Editing Bifocals

{{Short description|Eyeglasses with two distinct optical powers}}
{{For|the bifocal tensor|Fundamental matrix (computer vision)}}
[[File:Bifokalbrille (fcm).jpg|thumb|A bifocal lens with areas of differing magnification]]
[[File:Jan-Öjvind Swahn vid doktorspromovering 2006-06-02.jpg|thumb|Bifocals with separate lenses. In this case, the Swedish ethnologist {{Ill|Jan-Öjvind Swahn|sv}}.]]
'''Bifocals''' are [[glasses|eyeglasses]] with two distinct [[optical power]]s correcting vision at both long and short distances. Bifocals are commonly prescribed to people with [[presbyopia]] who also require a correction for [[myopia]], [[hyperopia]], and/or [[astigmatism]].

==History==
[[Benjamin Franklin]] is generally credited with the invention of bifocals. He decided to saw his lenses in half so he could read the lips of speakers of [[French language|French]] at court, the only way he could understand them.<ref>{{Cite book|last=Bronowski|first=Jacob|url=http://archive.org/details/ascentofman0000bron_y1z2|title=The Ascent of Man|publisher=BBC Books|year=1990|isbn=978-0-563-20900-3|location=London|pages=271|author-link=Jacob Bronowski|orig-year=1973}}</ref> Historians have produced some evidence to suggest that others may have come before him in the invention; however, a correspondence between [[George Whatley]] and [[John Fenno]], editor of the ''[[Gazette of the United States]]'', suggested that Franklin had indeed invented bifocals, and perhaps 50 years earlier than had been originally thought.<ref>{{cite journal |last1=Letocha |first1=Charles E. |title=The invention and early manufacture of bifocals. |journal=Survey of Ophthalmology |date=November 1990 |volume=35 |issue=3 |pages=226–35 | doi=10.1016/0039-6257(90)90092-A |pmid=2274850}}</ref> On the contrary, the College of Optometrists concluded:
:Unless further evidence emerges all we can say for certain is that Franklin was one of the first people to wear split bifocals and this act of wearing them caused his name to be associated with the type from an early date. This no doubt contributed greatly to their popularisation. The evidence implies, however, that when he sought to order lenses of this type the London opticians were already familiar with them. Other members of Franklin's circle of British friends may have worn them even earlier, from the 1760s, but it is at best uncertain (and arguably improbable?) that split bifocal lenses had a famous gentleman inventor. Since many inventions are developed independently by more than one person, it is possible that the invention of bifocals may have been such a case.<ref name=collegeoptometrists>{{cite web|title=The 'Inventor' of Bifocals?|author=The College of Optometrists|url=http://www.college-optometrists.org/en/knowledge-centre/museyeum/online_exhibitions/artgallery/bifocals.cfm|url-status=dead|archive-url=https://web.archive.org/web/20110613044912/http://www.college-optometrists.org/en/knowledge-centre/museyeum/online_exhibitions/artgallery/bifocals.cfm|archive-date=2011-06-13}}</ref>

[[John Isaac Hawkins]], the inventor of [[trifocal lenses]], coined the term ''bifocals'' in 1824 and credited Benjamin Franklin.{{Citation needed|date=October 2024}}

In 1955, Irving Rips of Younger Optics created the first seamless or "invisible" bifocal, a precursor to [[progressive lens]]es.<ref>{{cite news |title=Hall of Fame Honors 12 Industry Veterans |url=https://www.visionmonday.com/article/hall-of-fame-honors-12-industry-veterans/ |work=Vision Monday |date=12 November 2008 |language=en}}</ref> This followed Howard D. Beach's 1946 work in "blended lenses",<ref name="Fanin and Grosvenor">[https://books.google.com/books?id=IbQ3BQAAQBAJ&dq=%22howard+beach%22+bifocal&pg=PA276 Clinical Optics], p. 276, by Troy E. Fannin and Theodore Grosvenor; published 2013 by [[Butterworth–Heinemann]]</ref><ref name=EncyclopediaOfNY>[https://books.google.com/books?id=tmHEm5ohoCUC&dq=%22howard+d+beach%22+bifocal&pg=PA123 Encyclopedia of New York State], p. 123, by Peter Eisenstadt, published 2005 by [[Syracuse University Press]]</ref> O'Conner's <!-- Borish doesn't provide his full name or, really, any other details! This site - https://entokey.com/prescribing-multifocal-lenses/ - says his name was "Connor" and he was in Indianapolis, but I don't know how useful that really is.--> "Ultex" lens in 1910,<ref name="Borish, p 1109">[https://books.google.com/books?id=uxHODAAAQBAJ&dq=ultex+o%27conner+borish&pg=PT1777 Borish's Clinical Refraction], p 1109, by William J. Benjamin; published 2006 by [[Elsevier]] Health Publishing</ref> and Isaac Schnaitmann's single-piece bifocal lens in 1837.<ref name="Borish, p 1108">[https://books.google.com/books?id=uxHODAAAQBAJ&q=schnaitmann&pg=PT1777 Borish's Clinical Refraction], p 1108 by William J. Benjamin; published 2006 by [[Elsevier]] Health Publishing</ref>

