not drop or allow your Big Eyes® to fall. Not only will damage, such as de-alignment, occur to the binocular but serious physical injury may also result if they land on people, pets, or property. Remember, never set your Big Eyes® down in any way that will allow surface material to make contact with lenses. Leave your Big Eyes® n their case until you are ready to mount them on your secured stand.


Big Eyes® Stand Options

Big Eyes Binoculars provide a wooden tripod which includes a small removable metal tripod, you will need to follow these simple guidelines to avoid damaging your Big Eyes®, property and/or to avoid injuring people and pets.

1. Do not setup your stand, make adjustments, or move your stand when Big Eyes® are attached.

2. Do not set up your stand in a way that will allow it to become unbalanced easily. Achieve stability by spreading tripod legs far apart and screwing pedestals firmly to the floor.

3. Make sure all screws and locking mechanisms on your stand are firmly tightened.

Setting Up the Wooden Tripod

Your Big Eyes® wooden tripod was designed for use on soft materials such as dirt, sand, or gravel and will tend to slide on hard smooth surfaces such as wood flooring, ceramic tile, linoleum, or concrete. To avoid damage, care must be taken to ensure that the tripod does not slide or become unbalanced. This can be achieved by not trying to push or pull the tripod in an attempt to move it and by placing the tripod on an area rug or rubber padding. If the tripod must be moved after the binoculars are mounted to it, remove the binoculars before you attempt to move the tripod.

To avoid damaging your Big Eyes®, property, or person it is important that the binoculars are not attached to the tripod during the setup and adjustment of the tripod. Begin by setting up the large tripod and making the necessary adjustments to the leg length. The leg length should be adjusted so that the male shaft on the tripod is vertical. To be sure that the legs of the tripod will not collapse once the binoculars are mounted on it, firmly tighten the THUMBSCREWS (Fig. 1.12) and BASE RING (Fig. 1.5) to ensure stability and balance before attaching the binoculars. Pull the tripod legs apart until the chain and ring that connects them is taunt and there is no sag in it. Next, check that the points of the small tripod are centered over the CHAIN RING (Fig. 1.11) which connects the three chains.

If the binoculars are to be used with the SMALL TRIPOD (Fig. 1.10), loosen the SMALL TRIPOD RELEASE/ATTACHMENT RING (Fig. 1.9) by turning it counterclockwise without the binoculars attached. When the small tripod is loose, remove it. Your Big Eyes® and small tripod can then be reassembled as a table model.

Attaching Big Eyes® to the Tripod

Once you have located, setup, secured and adjusted the height of your stand you are ready to mount your Big Eyes®. As you remove the binoculars from the case, fold down the female shaft on the underside and place it over the male shaft of the stand. Only when using the Wooden Tripod or Aluminum Pedestal should you tighten the BINOCULAR/STAND FASTENING KNOB (Fig. 1.2) to lock the binocular and stand together or to restrict the binocular from rotating horizontally. If the BINOCULAR/ STAND FASTENING KNOB (Fig. 1.2) is tightened when using the Bogen 3236, 3258 tripods or the Mahogany Pedestal or any other stand using the spindle adapter you risk loosening the aluminum spindle, which will cause the binocular and spindle to become detached from the mounting plate, resulting in the binocular falling from its mount and becoming damaged. Check the aluminum spindle often to ensure that it is tightly fastened to the tripod!

Operating Your Big Eyes®

Big Eyes®, like most precision instruments, requires experience and a thorough understanding of operation and application before they can be used accurately and efficiently. When you are familiar with the following operating procedures, explain to first time users and those unfamiliar with Big Eyes® the following three steps that must be taken in order to enjoy the viewing experience and avoid frustration. Note: Do not use Big Eyes® to look at the sun. Severe and permanent eye damage will result! Do not expose Big Eyes® to moisture such as rain and/or snow.

1. Selecting the Magnification

    The magnification of Big Eyes® can be changed, from 25x to 40x or visa versa, by turning the RIGHT and LEFT ALUMINUM BANDS (Fig. 2.7 and 2.8) 180 degrees in either direction. A subtle click and a clear line of sight will indicate a successful change of magnification. We suggest you start with 25x.

2. Setting the Interocular Distance

    In order to adapt the binoculars to the observer's interocular distance, hold the RIGHT and LEFT PRISM ASSEMBLIES (Fig. 2.5 and 2.6) and gently pull them apart or push them together until a near perfect circle is created and the target is centered.

