Lens speed

Light intensity of a lens of the quotient of the maximum possible diameter of the entrance pupil and the focal length of a lens is known. It is this ratio usually as the reciprocal of the f-number, eg as a 1/2, 8 or f / 2.8 (2.8 is the f-number, and f is the focal length ).

The largest relative aperture zoom lenses can be dependent on the selected focal length. For example, the term means 70-300 mm ƒ/4-5, 6, that in the short focal length of 70 millimeters, the maximum aperture F-number at 4.0 and at the long focal length of 300 millimeters, the maximum aperture F-number at the 5 can be adjusted 6.

Since the image density increases with the square of the relative aperture, the time required for exposure is inversely proportional to the square of the relative aperture. The maximum relative aperture is adjacent to the focal length and the image circle diameter of one of the three fundamental properties of a lens.

Usual values ​​are between ƒ / ƒ 1.4 and / 5.6, occasionally one still finds ƒ / ƒ 1.25 and / or 6.3 ƒ / 8 Have extreme fast lenses partly light intensities to ƒ / 0.7, in older or simple super telephoto lenses are also found values ​​up ƒ/16. Theoretically any value to ƒ / 0.5 possible pinhole cameras have values ​​between ƒ/100 and ƒ/1000.

The " Super -Q Gigantar ƒ / 0,33 / 40 mm " was built in 1960 by Zeiss for PR purposes and never functioned as a lens.

The 1966 also made ​​by Zeiss " Planar ƒ / 0,7 / 50 mm " is considered the fastest lens in the world.

Comments

Typical and maximum light intensities

Normal lenses (50 mm in small picture format ) move normally at light intensities of 1:1.2 to 1:2.8. However, in some cases they may also reach intensities up to 1:1.0 and more. The brightest photographic lens is designed with Zeiss: The Planar 1:0.7 / 50 mm film footage of moving scenes could be turned with candlelight, such as in the film Barry Lyndon by Stanley Kubrick.

Pros and Cons

• Manual focus: The high speed benefits for SLR cameras to the brightness of the viewfinder and facilitates the focusing. In addition, the larger aperture with a shallower depth of field is accompanied whereby the position of the focus level in the viewfinder can be assessed more accurately. Installation aids for manual focusing, such as sectional image indicators, work even in weak lenses ( 1:5.6 or less) no longer only limited or. Cameras with electronic viewfinder or Live View on a screen can display an electronically brightened image that can optionally be displayed even increased with the help of a software magnifying glass. With cameras in which focusing is performed without the lens, this aspect is irrelevant.
• Autofocus (AF): SLR some have autofocus phase comparison sensors with different sized measuring base. For sensors with larger measurement basis, the camera can focus in principle more precise. The lenses require a higher set, for each sensor suitable light intensity.
• Image design: Lenses with high light intensities increase the design freedom. For example, enabling high- speed lenses in moderate wide-angle and telephoto one isolate a scene in front of a blurred fore - or background. But here also applies: the larger the recording format, the more prominent the game with focus and blur (important in the selection of digital cameras, with their different sensor formats).
• A high speed permits shorter exposure times or the use of lower film speeds with higher resolution and finer grain, or with digital cameras lower noise by using a lower ISO setting.
• Bigger, heavier and more expensive than comparable optics lower light intensity.
• Due to the low depth of field ( DOF also called English depth-of -field ), high demands are placed on the auto focus and slight misalignments faster visible (see focusing errors).
• For very bright lenses can not be used with open aperture when large film or image sensor larger the depth of field for the desired subject is too small.

History and Development

In general, the intensity of lenses could be increased significantly. While the box cameras of the 1920s and 1930s had a typical maximum aperture of 1:11 models from the 1950s already had 1:4 or even 1:2.8.

Comparatively fast lenses there were some time. An example of this is the Petzvalobjektiv, which was constructed jointly by Josef Maximilian Petzval and Peter Wilhelm Friedrich von Voigtländer in 1840. With a maximum aperture of 1:3.6, it was 22 times more light compared to Daguerre's lens from 1839, which under favorable conditions for the first time allowed portraits with exposure times of less than a minute. The Petzvalobjektiv was produced by Voigtländer and sold with great success worldwide. Until 1862 he produced 60,000 pieces.

Significant influence on the ability to produce lenses with high light intensity, the lens design. By using lens combinations of different types of glasses such as crown and flint glass, CaF2 lenses, ED glass and aspherical lens elements integration of aberrations could be kept small despite large numerical aperture.

A milestone represented the Cooke triplet, which was developed in 1893 by Harold Dennis Taylor. It makes possible, at low-cost lenses a light intensity of up to 1:3.6 ( mid-1930s ) and after the introduction of lanthanum glass up to 1:2.8. It is still used today.