It is called target device containing the set of converging and diverging lenses and, in some cases, the focusing system and / or sealing, which are part of the optics of camera both camera as video .
Its function is to redirect the light beams to create an “optical” image on a photosensitive support, allow a focus as accurate as possible and maintain a constant collimation of the optical elements. This support evolved from the early stages of chemical photography , to image sensors in the case of a digital camera .
History of the lens
The orifice of the camera obscura is considered as the first lens, since it allows to pass through it the light coming from an exterior scene and projects it on the interior walls or on a canvas (see pinhole camera ).
Subsequently, this hole was replaced by a spherical lens, which concentrated a greater amount of rays in the same point; despite the advantage of the amount of light, a lens has several disadvantages: to be shaped as a prism, has the disadvantage of scattering light, a phenomenon known as chromatic aberration ;
In addition, the spherical surface of a lens is not the ideal way to converge the light beams at a single point; this is known as spherical aberration.
Charles Chevalier developed the system known as an achromatic doublet, which corrects chromatic aberration through the use of two lenses, with different levels of refraction but with the same level of dispersion;
The next advance would come from the hand of József Miksa Petzval , who opted for a design with several concave and convex lenses that working together correct optical aberrations more effectively and allow greater openness .
During the late nineteenth and early twentieth century, the foundations of current lenses were laid. Names like those of Carl Zeiss , Paul Rudolph , Harold Dennis Taylor , or the mathematician and physicist Peter Barlow (creator of the lens of Barlow ) and John Dallmeyer figure as great contributors to lay the foundations of the current lenses, which remain almost alterations since those years in the field of optics, except for the appearance of the zum lenses in 1959 and mechanical stabilization systems in the last decade of the 20th century and the first decade of the 21st century.
Main features of the lens
The luminosity of an lens is conditioned by: the number of lenses that compose it, its chemical compositions, the type of coatings on its faces and its diameters. This defines the maximum opening of your diaphragm , known as effective aperture .
The use of the number f as a comparative indicator between lenses is generally widespread , consisting of a quotient between its maximum aperture and its focal length.
The number f is inversely proportional to the opening: the lower the number f, the greater the luminosity. The lenses can be of fixed number f (usually as a characteristic of the catadioptric lenses) or variable and their opening is regulated by the diaphragm .
Generally, the effective aperture is graphically labeled on the lens, relative to the focal length (for example, as “f / 2.8 ” or “1: 2.8“) In the case of zoom lenses, it can be represented by two values, thus indicating the availability of diaphragm aperture according to the operating ends for variable focal length lenses.
It indicates the distance (usually in millimeters, although until the 1950s the centimeter was the unit) from the optical center of the lens to the focal plane .
It defines the “magnification” or zoom of the lens, or how close the image is with respect to the observer’s subjective point of view, and at the same time its angular coverage.
Types of lenses
According to characteristics of the focal length
Fixed focal length : They stand out for having a superior optical quality, since they are built with fewer elements.
They tend to be brighter at equivalent focal lengths; they have less geometric and chromatic aberrations, which damage the quality of the image with respect to zoom-type lenses, and are lighter and more compact than the latter.
As a disadvantage, they make replacement by other lenses necessary when a different focal length is required, since their focal length can not be changed.
Variable focal length : They have the advantage of providing several focal lengths grouped in a single lens body, which is achieved by the movement of certain elements within it.
This makes them more versatile for daily use since it does not require changing the lens to obtain a different focal length. As a disadvantage, they have more optical elements, with which there is a higher probability of aberrations and greater loss of light, which makes them less bright than their fixed focal counterparts.
On the other hand, they are heavier and more fragile than a fixed lens in the same luminosity ratio.
There are three types of variable focal length lenses:
- Multifocal lenses
- Parfocales (true zum ) lenses
- Varifocal lenses.
In the three target classes, their focal length can vary depending on the user; however, the difference between them lies in the fact that the parfocals maintain the focus during the focal length change, while in the multifocal and varifocal ones the focusing distance changes.
The multifocal lenses are those that can be adopted by a limited number of focal lengths, but never an intermediate position between them. It is said that the passage from the maximum focal distance to the minimum focal distance is done discreetly. A multifocal lens is not a zoom lens, making focus mandatory at every focal length change.
The parfocal lenses increases under a microscope does not require refocusing the sample when lenses change. The lenses of the revolver of a microscope are parfocal, that is to say, when changing of a target of low increases to one of more high increases it is only necessary to touch up lightly the focus with the micrometric adjustment.
The varifocal lenses are those lenses that between the minimum focal distance and the maximum focal distance can be placed in any intermediate position going from one to one continuously.
This characteristic was considered important during the early stages of video shooting, since it was necessary for the focus to remain stable during the change of focus (do zum); Nowadays, thanks to the autofocus systems, its relevance has diminished among the manufacturers, which is why most designs of lenses called zum are varifocal.
