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General Questions about Optics


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General Questions about Optics


Which is better, a binocular or spotting scope?

Both eyes are used to look through a binocular while one eye is used to look through a spotting scope.

Generally, spotting scopes have a much higher magnification than binoculars, so far away images appear larger. But a better three-dimensional image can be expected from binoculars, and they are easier to handle and have better portability.


What is magnification?

Magnification describes how many times closer an image appears. Magnification is also called “Power” and is abbreviated by the symbol “X”. A magnification of 20 (20 power) means that an object appears twenty times closer than you would see it with the naked eye.

When looking through an optic with a magnification of 20, an object at a distance of 100 yards/meters appears to be only 5 yards/meters away.


What is the actual field of view (angle of view)?

Actual field of view is the view when looking through an optical device indicated by the angle from the center of the objective lens. An optical device with a wide angle of view makes it easier to find an object. In general it is more difficult to find an object with an optical device with a higher magnification.


What is a field of view at 1000m (1000yds)?

Field of View (FOV) is the width in yards/meters that can be seen at 1,000 yards/meters distance. FOV equals the real angle of view x 52.4′. Every degree of real angle of view can see 52.4′ at 1,000 yards/meters.


What is objective lens diameter?

The measurement in mm of the diameter of the objective lens of an optical instrument. As the diameter of the objective lens increases, the optic will have better light gathering and higher resolution.


What is exit pupil and relative brightness?

Exit pupil is the bright circle that is visible when the ocular lens is viewed at a distance of about 25cm (10 inches) with the objective lens facing towards a bright background or light source. The larger the exit pupil, the brighter the image will be under low-light conditions.

Exit pupil = Objective lens diameter ÷ Magnification

Relative brightness is a number used to compare the brightness of binoculars or spotting scopes of similar magnification. The larger the relative brightness number, the brighter the image.

Relative Brightness = Exit Pupil2 (or Exit Pupil size in mm, multiplied by itself).


What is resolution?

Resolution, or ultimate sharpness, is the measure of the ability of a spotting scope or binocular to distinguish fine details. Optical glass is at its sharpest in the center, while resolution decreases toward the edges in relationship to the quality level of the lens. Larger objective lenses (with quality as a constant) tend to improve resolution. This is because they have a larger amount of sharp, center objective lens area, as opposed to the amount of lesser sharpness at the edge of the lens surface.


What is minimum focus distance?

Close focus is the closest distance to an object that the binocular/spotting scope will focus sharply.


What is eye relief?

Eye relief is the distance, measured in mm, between the eyepiece lens to where the exit pupil is actually positioned in space (where the pupil of the eye must be positioned for a full view). A long eye relief makes observing for long periods of time easy and tireless. Eyeglass wearers typically need an eye relief of 15mm.


What is multi-coating?

When light enters a lens or prism, light is reflected from their surface at a rate of about 4%. This reflection diminished the brightness of the optic and causes flare or ghosting. By coating the surface of the lens, the amount of reflection can be minimized, decreasing flare and ghosting.

Different Types of Coatings:

Coated optics: one or more surfaces are coated.

Fully coated: all air-to-glass surfaces are coated.

Multi-coated: one or more surfaces are multi-layer coated.

Fully multi-coated: all air-to-glass surfaces are multi-layer coated.


What are the benefits of a gas filled optic?

Spotting scopes and binoculars are used in all types of weather, so it is important to protect the instrument from moisture. To keep the inside dry and keep moisture from penetrating from the outside, all of the air is purged from the inside of the optic and replaced with nitrogen gas. The nitrogen gas prevents moisture from entering the scope and keeps the inside dry.


*If condensation appears on the optic, it is important to wipe the moisture from the optic immediately. Failure to wipe moisture from the optic might stain or promote the growth of mold or rust.

*In extremely high temperatures, materials that keep the optic airtight (rubber rings, oil, etc.) might degrade and the airtightness may be compromised.


What is the difference between Achromatic and Apochromatic?

* Achromatic

An Achromatic lens system compensates the wavelength differences of two colors to diminish chromatic aberration. For observation, the two colors usually compensated are blue and red, and for photography, purple and red. An achromatic optic diminishes chromatic aberration so that only a little remains.

* Apochromatic

An apochromatic lens system compensates the wavelength differences of three colors or more. For example, an apochromatic system is designed to make purple, green and red focus at the same point so that chromatic aberration is significantly reduced. It has become standard to use Fluorite Crystal, or a kind of XD (ED) lens for this type of lens system. All Kowa Prominar lenses are apochromatic.


The above are general explanations and differ among optics manufacturers. Recent refraction telescopes are called apochromat when Fluorite or XD (ED) lenses are used and achromat when other lens materials are used.


What is needed to attach a digital camera to a spotting scope?

A Digital Camera Adapter. Connecting a digital camera to a spotting scope produces super-telephoto images. Inaccessible wildlife and scenery can be photographed with amazing detail.


Is it possible to attach a video camera to a spotting scope?

Yes, it is possible to attach a video camera to a spotting scope in the same way as a digital camera.


Which spotting scopes can be used for digiscoping?

Kowa offers a complete assortment of digiscoping accessories designed to fit virtually any digital camera; Digital SLR cameras, video cameras and smart phones. Dedicated smartphone adapters are available for iPhone and Samsung smartphones.

All Kowa Spotting Scopes are well suited for digiscoping.


What is the focal length of the digiscope system?

The focal length of a digiscope system can be found using the following equation:

Focal Length = Focal Length of the Digital Camera x Magnification of the eyepiece.


What is the collimation method?

The method of taking photographs through a digital camera that is attached to the eyepiece of a spotting scope is called the collimation method. Collimate means the aligning of the optical axis of the optical system. Usually, we look through a telescope/spotting scope through the eyepiece with our eye. When the human eye is replaced by a digital camera to look through the eyepiece to take a photograph, it is called the collimation method.


Why does vignetting occur?

The darkening phenomenon that appears around an image is called vignetting. When digiscoping, it is important to choose the appropriate adapter and eyepiece to reduce vignetting as much as possible.

Vignetting occurs when the angle of view of the camera is larger than the apparent field of view of the eyepiece. A wide angle eyepiece is effective in reducing vignetting.

Vignetting can also occur when the eye relief of an eyepiece is not sufficient for the camera to capture the full image. A long eye relief eyepiece is effective in reducing vignetting.


Is it possible to attach a digital SLR camera to a spotting scope?

Yes,  attach a digital SLR to the spotting scope with a Photo Attachment and Camera Mount. Kowa offers a variety of camera mounts for various makes of cameras.

Finding Products


Where to buy? /How to buy?

Check our Where to Buy page for an authorized retailer near you. Most of our retail partners also have online stores for your convenience.  We recommend buying from only authorized retailers with direct accounts with our manufacturers.  Be aware there are re-sellers in the marketplace that may offer cheap prices online, but consider the additional costs like shipping, brokerage and warranty/repair service before making your purchase. Ideally, locate a shop near you where you can look through several different models and brands to find the optic right for you.