Telescopes, Binoculars and Accessories
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Wikipedia-Article "Binoculars"
Binocular telescopes or binoculars are two identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, one to be viewed through each of the user's eyes. By contrast, relatively small single-tube telescopes are often called "monoculars".
The advantages of a binocular over a monocular telescope are:
- it gives a 3-dimensional image with depth: the two distinct views presented from slightly different viewpoints to each of the viewer's eyes merge to produce a single perceived view with a sensation of depth, allowing distances to be estimated
- it is more comfortable to use both eyes for viewing, without the need to close or obstruct one eye to avoid confusion
- it is easier and more comfortable to steadily hand-hold and move a pair of binoculars than a single tube—the two hands and the head form a steady 3-point platform.
The most commonly seen binoculars are of a size to be held by hand, and contain optical elements to fold the optical path so that the physical length of the binoculars is less than the focal length of the lenses. The folding of the optical path allows the separation between the objective lenses to be increased, allowing larger lenses to be used and giving a better sensation of depth.
All practical binoculars display an erect image, obtained either by using simple Galilean optics ("field glasses", "opera glasses"), or by using optical prisms both to erect the image and to fold the optical path.
Larger binoculars become uncomfortable and difficult to hold steady, and are mounted on tripods or other supports. Very large binoculars with a very wide separation (up to 15 meters, weight 10 tonnes, for ranging Second World War naval gun targets 25km away) have been used for accurate rangefinding, although late twentieth century technology made this application redundant.
An extreme example, although not one would that normally be called binoculars, is the Large Binocular Telescope in Arizona, USA, which produced its "First Light" image on 26 October 2005. The LBT comprises two 8-meter reflector telescopes. While not intended to be held to the eyes of a viewer (!), the use of two telescopes to view the same object gives additional information due to the separation of the objective mirrors.
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Prismatic binoculars
Binoculars with prisms to shorten the optical path and erect the image may have double Porro prism design which gives a Z-shaped optical path. This results in a set of binoculars which is wide, with objective lenses which are well-separated but offset from the eyepieces. Binoculars which use roof prisms (either the Abbe-Koenig or Schmidt-Pechan designs) are narrower, more compact, lighter, and more expensive than those which use Porro prisms. They have objective lenses which are approximately in line with the eyepieces.
Design details
Binoculars to be used to view objects which are not at a fixed distance must have a focussing arrangement. In some cases the two telescopes are focused independently by changing the distance between ocular and objective lenses. It is more convenient for the viewer to focus both tubes with one action (usually rotation of a central focussing wheel), and for one of the two eyepieces to be adjustable to compensate for differences between the viewer's eyes (usually by rotating the eyepiece in its mount). Once this adjustment has been made for a given viewer, the binoculars can be refocussed on an object at a different distance by using the focussing wheel to move both tubes together without eyepiece readjustment.
The distance between the eyepieces on most binoculars can be adjusted to accommodate viewers with different eye separation.
Optical parameters
The diameter of the objective lenses determines the light-gathering power and the ultimate resolving power of the binoculars. The ratio of the focal lengths of the objective and the ocular lenses gives the linear magnifying power (expressed in "diameters"). It is customary to categorise binoculars by the magnification × the objective diameter in mm; e.g. 7×50.
The magnification required depends upon the application, but with the major proviso that large magnifications give an image much more susceptible to shake when hand-held. The objective lens needs to be large enough to give acceptable resolution in all circumstances, but must be larger for low-light and night use.
The field of view depends upon the optical construction of the binoculars. Simple Galilean binoculars have the disadvantage of a narrower field of view—this is the reason for the prevalence of the more complex optical arrangements used.
For general hand-held use, subject to shake, 7 diameters is a good compromise between power and image steadiness for most people. 7×30 is good for daytime use. For general night use, a 50mm objective gives maximum brightness for 7 diameters magnification; objective diameter must be increased for higher magnifications at night.
Hand-held binoculars range from small 3x10 Galilean opera glasses used in theaters, to glasses with 7 to 12 diameters magnification and 30 to 50mm objectives for typical outdoor use. Larger models with objectives of up to about 120mm are used on supports, typically for amateur astronomy. Much larger binoculars have been made by dedicated amateur astronomers, essentially using two refracting or reflecting astronomical telescopes, with results claimed to be impressive.