3. Setting the Focus

    The next step is to adapt the binoculars to the diopter of the observer's eyes, commonly referred to as focusing. To focus, turn the SCALE RINGS (Fig. 2.9 and 2.10) separately on each eyepiece. To accurately make a dioptric correction, block off one tube/eyepiece by placing a cover over it or close one eye and focus the other. Repeat this procedure for your other eye.

Note: Most eyeglass users need not wear their corrective lenses when using Big Eyes®. By wearing eyeglasses when looking through Big Eyes® you will likely diminish your viewing experience.

Individual focusing binoculars such as Big Eyes® are adjusted for each eye at the eyepiece. Individual focusing binoculars are more adaptable to each individual viewer and are more sturdy and rugged in construction than all other focusing mechanisms, making them more dependable. Military services around the world require individual focusing binoculars for these reasons.

Elevation and Azimuth Adjustments

Elevation adjustments can be made when the ELEVATION KNOB (Fig. 1.7) is loosened. Course elevation adjustments are made by holding the RIGHT and LEFT PRISM ASSEMBLY (Fig. 2.5 and 2.6) with each respective hand and smoothly pushing up or pulling down. Fine elevation adjustments are made when the FINE ELEVATION KNOB (Fig. 1.1) is turned. Horizontal (azimuth) movements can be made when the BUTTERFLY BOLT (Fig. 1.3) is loosened and your hands are placed on the RIGHT and LEFT PRISM ASSEMBLY (Fig. 2.5 and 2.6) and moved right or left.

Distance Measurement

Except for a reticle in the focal plane of the right tube, the optical system of the right and left tube is identical. The reticle is vertically and horizontally graduated in 1.0 Mil increments for the bigger divisions and in 0.2 Mil increments for the smaller divisions. A rough measure of the distance between the observer and the target can be calculated when the height or width of the target is known by following the procedure below. If you can't see the reticle in the right tube, adjust the focus knob until visible.

Enter the known length (height or width) of the target in the appropriate space in the formula below. Count the number of Mil graduations needed to cover the targets height or width and enter this sum as the included angle in the formula below. If you are using the known height of the target instead of its width, make sure your calculation of the Included Angle also refers to height and vise versa. With values entered for 'Known Height or Width of Target' and 'Included Angle', solve the equation to determine approximate distance.

Distance (meters) = Known Height or Width of Target (meters) x1000

Included Angle (Mil)

Measurement of Angle Between Targets

To measure the angle between two targets you must use the Wooden Tripod offered by Big Eyes Binoculars because it has both the SCALE RING (Fig. 1.4) and VERNIER SCALE (Fig. 1.8) necessary for such a calculation. First, loosen the BUTTERFLY BOLT (Fig. 1.3). Next, aim the center of the cross on the reticle at the first target. Align the "0" line of the SCALE RING (Fig. 1.4) with that of the VERNIER SCALE (Fig. 1.8). Then clamp the BASE RING (Fig. 1.5) tightly. Now, aim the center of the reticle at the second target and read the SCALE RING (Fig. 1.4). Finally, calculate the difference to determine the angle. If the rings are set to "0" when the reticle is placed over the left target and the binocular is moved to the right to mark the second target, the rings will read the angle between two targets directly without any calculation.

Use of Contrast Enhancing Filters

Big Eyes® availabe filters that fit over the eyepieces. The BLUE-GREEN FILTERS reduce the glare associated with bright light and reflective surfaces. The ORANGE-YELLOW FILTERS improve the contrast of objects in thin fog or flat-light conditions. The filters are wrapped in tissue and recessed in channels cut into the wood mounting brackets of the STORAGE/ TRANSPORT CASE (Fig. 1.13).

Storage and Maintenance

Big Eyes® binoculars are a precision optical instrument that must be properly stored and maintained to ensure that they remain in good working order.


Before and after each use, clean the lens surfaces to prevent scratching. Use the brush followed by the lens cloth to lightly wipe off each lens. Never wipe the lenses with your hand, shirt, paper towel, napkin, or with a dirty cloth. During cleaning be careful not to remove the sealant between the joints of the metal and lenses.

Also, wipe off any dust or moisture on all metal surfaces after use to prevent corrosion. If chipping of the paint occurs, lightly sand the edges to blend and prevent further flaking. Touch up with quality paint specifically made for metal.

When your binoculars are not in use cover them with the dust cover to protect them from exposure to sunlight, temperature, dust, sand and moisture.

Whenever you are transporting your Big Eyes® you should secure them in their protective STORAGE/ TRANSPORT CASE (Fig. 1.13) or optional Pelican Protector case.

