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[[Berkas:Aperture diagram.svg|rightka|thumbjmpl|320px|Diagram penurunan nilai tingkaptingkapan, dan juga, penaikan nilai bukaan, pada jeda 1 ''stop''.]]
Dalam [[optika]], '''bukaan''' atau '''nilai bukaan''' ({{lang-en|'''f-number, focal ratio, f-ratio, relative aperture'''}})<ref>Smith, Warren ''Modern Lens Design'' 2005 McGraw-Hill</ref> adalah bilangan yang menunjukkan korelasi [[panjangjarak fokuspumpun]] [[lensa]] terhadap [[tingkap|tingkapan]]. Sebagai contoh, lensa dengan panjangjarak fokuspumpun 100mm, pada pengaturan bukaan 4 (nilai tingkaptingkapan f/4), mempunyai arti bahwa diafragma pada lensa tersebut sedang terbuka dengan diameter 25mm.
Biasanya dilambangkan dengan huruf "f". Nilai bukaan umumnya merupakan urutan 1, 1.2, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, dan seterusnya. Karena bukaan adalah perbandingan antara panjangjarak fokuspumpun lensa dengan diameter dari diafragma yang terbuka saat itu, maka untuk satu nilai bukaan (misalnya 8) pada semua lensa (tidak tergantung dari panjangjarak fokuspumpun lensa tersebut), akan meneruskan [[intensitas cahaya]] yang sama.
 
Pada tahun 1867, [[Thomas Sutton]] dan [[George Dawson]] mendefinisikan '''nisbah tingkapan (''apertal ratio''''') yang pada dasarnya setara dengan '''nilai bukaan (''f-number'''''):<ref name="Sutton">Thomas Sutton and George Dawson, ''A Dictionary of Photography'', London: Sampson Low, Son & Marston, 1867, (p. 122).</ref>:
 
<blockquote>In every lens there is, corresponding to a given apertal ratio (that is, the ratio of the diameter of the stop to the focal length), a certain distance of a near object from it, between which and infinity all objects are in equally good focus. For instance, in a single view lens of 6 inch focus, with a 1/4 in. stop (apertal ratio one-twenty-fourth), all objects situated at distances lying between 20 feet from the lens and an infinite distance from it (a fixed star, for instance) are in equally good focus. Twenty feet is therefore called the 'focal range' of the lens when this stop is used. The focal range is consequently the distance of the nearest object, which will be in good focus when the ground glass is adjusted for an extremely distant object. In the same lens, the focal range will depend upon the size of the diaphragm used, while in different lenses having the same apertal ratio the focal ranges will be greater as the focal length of the lens is increased. The terms 'apertal ratio' and 'focal range' have not come into general use, but it is very desirable that they should, in order to prevent ambiguity and circumlocution when treating of the properties of photographic lenses.</blockquote>
 
Pada 1874, [[John Henry Dallmeyer]] menyebut rasionisbah <math>1/N</math> sebagai '''''intensity ratio''''' sebuah lensa:<ref name="Dallmeyer">John Henry Dallmeyer, ''Photographic Lenses: On Their Choice and Use—Special Edition Edited for American Photographers'', pamphlet, 1874.</ref>
 
<blockquote>The ''rapidity'' of a lens depends upon the relation or ratio of the aperture to the equivalent focus. To ascertain this, divide the ''equivalent focus'' by the diameter of the actual ''working aperture'' of the lens in question; and note down the quotient as the denominator with 1, or unity, for the numerator. Thus to find the ratio of a lens of 2 inches diameter and 6 inches focus, divide the focus by the aperture, or 6 divided by 2 equals 3; i.e., 1/3 is the intensity ratio.</blockquote>
 
Walaupun dia belum mengetahui teori [[Ernst Abbe]] mengenai ''stop'' dan ''pupil'',<ref>[http://books.google.com/books?vid=OCLC01942476&id=-r6LPy-nWPwC&pg=RA3-PA537&dq=theory-of-stops Theory of stops]</ref>, yang dipopulerkan oleh [[Siegfried Czapski]] pada tahun 1893,<ref name="Czapski">Siegfried Czapski, ''Theorie der optischen Instrumente, nach Abbe,'' Breslau: Trewendt, 1893.</ref>, Dallmeyer menyadari bahwa tingkap tidaklah sama dengan diameter sebuah ''aperture-stop''.<ref name="Dallmeyer"/>
 
<blockquote>It must be observed, however, that in order to find the real ''intensity ratio'', the diameter of the actual working aperture must be ascertained. This is easily accomplished in the case of single lenses, or for double combination lenses used with the full opening, these merely requiring the application of a pair of compasses or rule; but when double or triple-combination lenses are used, with stops inserted ''between'' the combinations, it is somewhat more troublesome; for it is obvious that in this case the diameter of the stop employed is not the measure of the actual pencil of light transmitted by the front combination. To ascertain this, focus for a distant object, remove the focusing screen and replace it by the collodion slide, having previously inserted a piece of cardboard in place of the prepared plate. Make a small round hole in the centre of the cardboard with a piercer, and now remove to a darkened room; apply a candle close to the hole, and observe the illuminated patch visible upon the front combination; the diameter of this circle, carefully measured, is the actual working aperture of the lens in question for the particular stop employed.</blockquote>
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== Notasi ==
Pada [[fotografi]], karena [[lensa fotografi]] tidak hanya terdiri dari sebuah [[lensa]] melainkan dari ''exit pupil'', ''entrance pupil'' dan beberapa bilah lensa di antaranya, '''bukaan''' kemudian didefinisikan sebagai rasio antara panjang fokus lensa dan diameter ''entrance pupil''<ref>{{cite book|first=Ralph (et al.)|last=Jacobson|year=1988|title=The Manual of Photography|url=https://archive.org/details/manualofphotogra0000unse|edition=8th ed.|publisher=Focal Press|isbn=0-240-51268-5}} p.49</ref> dengan persamaan:
 
: <math>N = f/D \,</math>
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=== [[T-stop]] ===
Pada lensa kamera sinematografi, terdapat istilah '''T-stop''', kependekan dari '''''Transmission stop'''''',<ref>[[Eastman Kodak]], [http://www.kodak.com/US/en/motion/support/h2/intro01P.shtml "H2: Kodak Motion Picture Camera Films"] {{Webarchive|url=https://web.archive.org/web/20021002095739/http://www.kodak.com/US/en/motion/support/h2/intro01P.shtml |date=2002-10-02 }}, November 2000 revision. Retrieved 2 September 2007.</ref>, yaitu ukuran digunakan untuk kalibrasi berdasarkan tingkat sinar luminasi yang melalui lensa. Kalibrasi ini diperlukan karena lensa kamera sinematografi juga terdiri dari banyak bilah lensa dan tiap bilah lensa dapat meredam dan memantulkan sejumlah intensitas cahaya. T-stop mirip dengan ''exposure value'', luminasi yang terjadi di atas bidang fokal fotografi, namun pada T-stop tidak terdapat komponen ''shutter-stop'', jadi pada praktiknya sebuah T-stop setara dengan f-stop sebuah lensa ideal dengan 100% transmisi luminasi.
 
== Referensi ==
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* {{en}} [http://fcalc.net/manual/fnumbers.html f Number Arithmetic]
* {{en}} [http://www.largeformatphotography.info/fstop.html Large format photography—how to select the f-stop]
{{Fotografi-stub}}
 
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