Of particular relevance for low-light and astronomical viewing, as against astrophotography, is the ratio between magnifying power and objective lens diameter. Binoculars concentrate the light gathered by the objective into a beam, the exit pupil whose diameter is the objective diameter divided by the magnifying power. For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated human eye—about 7mm, reducing with age. Light gathered by a larger exit pupil is wasted. However, for viewing stars and small astronomical objects, a large exit pupil will mostly image the night sky background, effectively decreasing contrast, making the detection of faint objects more difficult except perhaps in remote locations with negligible light pollution. A large exit pupil facilitates viewing larger objects such as nearby galaxies, though. The current trend favours models with 5mm exit pupil, such as 10x50, or 8x40; 7x50 is falling out of favour. For daytime use an exit pupil of 3mm—matching the eye's contracted pupil—is sufficient.
Optical construction
When light strikes an interface between two materials of different refractive index (e.g., at an air-glass interface), some of the light is transmitted, some reflected. In any sort of image-forming optical instrument (telescope, camera, microscope, etc.), ideally no light should be reflected; instead of forming an image, light which reaches the viewer after being reflected is distributed in the field of view, and reduces the contrast between the true image and the background. Reflection can be reduced, but not eliminated, by applying optical coatings to interfaces; this is of great importance for any optical instrument with multiple interfaces. Light can also be reflected from the interior of the instrument, but it is simple to minimise this to negligible proportions.
Phase-corrected prism coating and dielectric prism coating are recent (in 2005) effective techniques for reducing reflections.
When light traverses an optically transmissive material, some light is absorbed. This reduces brightness, and is also undesirable, although less of a problem than reflections in most cases. (The advanced naval binocular rangefinders of the mid-twentieth century had perhaps 150 glass elements; absorption of light would have been significant.)
Different optical construction affects reflections and brightness. A Porro prism binocular will inherently produce an intrinsically brighter image than a roof prism binocular of the same magnification, objective size, and optical quality, as less light is absorbed along the optical path. However, as of 2005, the optical quality of the best roof-prism binoculars with up-to-date coating processes as used in Schmidt-Pechan models is comparable with the best Porro glasses, and it appears that roof prisms will dominate the market for high-quality portable binoculars in spite of their higher price. The major European optical manufacturers (Leica, Zeiss, Swarovski) have discontinued their Porro lines; Japanese manufacturers (Nikon, Fujinon, etc.) may follow suit.
When buying binoculars of lower price, Porro prism binoculars can be expected to give more image quality for money.
Image stabilisation
Shake can be much reduced, and higher magnifications used, with binoculars using image stabilisation technology. Parts of the instrument which change the position of the image may be held steady by powered gyroscopes or by powered mechanisms driven by gyroscopic or inertial detectors, or may be mounted in such a way as to oppose and dampen sudden movement. Stabilisation may be enabled or disabled by the user as required. These techniques allow binoculars up to 20× to be hand-held, and much improve the image stability of lower-power instruments. There are some disadvantages: the image may not be quite as good as the best unstabilised binoculars when tripod-mounted, and stabilised binoculars contain more advanced technology to go wrong, and to become obsolete. They are also more expensive, heavier, and battery life tends to be short. Stabilisation is not suitable when tracking moving objects.
Maintenance
If the binoculars are not collimated properly, i.e., if the images from the two tubes are not properly aligned, then they will give poor results and can be uncomfortable and tiring to use. This may be due to poor manufacturing quality control (more likely with cheaper binoculars) or to a shock (being dropped) or drift over time. If the binoculars are basically sound, this can be remedied by small movements to the prisms, often by turning screws accessible without opening the binoculars. While it is inadvisable for the non-expert to try to repair quality instruments, collimation by the owner may be justified for maladjusted binoculars which are not good enough to merit the expense of professional attention. Instructions for checking binoculars for collimation errors, and for collimating them, can be found on the Internet (search for collimation binoculars and the model).
A well-collimated pair of binoculars should produce, when viewed through human eyes and processed by a human brain, a single circular, apparently three-dimensional, image, with no visible indication that we are actually viewing two distinct images from slightly different viewpoints. Departure from the ideal causes, at best, vague discomfort and visual fatigue, but the perceived field of view will be close to circular anyway. The cinematic convention to represent a view through binoculars as two circles partially overlapping in a figure-of-eight shape is not true to life.
