A Chronological History of Smyth & Barlow Lenses
Rodger W. Gordon
by Rodger W. Gordon
Smyth & Barlow lenses originated in the C19th. These lenses serve several purposes, including field flattening, aberration correction, and either decreasing or increasing effective focal length respectively. The Barlow lens, when used as an accessory to an eyepiece, can increase its magnification usually between 2x & 3x.
Peter Barlow (1776-1862), English mathematician and engineer, developed his "Barlow Lens" in collaboration with George Dollond. Barlow calculated a concave achromatic lens which Dollond made in 1833. The invention of this new optical element was presented to the Royal Society by Dollond. (Phil. Trans., 124, pp. 199-207, 1834)
Shortly after the introduction of Daguerreotype photography (1839), in 1841, Joseph Petzval (1807-1891) designed a portrait lens comprising a pair of crown-flint doublets; a cemented doublet and an air-spaced doublet. Charles Piazzi Smyth (1819-1900) Astronomer Royal for Scotland, surveyed and photographed the Gizeh Necropolis between the years 1860 and 1868. Smyth, dissatisfied by the poor edge definition of the Petzval portrait lens, modified the lens in 1873 by adding a negative field flattener element. Smyth realized that if astigmatism was corrected to leave only Petzval curvature, then a negative lens of suitable power close to the focal plane to act as a field lens would offset field curvature. (Brit. J. Phot. 22, 208 (1875). See also Brit. J. Phot. Almanac, 1874, p.43.)
Ernst Abbé proposed a negative element for microscope eyepieces in 1878. (E. Abbé, Fernrohrocular mit weit abliegendem Augenpunkt. J. Roy. Microsc. Soc., 1878)
The field flattener idea of Smyth was not further developed in camera lenses until 1911, when Moritz von Rohr used it to flatten the field of the Zeiss Biotar f/1.9 lens. In 1917 H. Dennis Taylor designed an f/2 Petval type camera lens for Taylor Hobson with a negative element field flattener. A 4-inch f/2 lens of this type was used in the 1920's at Mount Wilson Observatory for wide field stellar photography. (Zeits. für Instkde. 31, 265 (1911) Brit. Pat. 127,058 (1917). See also Trans. Opt. Soc. (London) 24, 148 (1923))
Other photographic lenses with a Smyth field flattener were manufactured by Kodak in their Ektar range, including the Projection f/1.0 Ektar. A negative meniscus lens was placed immediately ahead of the image plane. (U.S. Pat. 2,076,190 (1934). See also JSMPTE 54, 337 (1950)) The lens arrangements of some of these systems may be found in Rudolph Kingslake, 'A History of the Photographic Lens', Academic Press ISBN 0-12-408640-3, 1989.
The need for hyper-wide angle eyepieces in WWII led to some astounding developments. In 1944, Albert König, Zeiss' chief optical designer, developed a 120¼ eyepiece with a Smyth lens, for use on U-Boat observation periscopes. Hörst Köhler, König's former assistant at Zeiss, designed for Zeiss Oberkochen, an improved version incorporating a triplet Smyth lens. The afov was listed as 110°-120°, focal length 26.1mm. It was used in a Zeiss Oberkochen aircraft spotting telescope, 15x75. ('A New Telescope Eyepiece with Extremely Large Field of View', Hšrst Kšhler, Oberkochen, Optik, J. Light & Electron Optics Special Reprint, Optik 17 (1960), pp500-509. See also 'Military Binoculars and Telescopes for Land, Air, and Service', Hans Seeger (1995; 2002; 2005))
Information on the König - Köhler 120° eyepiece can be found in the recent publication, 'Handbook of Optical Systems', Vol.4, edited by Herbert Gross, Wiley-VCH, ISBN 3-527-40382-5. Gross considers the Köhler 1960 eyepiece to be the predecessor to the Nagler ultra-wide angle eyepiece which will be mentioned later.
(Jos. Schneider & Co., Optische Werke, Kreuznach) 25x105 (45¡) binoculars for aircraft identification and general observation. Earlier models (with a retractable section to the hood) had weak attachment of the OG barrels to the prism housings but this was later greatly improved.