The DRY VALVE(S) (Fig. 1.6) are used for draining and filling the binocular housing with nitrogen gas as this prevents fogging of the internal optical surfaces. Filling the binocular housing with nitrogen gas requires special equipment. Filling Big Eyes® with nitrogen, as with any repair or service, must be performed by a Big Eyes® Service Center; otherwise the enclosed limited warranty is rendered null and void. The user should NOT open the DRY VALVES as contamination and/or moisture may enter the binocular housing and cause corrosion and/or impair optical performance. Contact Big Eyes Binoculars, L.L.C. if dry valves have been opened. There is a fee for refilling the binocular housing with nitrogen.


To maintain the integrity and longevity of the mechanism, store your Big Eyes® in a well-ventilated area and in temperatures ranging between 0° F and +100° F with a humidity of not more than 70%. Also, store away from acids, alkaline batteries, salt substances and other chemicals, and at least five feet away from any heat source or exterior wall.


If a problem is not readily discernible or you have questions or comments call Big Eyes Binoculars @ 250 415 2063.

Please keep all packaging materials and hardware in case you must return products. If damage does occur as a result of insufficient packaging, the sender, you, will be liable for such damage. You must acquire a return authorization number before returning! Include your name, telephone number, and a valid copy of the proof of purchase (bill of sale) and service request inside the shipping container. Upon receiving a return authorization number write the number on the outside of the cardboard shipping container and return the product, freight pre-paid and insured for the purchase price to:

BIG EYES™ Product Repair
#14 - 2042 Mills Road West, Sidney B.C. V8L 5X4
Repair and service phone: (250) 415 2063

Parts and Accessories Included with all Big Eyes®

1. Big Eyes® 25/45x100mm Binocular 1 Piece

2. Blue-Green Filters 1 Pair (optional)

3. Yellow-Orange Filters 1 Pair (optional)

4. Flannelette Cleaning Cloth and Lens Brush 1 Each

5. Dust Cover 1 Piece

6. Instruction Manual 1 Copy

7. Limited Warranty 2 Copies

8. Storage/ Transport Case 1 Piece


(All values are approximate)

1. Optical Specifications of Binocular:

    -Two Magnifications: 25x 40x

    -Real Angle of View: 2° 30' 1° 30'

    -Real Field of View in Meters @ 1000 Meters 43.64 m 26.18 m

    -Apparent Angle of View: 62.5° 60.0°

    -Minimum Focal Distance (Varies for each user) 50 m 140 m

    -Exit Pupil Diameter: 4 mm 2.5 mm

    -Eye Relief: 14 mm 8.2 mm

    -Interpupillary Distance Range: 58 mm 72 mm

-Relative Brightness: 16 6.5

-Twilight Factor: 50 60.25

-Objective Lens Diameter: 100 mm

-Lens Coating Material: Magnesium Fluoride

-Lens Coating Type: Multi-coated

-Elevation Angle: 57°

-Depression Angle: 30°

-Azimuth Angle: 360°

-Dimensions: L= 25" W= 8.6" H= 10.9"

-Weight: 27 Lbs.

2. General Specifications of Wooden Tripod w/ inset Small Tripod:

-Maximum Elevation: 66"

-Minimum Elevation: 44"

-Weight: 16.6 Lbs.


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. Big Eyes® have three, or triplet objectives.

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. The apparent angle of view for Big Eyes® is 62.5° when using the 25x eyepiece and 60° when using the 40x eyepiece. Since the result of this calculation is greater than 60° , these binoculars qualify as a wide-angle instrument.

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. Big Eyes® have a very high twilight factor rating relative to other binoculars.

Vernier: A small, movable, auxiliary scale for obtaining fractional parts of the subdivisions of a fixed scale.

Limited Warranty

All BIG EYES™ binoculars are covered by a one year warranty (from date of purchase) for problems due to manufacturing defects. Repair or exchange at our discretion.

Problems caused by accident or misuse, regardless of purchase date, shall be treated as "Out-of-Warranty Repair".

Warranty Repair Procedure-

Package the binocular to protect it during shipping.
Enclose a copy of the Sales Receipt in the box.
Enclose a note explaining the problem for repair department. Include your name, shipping address, phone number (required), and email address (optional).

Enclose payment for return shipping ($125 can - $85 us ). Payment can be made by check, money order, or credit card. If paying by credit card, please include credit card number, name on card, billing address for card (if different from shipping address).

Out-of-Warranty Repair Procedure-
Follow same procedure as "Warranty Repair" above.

Shipping Address for All Repairs-

BIG EYES™ Product Repair
#14 - 2042 Mills Road West, Sidney B.C. V8L 5X4
Repair and service phone: (250) 415 2063

NOTE: Depending on workload, turn-around time for repairs is typically one to three weeks.
For any questions support@bigeyes.ca


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