Perhentian aperture dari lensa fotografi dapat disesuaikan untuk mengontrol jumlah cahaya yang masuk ke film atau sensor gambar. Dalam kombinasi dengan variasi dari kecepatan rana, ukuran kecepatan rana akan mengatur film atau gambar tentang derajat sensor terkena cahaya. Biasanya, kecepatan rana yang tinggi akan memerlukan aperture yang lebih besar untuk memastikan pemaparan cahaya yang cukup, dan kecepatan rana yang rendah akan memerlukan aperture yang lebih kecil untuk menghindari paparan berlebihan.
perangkat yang disebut diafragma biasanya berfungsi sebagai stop aperture, dan kontrol kecepatan rana. Fungsi diafragma mirip dengan iris mata – mengontrol diameter efektif pembukaan lensa. Mengurangi ukuran aperture meningkatkan kedalaman lapangan, yang menggambarkan sejauh mana subyek berbaring dekat dari atau lebih jauh dari pesawat yang sebenarnya fokus tampaknya berada dalam fokus. Secara umum, semakin kecil aperture (angka semakin besar) jarak, semakin besar dari bidang fokus pokok mungkin tetap muncul di fokus.

Bukaan lensa biasanya ditetapkan sebagai nomor-f, rasio panjang fokus dengan diameter diafragma. lensa biasanya memiliki seperangkat “ditandai f-berhenti” bahwa jumlah-f dapat diatur ke. A f rendah-nomor menunjukkan pembukaan aperture yang lebih besar yang memungkinkan lebih banyak cahaya untuk mencapai film atau sensor gambar. Istilah fotografi “satu f-stop” mengacu pada faktor √ 2 (sekitar 1,41) perubahan f-nomor, yang pada gilirannya berhubungan dengan faktor 2 perubahan intensitas cahaya.

Prioritas bukaan adalah modus bidikan semi-otomatis yang digunakan dalam kamera. Hal ini memungkinkan fotografer untuk memilih pengaturan aperture dan memungkinkan kamera untuk menentukan kecepatan rana dan kadang-kadang sensitivitas ISO untuk eksposur yang tepat. Ini kadang-kadang disebut sebagai Auto Exposure,  Aperture Prioritas, mode A, Av mode, atau mode semi-otomatis.

Maksimum dan minimum diafragma

Spesifikasi untuk suatu lensa biasanya meliputi diafragma maksimum dan minimum, seperti misalnya f/22 – f/1.4. Dalam hal ini adalah yang terkecil f/22, atau minimal membuka aperture, dan f/1.4 adalah terluas, atau maksimum bukaan. Bukaan maksimum cenderung paling menarik, dan selalu disertakan saat menjelaskan sebuah lensa. Nilai ini juga dikenal sebagai kecepatan lensa, karena secara langsung mempengaruhi waktu pemaparan. bukaan sangat proporsional dengan akar kuadrat cahaya diterima, dan dengan demikian berbanding terbalik dengan akar kuadrat dari waktu bukaan, yang seperti aperture dari f /2 memungkinkan untuk eksposur kali seperempat yang dari f /4.

Nilai Aperture f/2.8 lebih lebar dari biasanya dikenal sebagai “cepat” lensa. Dalam lensa keadaan luar biasa dapat memiliki f-nomor di bawah ini f/1.0. Misalnya, dalam fotografi, baik arus Leica Noctilux-M 50mm ASPH dan Canon 50mm 1960′s era lensa pengintai memiliki diafragma maksimum f/0.95. lensa tersebut cenderung optik eksotis dan sangat mahal; di peluncuran, pada bulan September 2008, Leica Noctilux ritel untuk $ 11.000  lensa Profesional untuk beberapa kamera film memiliki f.-angka serendah f/0.75. film Stanley Kubrick’s Barry Lyndon telah ditembak adegan dengan bukaan relatif terbesar dalam sejarah film: f/0.7.