== Construction ==
Original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper. Up until the beginning of the 20th century two separate lenses were cut in half and combined in the rim of the frame. The mounting of two half-lenses into a single frame led to a number of early complications and rendered such spectacles quite fragile. A method for fusing the sections of the lenses together was developed by [[Louis de Wecker]] at the end of the 19th century and patented by John Louis Borsch Jr. (1873–1929)<ref>{{Cite journal |last=Ravin |first=James G. |date=November 2009 |title=The multifaceted career of Louis Borsch |url=https://pubmed.ncbi.nlm.nih.gov/19901225/ |journal=Archives of Ophthalmology  |volume=127 |issue=11 |pages=1534–1537 |doi=10.1001/archophthalmol.2009.264 |issn=1538-3601 |pmid=19901225}}</ref> in 1908. In 1915, Henri (Henry) A. Courmettes (1884-1969), a French immigrant to the US, patented the “Flat Top” (or “D Segment”) reading portion of the bifocal.<ref>{{cite web | url=https://patents.google.com/patent/US1160383A/en? | title=Bifocal lens and method for making the same }}</ref> The advantages were wide reading area, less prismatic effects and no image jump between distance and close viewing.  This was first introduced in mass production by the Univis Lens Co. of Dayton, OH. in 1926.<ref>{{cite web | url=https://entokey.com/prescribing-multifocal-lenses/ | title=Prescribing Multifocal Lenses | date=10 July 2016 }}</ref>  In 1935, Courmettes went on to patent the Tilted Bifocal Lens,<ref>{{cite web | url=https://patents.google.com/patent/US1991544A/en? | title=Bifocal lens }}</ref> in 1936, a method of grinding two prescriptions simultaneously on that Tilted Bifocal Lens,<ref>{{cite web | url=https://patents.google.com/patent/US2040242A/en? | title=Method and means for surfacing solid bifocal lenses }}</ref> and in 1951, the Cataract Bifocal Lens.<ref>{{cite web | url=https://patents.google.com/patent/US2574960A/en? | title=Cataract bifocal lens }}</ref> 

Today most bifocals are created by moulding a reading segment into a primary lens and are available with the reading segments in a variety of shapes and sizes.

==Problems==
Bifocals can contribute to falls,<ref>{{Cite web |date= |title=LET'S TALK ABOUT . . . Balance and Dizziness: The Visual Connection |url=https://balanceanddizziness.org/wp-content/uploads/2018/05/balance-dizziness-visual-connection.pdf |access-date=16 October 2024 |publisher=BC Balance & Dizziness |page=2}}</ref> cause headaches, and even dizziness for some wearers. Adaptation to the small field of view offered by the reading segment of bifocals can take some time, as the user learns to move either the head or the reading material rather than the eyes. [[Computer monitor]]s are generally placed directly in front of users and can lead to muscle fatigue due to the unusual straight and constant movement of the head.<ref>{{Cite web |last=Kiani |first=Tamkeen |date=2021-10-27 |title=Bifocals at the Monitor Liberation Day |url=https://nationaltoday.com/bifocals-at-the-monitor-liberation-day/ |access-date=2024-10-16 |website=National Today |language=en-US}}</ref> This trouble is mitigated by the use of monofocal lenses for computer use.

==Future==
{{Unreferenced section|date=October 2024}}
Research continues in an attempt to eliminate the limited field of vision in current bifocals. New materials and technologies may provide a method which can selectively adjust the optical power of a lens. Researchers have constructed such a lens using a [[liquid crystal]] layer applied between two glass substrates.{{citation needed|date=September 2021}}

==Bifocals in the animal world==
The aquatic larval stage of the [[Dytiscidae|diving beetle]] ''[[Thermonectus marmoratus]]'' has, in its principal eyes, two [[retina]]s and two distinct focal planes that are substantially separated (in the manner of bifocals) to switch their vision from up-close to distance, for easy and efficient capture of their prey, mostly [[mosquito]] larvae.<ref>{{Cite web |last=Dawn Fuller |first=Dawn  |date=24 August 2010 |title=Bug With Bifocals Baffles Biologists |url=https://www.sciencedaily.com/releases/2010/08/100823121935.htm |url-status=live |archive-url=https://web.archive.org/web/20100825140335/https://www.sciencedaily.com/releases/2010/08/100823121935.htm |archive-date=25 August 2010 |access-date=25 August 2010 |website=ScienceDaily |publisher=ScienceDaily LLC}}</ref>

==See also==
* [[Binocular vision]]
* [[Binocular rivalry]]
* [[Glasses]]
* [[Eyewear]]
* [[Lens]]

==References==
{{Reflist}}

==Sources==
* {{cite journal
    | author = G. Li
    | title  = Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications
    |date=April 2006
    | journal= Proc. Natl. Acad. Sci. USA
    | volume = 103
    | pages = 6100–6104
    | doi = 10.1073/pnas.0600850103
    | pmid  = 16597675
    | issue = 16
    | pmc = 1458838
|display-authors=etal| bibcode = 2006PNAS..103.6100L
 | doi-access = free
 }}

==External links==
* [http://eyehistory.wordpress.com/letters-on-double-spectacles-by-benjamin-franklin/  Franklin's letters to Whatley concerning double spectacles.]

{{Eyewear}}
{{Benjamin Franklin}}
{{Authority control}}

[[Category:Corrective lenses]]
[[Category:Glasses]]
[[Category:Inventions by Benjamin Franklin]]
[[Category:American inventions]]
[[Category:18th-century inventions]]