Choosing binoculars
Ideally a pair of binoculars will produce two uniformly sharp images, each of perfect quality, with no errors of geometry or colour-correction and no internal reflections. The two images will be identical (apart from the slightly different viewpoint), with no differences in size, orientation, aberrations, etc. Real binoculars depart to a greater or lesser extent from the ideal.
All binoculars should be accurately aligned and collimated, comfortable to use, and robust. Roof-prism models will be lighter and more compact for a given size, but more expensive than equivalent Porro models.
Hermetically sealed binoculars filled with dry gas (usually nitrogen) will not be susceptible to clouding due to condensation at low temperatures; this will also help to prevent mildew, although air may leak in over a period of years if the binoculars are not overhauled. Completely waterproof (submersible) binoculars are available.
All binoculars should be reasonably free from reflections. Fully multi-coated (FMC) models should be better in this respect than others. For roof-prism models, phase coating is better.
The magnification and objective diameter must be chosen to suit the requirement, remembering that higher magnification exaggerates shake when hand-held, and that larger objective lenses increase the weight and size.
For general-purpose use, 8x40 is a good combination. 7x50 is brighter for night use. Larger objective diameters have better light-gathering power, and can view fainter objects for astronomical use. If more compact binoculars are required, smaller objectives may be used at some loss of performance and increase in price.
Image stabilisation much improves image steadiness and allows the use of higher magnification in hand-held applications. The trade-off is that compared to unstabilised binoculars of the same parameters, stabilised binoculars are more expensive, larger and heavier, less reliable due to their complexity, more subject to obsolescence, and consume batteries.
Zoom binoculars, while in principle a good idea, do not perform very well.
Some binoculars (and cameras) claim to be "focus-free". This is an example of marketing departments making a virtue of necessity. Such models would have been called "fixed-focus" in more honest times: they have a depth of field from a relatively large closest distance, to infinity, and perform exactly the same as a focussing model of the same optical quality (or lack of it) focussed on the middle distance.
Binoculars of the same make and model may vary from unit to unit, although hopefully less so for the more highly priced models from quality manufacturers, so the experienced user may benefit from trying several samples. By the same token, many cheaper types of generally mediocre quality but basically sound design may have a few exceptionally good units.
Some reputable binocular manufacturers as of 2005:
1. European Brands
- Leica GmbH (Ultravid, Duovid, Geovid: all are Roof)
- Swarovski Optik (SLC, EL: all are Roof; Habicht: Porro, but to be discontinued)
- Zeiss GmbH (FL,Victory, Conquest: all are Roof; 7x50 BGAT/T, 15x60 BGA/T:Porro, but to be discontinued)
- Docter Optik (Nobilem: Porro)
- Optolyth (Royal: Roof; Alpin: Porro)
- Steiner (Commander, Nighthunter: Porro; Predator, Wildlife: Roof)
- Russian Military Binoculars (BPOc 10x42 7x30, BKFC series)
2. Japanese Brands
- Canon Co. (I.S. series, Porro variants?)
- Nikon Co. (High Grade series, Monarch series,RAII, Spotter series: Roof; Prostar series, Superior E series, E series, Action EX series: Porro)
- Fujinon Co. (FMTSX, MTSX series: Porro)
- Kowa Co. (BD series: Roof)
- Pentax Co. (DCFSP/XP series; Roof, UCF series: Inverted Porro; PCFV/WP/XCF series: Porro)
- OLympus Co. (EXWPI series: Roof)
- Minolta Co. (Activa, some are Roof, some are Porro)
- Vixen Co. (Apex/Apex Pro: Roof; Ultima: Porro)
- Miyauchi Co. (Specialized in over-sized Porro binocualars)
P.S. Many of the above are OEM products of Kamakura or Chinese manufacturing plants.
3. Chinese Brands In the early years of the 21st century some mid-priced glasses have become available on the internal Chinese market. A few of them are said to be comparable both in performance and in price to some of the better brands; the great majority of them are inferior.
- Sicong (from Xian Stateoptics. Navigator series: Roof; Ares series: Porro)
- WDtian (from Yunnan State optics, all Porro)
- Yunnan State optics (MS series: Porro)
Binoculars are widely used by amateur astronomers, their wide field of view making them useful for comet and supernova seeking (giant binoculars) and general observation (portable binoculars). The major market is amongst bird watchers and hunters, who mostly prefer, and are prepared to pay for, the lighter but more expensive roof-prism models.