lensa zoom biasanya memiliki aperture maksimum (minimum f-angka) dari f/2.8 sampai f/6.3 melalui berbagai mereka. Lensa high-end akan memiliki aperture konstan, seperti f/2.8 atau f /4, yang berarti bahwa apertur relatif akan tetap sama di seluruh rentang zoom. Seorang konsumen lebih khas zoom akan memiliki aperture relatif variabel, karena lebih keras dan lebih mahal untuk menjaga diafragma sebanding dengan panjang fokus pada panjang fokus yang panjang; f/3.5 ke f/5.6 ini adalah contoh berbagai variabel aperture umum di lensa zoom konsumen.

f/32 – narrow aperture and slow shutter speed

f/5.6 – wide aperture and fast shutter speed

Source : http://en.wikipedia.org/wiki/Aperture

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Image by : http://media.wiley.com/assets/1007/41/0-7645-9802-3_0213.jpg

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Canon, a leader in photographic and imaging technology, today announces the EF-S 17-85mm f/4-5.6 IS USM and EF-S 10-22mm f/3.5-4.5 USM wide-angle EF-S zoom lenses. Targeted at advanced digital photographers, both lenses are designed to work exclusively with Canon EOS 20D and EOS 300D digital SLR cameras. The focal length ranges of the respective lenses translates to 27-136mm and 16-35mm respectively in 35mm film format “ a spread carefully chosen by Canon to reflect the needs of its rapidly growing base of advanced D-SLR photographers. Both lenses feature Canon ring-type Ultra Sonic Motors (USM) for fast, near-silent auto focus, and a range of features designed to deliver a level of image quality approaching that of Canon revered L-Series professional lenses.

Perfect for digital photography Wide Angle

The EF-S 10-22mm f/3.5-4.5 USM is the market lightest and smallest 16-35mm (35mm equivalent) zoom lens available; ideal for landscape or indoor photography, or for photographers wishing to experiment with pan-focus and dramatic perspective effects. It also boasts a remarkably close focusing distance of just 24 cm. With similar high build quality to the EF-S 17-85mm f/4-5.6 IS USM, the EF-S 10-22mm f/3.5-4.5 USM includes a Super UD (ultra-dispersion) glass lens and two aspherical elements in its 13 element, 10 group construction. The non-rotating front element makes it ideal for use with filters such as polarizing and graduated, both of which are popular for the type of landscape photography for which the lens is suited.

Fast auto-focus

The ring-type USM motors found in both lenses employ ultra-sonic frequency vibrations to drive focussing with unrivalled speed. Canon ring-type USM holds the lens with precision and accuracy, the instant it arrives at the correct focus point and without overshoot. To further improve accuracy and speed, the drive patterns of both lenses are tailored to specifically match that of the EOS 20D and EOS 300D. Both lenses also feature full-time manual focus override without risk of motor damage. This 5x zoom lens offers the widest range of any lens in this class, and features Canon latest optical Image Stabilizer technology, providing up to 3-stop compensation for image blur caused by camera shake and slow shutter speeds. With superb quality suited to semi-professional use, the lens has a 17 element, 12 group construction, including one double-sided aspherical element for crisp, high contrast corner-to-corner detail.

Appealing background blur

The lenses both incorporate an EMD (electromagnetic diaphragm), which employs an almost perfectly circular barrel aperture. This helps create an attractive, even background blur when the photographer minimises depth of field to isolate a subject. This describes the system that allows the rear lens element of EF-S lenses to be positioned closer than normal to the image sensor of APS-C sensor cameras. A white lens mount index differentiates the lenses from other EF lenses, which retain their traditional red marking. A rear rubber ring prevents any damage in the event that an inadvertent attempt is made to mount the lenses on non-compatible cameras.

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Features

Canon, a leader in photographic and imaging technology, today announces the EF-S 17-85mm f/4-5.6 IS USM and EF-S 10-22mm f/3.5-4.5 USM wide-angle EF-S zoom lenses. Targeted at advanced digital photographers, both lenses are designed to work exclusively with Canon EOS 20D and EOS 300D digital SLR cameras. The focal length ranges of the respective lenses translates to 27-136mm and 16-35mm respectively in 35mm film format “ a spread carefully chosen by Canon to reflect the needs of its rapidly growing base of advanced D-SLR photographers. Both lenses feature Canon ring-type Ultra Sonic Motors (USM) for fast, near-silent auto focus, and a range of features designed to deliver a level of image quality approaching that of Canon revered L-Series professional lenses.