A. Tronnier, senior executive and chief optical designer at Schneider Kreuznach during WWII, received a request in 1943 to supply 15x105, 18x105 & 25x105 aircraft spotting binoculars, to be used by observers from tethered balloons. Tronnier designed 82° & 110° eyepieces for the 25x105 & 18x105 respectively. The 110¼ eyepiece had eight elements in five groups, including a Smyth lens. The 82° had a six element Erfle, a wedge prism placed in front of the field lens, and a two element Smyth lens in front of the prism. The optical layout of the 25x105 is detailed in Seeger's book. Tronnier emmigrated to the USA after WWII and designed the T14 US military binocular for Farrand Optical Company. The Frankford Arsenal was also involved in the development of the T14 7x50 binocular. (Opto-Mechanical Systems Design p28 3rd Ed. Paul R. Yoder, Jr., Taylor & Francis, ISBN 0-8247-8754-4, (1993))
In 1965 Wright Scidmore, employee of Frankford Arsenal, applied for a patent on a 80° eyepiece that used a cemented field doublet with a dedicated leading negative element. (US Pat 3,390,935 July 2, 1968). Whether there was a connection between Tronmier and Scidmore in these eyepiece developments cannot be ascertained at present (2009).
The field curvature of the Huyghenian eyepiece is well known and has been occasionally employed on Schmidt-Cassegrain telescopes to compensate for its inherent field curvature. A compensating Huyghenian eyepiece incorporating a Smyth lens was manufactured by Goto Optical c1960. At least two were made, a combination 10mm/20mm and 12.5mm/25mm. The Smyth lens was used as both a field flattener and Barlow. The 10mm/20mm was sold by Edmund Scientific in its May 1962 catalogue but does not appear in an earlier 1960 catalogue. It was sold by Edmund Scientific for several years. The Smyth lens could be unscrewed, hence the twin focal lengths.
C.R. Hartshorn, in his article on the Barlow lens in Amateur Telescope Making Book III (1953) mentions the possible use of a Barlow and eyepiece designed together to cancel aberrations of each other. In the same book Dr. Henry Paul mentions, briefly, Piazzi Smyth and his field flattener, citing Kingslake. Kingslake was the son-in-law of A. E. Conrady. Conrady also mentions what would be referred to today as a Smyth lens, in a 1918 British publication.
The 1955 Edmund booklet, 'How to Condense and Project Light with Lenses', shows a Petzval telescopic system with and without a field flattener. The term Smyth lens is not mentioned but it is clear that is what is shown. <http://www.astromart.com/articles/article.asp?article_id=467>
In a 1950's instruction sheet on the Edmund Barlow lens (which at the time was a singlet negative lens) it is stated that many amateurs use a Barlow - eyepiece combination in the same housing. Strictly speaking these are not dedicated Barlow type eyepieces as the common focal ratios of telescopes then were typically f/8 for Newtonians and f/15 for achromatic refractors. It is only when focal ratios are faster than f/6 that it becomes necessary to design the eyepiece - Barlow combination to specifically balance the aberrations of the other.
In 1971, Franz Schlegel, optical designer at Rodenstock filed a patent (US3,738,735 - issued 12Jun1973) for an objective intended for a night vision apparatus. A pair of Smyth lenses introduced positive field curvature to match the back focal distance.
In 1973, Klein, an optical designer at Leitz, developed a zoom Pancratic microscope eyepiece. This eyepiece used a dedicated Barlow lens. A later development c1990 by Zeiss gives a constant 50° field over the zooming range. Some Japanese zoom lenses for astronomical purposes having Smyth - Barlow lenses, were imported into the United States during the 60's & 70's. Many were unsatisfactory and had numerous ghost images owing to uncoated internal elements.
