Explanation Of Terms Used In Specifying Zeiss Monoculars And Binoculars

MAGNIFICATION

Magnification is a measure of how many times better you can see an object through your magnifier than with your naked eye. The magnification of a monocular or binocular is stated in the first number of the two numbers which make up the model number. For instance, the Zeiss 8x20BT* MonoDesign monocular has a specified magnification of "eight-power" - the object appears eight times closer than with the naked eye.

Single lens magnifiers, such as magnifying glasses and loupes, have a magnifying power which depends on how they are used. For a more thorough discussion of magnification, see our page...

OBJECTIVE LENS DIAMETER

The objective lenses are the outer lenses of monoculars or binoculars that are directed toward the object being viewed. The diameter of the objective lens is expressed in millimeters and is specified in the second of the two numbers of the model number. For the Zeiss 8x20BT* monocular, the objective lens is 20mm in diameter.

The size of the objective lens determines how much light can enter an optical instrument. Along with the magnification and the light-transmitting efficiency of the internal optical elements, it directly affects how bright and clear the image of a distant object appears.

FIELD-OF-VIEW

Field-of-view is the actual width of the scene you view through your magnifier at any given distance. Think of it as a "cone of view", getting wider as you move farther away from the objects being viewed. For the Zeiss 8x20BT* monocular, the view would be about 11-1/2 meters wide at a range of 100 meters or about 115 meters wide at a range of 1000 meters.

Generally speaking, field-of-view is proportional to the diameter of the objective lens and inversely proportional to magnification - that is, more magnification means less field-of-view. The upper limit on the useful field of view of a binocular or optical telescope is set by the eye's own useful field of view (actually the field of "perception", within which we can actually perceive and recognize objects; this is a cone of about 40 degrees included-angle), divided by the magnification of the scope. However, only scopes with very large objective lenses achieve this maximum cone of view, and internal optical elements of the scope, such as the erecting prisms, may also reduce the effective field-of-view.

MINIMUM FOCUS DISTANCE

All monoculars and binoculars have the ability to be focused at "infinity", or "very far away" - you can view the stars or distant mountain peaks with the scope set to "infinity". So a primary point of distinction between monocular models is the minimum focus range, i.e. how close an object can be to you and still be viewed clearly through the scope. Birders tend to favor models with a shorter minimum focus distance, sometimes as little as 5 meters. All Zeiss monocular scopes will focus to a distance of 5 meters or less.

The specified minimum focusing distance is a theoretical value based on the visual acuity and visual accommodation ability of a young person. Older people have slightly more distant near focus due to presbyopia. Throughout the population, there is a wide variance of minimum near focus distances from person to person. Also, eyeglass wearers who remove their glasses will notice changes in the minimum focus distance they obtain with an optical instrument.

LOW LIGHT PERFORMANCE

The formulas below give a basic evaluation of low light performance based on the physical dimension of the optical elements of the scope. However, they do not take into account some of the most critical features in optics; glass quality, number of lenses, precision of manufacture, and most importantly, the performance of anti-reflective coatings.

EXIT PUPIL

The exit pupil, expressed in millimeters, corresponds to the diameter of the cylinder of light as it exits the eyepiece of the scope and enters your eye. The diameter of the exit pupil is generally equal to the diameter of the objective lens divided by the magnification. When holding the monocular at arm's length, it's the small, circular image that's visible in the eyepiece lens.

It is known that the pupil of the average adult eye dilates to 7mm in diameter under low light conditions. Theoretically, one would like to have the exit pupil of the scope nearly equal to this dilated pupil diameter in order to deliver the maximum amount of light into the eye. Practically speaking, however, compact optical instruments will have a somewhat smaller exit pupil, since a 10-power scope would need an objective lens 70mm (2-3/4") in diameter to achieve a 7mm exit pupil. Most Zeiss compact monocular scopes have an exit pupil size of 3 to 4mm.

Note also that the eye must be positioned properly, both fore-and-aft and side-to-side, in order for your eye to capture all the light coming from the eyepiece and see the full field-of-view with maximum clarity. The rubber eyecup of the monocular aids in the proper positioning of the eye, and for users with eyeglasses, the cup can be folded back to allow them to use the scope without removing their glasses.

IMAGE ERECTING SYSTEM

With the exception of the 5x10T* MiniQuick model, Zeiss incorporates Pechan-type roof prisms to correct the upside-down image which is formed by the objective lens of a scope. Roof prism pairs allow straight-through optical layouts - there is no "dogleg" in the optical path as compared with older Porro prism binoculars.

The MiniQuick utilizes a relay-lens system, a simple design that is taken to the next level by Zeiss' painstaking craftsmanship. This provides the user with superior optics at less than half the weight of the prism-corrected monoculars.