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[[Berkas:Aperture diagram.svg|
Dalam [[optika]], '''bukaan'''
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
Pada tahun 1867, [[Thomas Sutton]] dan [[George Dawson]] mendefinisikan
<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
<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>
<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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== Pengaruh bukaan ==
Semakin besar angka bukaan, berarti semakin kecil diameter lubang diafragma di bagian dalam lensa. Besarnya diameter terbukanya diafragma akan membuat cahaya yang masuk menjadi lebih banyak, sehingga [[pajanan]] cahaya bertambah dan akibatnya tingkat keterangan foto bertambah, demikian pula sebaliknya.
Pengaruh lain dari bukaan adalah terjadinya perbedaan [[ruang ketajaman]]. Angka bukaan yang
== Notasi ==
Pada [[fotografi]], karena [[lensa fotografi]] tidak hanya terdiri dari sebuah [[
: <math>N = f/D \,</math>
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Satu contoh, misalnya panjang fokus adalah 16x diameter pupil, maka nilai bukaan N=16, sedangkan nilai tingkap = f/16.
Dengan bertambahnya panjang fokus lensa, nilai bukaan akan bertambah, karena [[sudut pandang]] ''entrance pupil'' yang mengecil dan menghasilkan proyeksi berupa [[lingkaran citra]] dengan diameter yang lebih kecil pula. Jika nilai bukaan tidak ditambahkan sebanding dengan panjang fokus lensa, maka diameter lingkaran citra yang terdifraksi dari tingkap yang diteruskan oleh ''exit pupil'' ke arah [[cermin refleks]] dan terpantul ke permukaan [[bidang fokal]], akan membuat [[vignet]]. Pada desain [[lensa cepat]] ([[bahasa Inggris|en]]:''fast lens'') nilai bukaan dapat dipertahankan kecil terhadap pertambahan panjang fokus karena terdapat mekanisme bilah
Nilai <math>1 \over N</math> disebut [[rasio tingkap]] ([[bahasa Inggris|en]]:''aperture ratio'').
== Penggunaan notasi stop ==
Istilah stop, selain digunakan sebagai padanan kata untuk diafragma, menjelaskan satu teknik ''bracketing'', juga digunakan untuk menjelaskan urutan geometrik nilai pajanan (
Menurut sistem [[APEX]] (''Additive System of Photographic Exposure'') dari [[ASA]] (''American Standards Association''), nilai pajanan EV adalah penjumlahan aritmatik dari '''''f-stop''''' atau '''''aperture-stop''''' dengan '''''time-stop''''' atau '''''shutter-stop''''' dengan persamaan:
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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>
== Referensi ==
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* {{en}} [http://www.largeformatphotography.info/fstop.html Large format photography—how to select the f-stop]
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[[Kategori:Istilah fotografi]]
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