Standard

Ideal for travel, mid-range nature/wildlife and general photography, the EF-S 17-85mm f/4-5.6 IS USM is a high quality, lightweight, all-purpose standard zoom lens. This 5x zoom lens offers the widest range of any lens in this class, and features Canon latest optical Image Stabilizer technology, providing up to 3-stop compensation for image blur caused by camera shake and slow shutter speeds. With superb quality suited to semi-professional use, the lens has a 17 element, 12 group construction, including one double-sided aspherical element for crisp, high contrast corner-to-corner detail.

Wide Angle

The EF-S 10-22mm f/3.5-4.5 USM is the market lightest and smallest 16-35mm (35mm equivalent) zoom lens available; ideal for landscape or indoor photography, or for photographers wishing to experiment with pan-focus and dramatic perspective effects. It also boasts a remarkably close focusing distance of just 24 cm. With similar high build quality to the EF-S 17-85mm f/4-5.6 IS USM, the EF-S 10-22mm f/3.5-4.5 USM includes a Super UD (ultra-dispersion) glass lens and two aspherical elements in its 13 element, 10 group construction. The non-rotating front element makes it ideal for use with filters such as polarizing and graduated, both of which are popular for the type of landscape photography for which the lens is suited.

EF-S mount

Both lenses feature Canon EF-S lens mount, matched to the EOS 20D and EOS 300D digital SLRs. The APS-C sized sensor of these cameras results in an effective multiplication factor of 1.6x the indicated focal length. While this creates an obvious advantage for telephoto lenses, it is less beneficial when meeting wide-angle requirements. To meet the needs of the new digital photographers, Canon developed the wide-angle EF-S lens mount. The EOS 20D and EOS 300D are both completely compatible with all existing EF lenses, while EF-S lenses only fit these two cameras. Optimized for Digital SLRs with APS-C size sensors, the EF-S mount enables economic wide-angle solutions often required by advanced photographers. The “S” in the EF-S nomenclature stands for the term short back focus. This describes the system that allows the rear lens element of EF-S lenses to be positioned closer than normal to the image sensor of APS-C sensor cameras. A white lens mount index differentiates the lenses from other EF lenses, which retain their traditional red marking. A rear rubber ring prevents any damage in the event that an inadvertent attempt is made to mount the lenses on non-compatible cameras.

Fast auto-focus

The ring-type USM motors found in both lenses employ ultra-sonic frequency vibrations to drive focussing with unrivalled speed. Canon ring-type USM holds the lens with precision and accuracy, the instant it arrives at the correct focus point and without overshoot. To further improve accuracy and speed, the drive patterns of both lenses are tailored to specifically match that of the EOS 20D and EOS 300D. Both lenses also feature full-time manual focus override without risk of motor damage.

Appealing background blur

The lenses both incorporate an EMD (electromagnetic diaphragm), which employs an almost perfectly circular barrel aperture. This helps create an attractive, even background blur when the photographer minimises depth of field to isolate a subject.

Perfect for digital photography

By optimising lens coatings, Canon engineers have been effective in suppressing flare and ghosting, which are more prone to occur with digital cameras due to reflection off the image sensor. Coatings reduce reflections off the lens surfaces to deliver crisp, undistorted digital images with natural colour balance.

Distant linked flash

Both lenses include new distance metering information, providing compatibility with the E-TTL II flash system of the EOS 20D. When used in conjunction with Canon range of EX Speedlite flash units, this allows for markedly improved flash exposure metering. An optional lens hood accessory is available for both lenses. In harmony with the environment In keeping with Canon stringent environmental policy, only lead free glass is used in the lens optics.

Canon EF-S 10 -22 mm lens specifications