In 1977, Albert Nagler, who was employed by Farrand Optical from 1957-73 and later Keystone Camera, founded TeleVue Optics. In 1982 the company launched the first of a series of 82° ultra-wide angle eyepieces employing a removable Smyth lens. Nagler's first series exhibited pronounced spherical aberration of the exit pupil which limited its satisfactory use to night viewing only. Spherical aberration of the exit pupil cannot be tolerated in military eyepieces as used in firing control, or in periscopic type applications. Later Nagler ultra-wide angle series 2, 4 & 5 were essentially free of this defect. In the early 80's, Meade Instruments followed with an 84° ultra-wide angle eyepiece also using a Smyth lens.
There is an interesting biography of Nagler written by David Levy in the June 1999 Sky & Telescope. ("An Eye to the Stars" Sky & Telescope, 1999, June, pp99-100)
In the 80's Bausch & Lomb developed a 100° eyepiece for use in the U.S. Army's M1 Abrams tank. Information is sketchy on this particular eyepiece and it may have employed a Smyth lens within the body of the telescope, located near the eyepiece field lens. One U.S. amateur owns three (probably slightly out of specification rejects). The field lens is 2"5/16 diameter and the eye lens 1"3/4 diameter - clear apertures.
100° eyepieces using a triplet Smyth lens were used on a 20x110 binocular c1990 and first manufactured in Poland and later in Russia by the firm RuBland. Some of the Russian 20x110's made their way to the United States and were sold by Deutsche Optik. These binoculars are quite compact for their aperture. <http://binofan.home.att.net/rus20x110.htm>
In 2007, TeleVue Optics introduced a 100° eyepiece. Several focal lengths are available. In 2009, the firm Explore Scientific introduced a 100° eyepiece. This particular eyepiece is dry nitrogen filled to prevent internal dewing, and is sealed to make it water resistant to a depth of about 3 feet. Lens arrangements for these eyepieces are not known at present as neither deisign has been Patented.
The TeleVue 'Powermate' telecentric amplifier is based on a similar principle to the Smyth lens, as recorded in US Patent 2,620,706 - Dec.1952, applied for by Aaron A. Levin on behalf of Bausch & Lomb on 23Sep1948. The negative doublet included in the objective group and the biconvex element between it and the image plane comprise a telecentric system. The eyepiece is basically a Kellner type married to a tele-negative amplifier.
Finally the Smyth lens is used in compensating eyepieces of which a classic example was the Klee Pretoria eyepiece, designed to correct the coma inherent to an f/4 Newtonian. (H. W. Klee and M. W. McDowell, The Pretoria Eyepiece, Telescope Making, #29 1986).
Those interested in optical patents are referred to ATM Book III (1953) where James Shean of Bausch & Lomb, in his article on 'Elementary Camera Lenses', gives three pages of information on obtaining copies of US Patents, and also a list of US City public libraries that have copies. The New York City Public Library has an extensive list of both US and foreign patents. If one is a member of a large public library this information is relatively inexpensive to obtain and in many instances can readily be found. Today patents can be searched via the Internet although there is usually a fee payable prior to download. Google have an advanced patent search website: <http://www.google.co.uk/advanced_patent_search>
It may be asked why the use of a Smyth element was not more heavily used immediately after WWII, especially for astronomical purposes. A likely explanation was the inventory of war surplus Erfle eyepieces at knock down prices, which satisfied the demand for wide angle eyepieces well into the 1970's. Because most telescopes at the time were slow focal ratio (f/8 - f/15), there was little need for more advanced types. Also during the 50's - 60's and much of the 70's, 2-inch eyepieces were uncommon, eyepieces were either 0".965 (24.5mm-Swift fitting) or 1".25-inch or RAS thread. When the tastes of amateur astronomers shifted to a preference for compact Dobsonian reflectors and faster apochromatc refractors, the demand for more highly corrected wide angle eyepieces with better edge definition grew. A number of 80° - 90° eyepieces in 1".25-inch standard were available in the late 60's - 70's, but were limited in the focal lengths offered. 90° eyepieces in fact were used by Zeiss (prior to and during WWII) on binoculars, using only 4 elements with one surface aspherised. Some 80° - 90° eyepieces were found on commercial binoculars in the 60's and 70's, but did not employ a Smyth lens. In some instances the listed afov fell markedly short of the claimed value. Viewed from a chronological perspective there is a clear hierarchy to the design of Smyth lens systems. The availability of modern high dispersion glasses and ray tracing software permit optical designers to optimise older designs and improve performance characteristics not possible in an earlier pre-computer era. Nevertheless it is abundantly clear the older designers understood basic principles quite satisfactorily and in most cases pushed their designs to the limit, whilst the current generation of optical designers have in some instances simply modified the older designs to satisfy the needs of today's market.