- Super Wide Angle : with focal lengths between 12 and 28 mm (for 35mm film) and a viewing angle greater than 80 °. They are usually used to get certain special effects that are obtained by their distortion of the image. It is the focal range most used in night photography. 3
- Fisheye lens : It is an extremely wide angle, exceeding 180 ° in some cases. They provide extreme depth of field, and the lines of the image are projected curves, as if they were reflected in a sphere. Two types are distinguished: those that cover the entire exposure surface (film or sensor) forming rectangular images, and those that form a circular image.
- Wide angle : from 28 to 40 mm focal length, and capture angles between 60 and 180 °. They are used for panoramic views of landscapes, architecture, sports.
- Normal : between 45-70 mm and with an angle between 40 and 65º. They are characterized by creating images that look similar to the vision of the human eye. Its depth of field is moderate.
- Telephoto lenses : They have focal lengths between 70 to 300 mm, and with a viewing angle of less than 40 °. They have a reduced depth of field compared to shorter focal lengths. As a characteristic of their image, they compress the perspective by showing relatively distant objects themselves to a similar comparative size.
- Super telephoto lenses: focal length greater than 300mm reaching even 1200mm. They allow to approach objects located at great distances; Their depth of field is minimal, and they are usually used in combination with large openings to obtain images of objects totally differentiated from their background. They are used in the coverage of artistic and sports events, and wildlife photography.
- Macro lenses: They allow focusing at very
closerange. The macro designation applies when the projected image, on the photosensitive surface, has at least the same size as the photographed object.
- Anamorphic lenses, usually used in the cinema (for example in Cinemascope) to narrow the images on the film and thus compress panoramic views. Obviously, then lenses of this type are also used in the projector to reconstruct the original relations.
- Shift or decentrale lenses, in which the optical axis can be displaced, thus controlling the perspective of the camera. They are widely used in architecture, for example to correct the leakage of lines that occurs when making a contrapicado of a building.
- UV lenses, built with quartz or quartz fluoride lenses to be able to photograph the ultraviolet light spectrum.
- Blur lenses, which have a certain level of spherical aberration that produces a certain degree of diffusion or halo effect, in some the degree of diffusion can be varied at will. They are used for portraits, nudes and to achieve a romantic and dreamy atmosphere. This effect can also be achieved through filters or other simple tricks.
- Underwater targets , which, in addition to being watertight, are designed to refract light optimally under water.
- Medical lenses, which are basically macro lenses with a built-in automatic annular flash to avoid shadows. They tend to be of high quality and their main use – as the name suggests – is medical photography.
- Normal or rectilinear : most conventional lenses are classified in it. They are designed with optical corrections that make an approximation to the parallel projection to create the image, in such a way that the straight lines in the scene appear equally straight in the image. Fixed focal length lenses are designed to meet this requirement, while in zoom lenses some degree of curvature distortion is inevitable, which is considered a defect for them.
- Spherical : they project the image as if it were inside a sphere; Due to the type of projection, it is possible to create lenses with a greater angular coverage than the lenses of rectilinear projection, at the expense of the distortion of the projected image. They have their application in surveillance and in the artistic field of photography.
Element of identification
To facilitate physical identification, optical performance, and compatibility between photographic systems (cameras from the same manufacturer or from other manufacturers) each lens has a series of informative data in its housing:
- Manufacturer of cameras : Canon, Leica, Nikon, Olympus, Pentax, Sony – Minolta.
- Target manufacturers: Carl Zeiss, Tamron, Tokina , Sigma, Vivitar.
- Frame system : Canon EF , Canon EF-S , Nikon F
- Manufacturing serial number: Unique identification number, in case of theft or loss.
- Focal length : expressed in millimeters. In the zum lenses , a range of values is expressed indicating the minimum and maximum focal distance.
- Luminosity or number f : In the case of the zum lenses, two different values are also expressed, indicating the luminosity for the minimum and maximum focal lengths.
- Filter diameter : an absolute value in millimeters, expressed with the symbol (Ø). Indicates the diameter of the filter that can be attached in front of the optical system.
- Correction of optical aberrations : By means of the expressions “aspheric” and “apochromatic” the manufacturer makes it known if it has applied a special degree of correction to the lenses.
- Surface treatment of the lenses: indicates the optical treatment of the surface of the lenses through the words “coated” or “multicoated“.
There are multiple parameters with which to attribute higher or lower quality to an lens:
- Definition, that is, the sharpness with which they can reproduce the images, determined by
- Number of elements, and groupings.
- Optical quality of the elements.
- Focal length range: at higher focal length ranges the optical quality of the lens decreases. Optically, it is convenient to have several lenses of fixed or low-range focus, and resort to the change of lenses according to the requirements of the frame.
- Metal frame, more resistant and durable.
- Greater weight, that although without apparent relation usually indicates the use of materials of greater quality in its construction.
- With correction mechanisms of certain optical aberrations : for example of “spherical” ( aspheric in English), or “anamorphic” type.
- Contrast: that is, that there is a linear behavior in the whole spectrum of light frequencies (without varying the intensity of their intensities).
- Type of Frame: Full (Full frame) capable of incising on 36x24mm; or against Cropped (Typically the so-called DX in Nikon)
- Manufacturer, since some manufacturers are key references in quality.