This article probably does not list all the publications on usage of Smyth field flatteners. It is clear however the Smyth field flattener principle is an essential feature of ultra wide angle eyepieces. US Patents issued up to 2000 for eyepieces using a negative element immediately in front of the image plane (a Smyth lens) are:
6,104,543 filed 17Sep1999 issued 15Aug2000 Inventor: Takaaki Yano Assignee: Asahi Kogaku
6,011,655 filed 17Jan1996 issued: 04Jan2000 Inventor: Moriyasu Kanai Assignee: Asahi Kogaku
6,008,949 filed 18Nov1998 issued 28Dec1999 Inventor: Saburo Sugawara Assignee: Canon Kabushiki Kaisha
5,973,847 filed 03Nov1997 issued 26Oct1999 Inventor: Saburo Sugawara Assignee: Canon Kabushiki Kaisha
5,969,873 filed 30Dec1997 issued 19Oct1999 Inventor: Saburo Sugawara Assignee: Canon Kabushiki Kaisha
5,812,324 filed 10Jun1996 issued 22Sep1998 Inventor: Satoshi Fukumoto Assignee: Nikon
5,757,553 filed 04Feb1997 issued 26May1998 Inventor: Saburo Sugawara Assignee: Canon Kabushiki Kaisha
5,748,380 filed 20Sep1996 issued 05May1998 Inventor: Mitsuhiro Yanari Assignee: Nikon
5,684,635 filed 14Jul1994 issued 04Nov1997 Inventor: Saburo Sugawara Assignee: Canon Kabushiki Kaisha
5,638,213 filed 02Jub1995 issued 10Jun1997 Inventor: Yasunori Ueno Assignee: Nikon
5,619,379 filed 15Jun1995 issued 08Apr1997 Inventor: Satoshi Fukumoto Assignee Nikon
5,612,823 filed 03Feb1995 issued 18Mar1997 Inventor: Noboru Koizummi Assignee Fuji Photo Optical
4,747,675 filed 01Jun1987 issued 31May1988 Inventor: Albert Nagler
cites Altman 2,423,676 & Bertele 2,549,158 ¤
4,720,183 filed 27Feb1986 issued 19Jan1988 Inventor: Donald C. Dilworth Assignee: Optical Systems Design
4,623,224 filed 23Sep1982 issued 18Nov1986 Inventor: Thomas L. Clarke - cites 108,822
4,286,844 filed 07Nov1979 issued 01Sep1981 Inventor: Albert Nagler
4,054,370 filed 09Jul1976 issued 18Oct1977 Inventor: Andor A. Fleischman Assignee: Bell & Howell
first computer optimised eyepiece
3,738,735 filed 21Jul1971 issued 12Jun1973 Inventor: Franz Schlegel Assignee: Rodenstock
3,384,434 filed 02Apr1965 issued 21May1968 Inventor: W. H. Scidmore & Mary D. Flanagan
3,352,620 filed 03Apr1964 issued 14Nov1967 Inventor: W. H. Scidmore & Robert J. Wolfe
3,233,513 filed 03Aug1962 issued 08Feb1966 Inventor: H. Wagner & Adolf Sterlepper
2,621,564 filed 07Sep1950 issued 16Dec1952 Inventor: L. J. Bertele
2,549,158 - p1 2,549,158 - p2 filed 17Apr1947 issued 17Apr1951 Inventor: L. J. Bertele ¤
2,423,676 filed 20Nov1943 issued 08Jul1947 Inventor: F. E. Altman ¤
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