Definitions

objective lens

Photographic lens

A photographic lens (also known as objective lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically. While in principle a simple convex lens will suffice, in practice a compound lens made up of a number of optical lens elements is required to correct (as much as possible) the many optical aberrations that arise. Some aberrations will be present in any lens system. It is the job of the lens designer to balance these out and produce a design that is suitable for photographic use and possibly mass production.

There is no major difference in principle between a lens used for a camera, a telescope, a microscope, or other apparatus, but the detailed design and construction are different.

A lens may be permanently fixed to a camera, or it may be interchangeable with lenses of different focal lengths, apertures, and other properties.

Theory of operation

Most photographic lenses can be thought of as modified pinhole lenses. A pinhole lens would be excellent except for a few serious limitations. They are limited in their resolution because, while geometric optics says that making the pinhole smaller improves resolution, this also reduces light; furthermore, diffraction limits the effectiveness of shrinking the hole. Most photographic lenses can be thought of as an answer to the question "how can we modify a pinhole lens to admit more light and give higher resolution?" A first step is to put a simple convex lens at the pinhole with a focal length equal to the distance to the film plane (assuming the camera will take pictures of distant objects). This allows us to open up the pinhole a bit. The geometry is almost the same as with a simple pinhole lens, but rather than being illuminated by single rays of light, each image point is illuminated by a focused "pencil" of light. Standing out in the world, you would see the small hole. This image is known as the entrance pupil: all rays of light leaving an object point that enters this pupil will be focused to the same point on the film. If one were inside the camera, one would see the lens acting as a projector. The image of aperture is the exit pupil.

Practical photographic lenses include more lens elements. The additional elements allow lens designers to reduce various aberrations, but the principle of operation remains the same: pencils of rays are collected at the entrance pupil and focused down from the exit pupil onto the image plane.

Construction

A practical camera lens will most often incorporate an aperture adjustment mechanism, usually an iris diaphragm, to regulate the amount of light that may pass. In early camera models a rotating plate or slider with different sized holes was used. These Waterhouse stops may still be found on relatively modern, specialized lenses. A shutter, to regulate the time during which light may pass, may be incorporated within the lens assembly (for better quality imagery), or may be within the camera, or even, rarely, in front of the lens. Some cameras with leaf shutters in the lens omit the aperture, using the shutter to perform this function.

The lens may usually be focused by adjusting the distance from the lens assembly to the image-forming surface, or by moving elements within the lens assembly. Some lenses include a cam system which may vary the distance between the groups to provide better performance when the lens is focused at different distances. This system is usually referred to differently by different manufacturers for marketing purposes. Nikon calls this technology CRC (close range correction) while Hasselblad uses the term FLE (floating lens element)

The lens elements are made of transparent materials. Glass is the most widely used material due to its good optical properties and resistance to scratching. Sometimes lenses are made of materials such as quartz or fluorite. Various plastics, such as acrylic (the material of Plexiglass) can also be used. Occasionally, even germanium and meteoritic glass have been used. Plastics allow the manufacture of strongly aspherical lens elements which are difficult or impossible to manufacture in glass, and which simplify or improve lens manufacture and performance. Plastics are not used for the outermost elements of all but the cheapest lenses as they scratch easily. Moulded plastic lenses have been used for the cheapest disposable cameras for many years, and have acquired a bad reputation: manufacturers of quality optics tend to use euphemisms such as "optical resin". However many modern, high performance (and high priced) lenses from popular manufacturers include molded or hybrid aspherical elements, so it is not true that all lenses with plastic elements are of low photographic quality.

The 1951 USAF Resolution Test Chart is one way to measure the resolving power of a lens in a standardized manner. The quality of the lens material, of the coatings and of the internal manufacture all influence the "resolution" of the lens, even at the same F-number and focal length. Lens resolution is limited by diffraction, and very few photographic lenses approach this resolution. Ones that approach this resolution with the aperture wide open are termed diffraction limited and are usually extremely expensive. Today, most lenses are multi-coated in order to minimize lens flare and other unwanted effects. Some lenses have a UV coating to keep out the ultraviolet light that could expose photosensitive materials and result in inaccurate colors. Most modern optical cements which are used for bonding two glass elements together block UV light quite efficiently, which negates the need to use UV filters with most lenses. UV photographers will go to great lengths to find lenses that they can use effectively for their work, converting lenses with no cemented elements for possible UV work by polishing away the lens coatings, if present.

Aperture and focal length

The two main optical parameters of a photographic lens are the maximum aperture and the focal length. The focal length determines the angle of view, and the size of the image relative to that of the object, while the maximum aperture limits the brightness of the image and the fastest shutter speed usable. A popular third consideration is close focusing distance.

The maximum usable aperture of a lens is usually specified as the focal ratio or f-number, which is equal to the focal length divided by the effective aperture (or entrance pupil) diameter in the same units. The lower the number, the more light per unit area is delivered to the focal plane. Larger apertures (smaller f-numbers) provide a much shallower depth of field than smaller apertures, other conditions being equal. Practical lens assemblies may also contain mechanisms to deal with measuring light, secondary apertures for flare reduction, and mechanisms to hold the aperture open until the instant of exposure to allow SLR cameras to focus with a brighter image with shallower depth of field, theoretically allowing better focus accuracy.

Focal lengths are usually specified in millimetres (mm), but older lenses marked in centimetres (cm) and inches are still to be found. For a given film or sensor size, specified by the length of the diagonal, a lens may be classified as

  • Normal lens: angle of view of the diagonal about 50° and a focal length approximately equal to the diagonal produces this angle.
  • Macro lens: angle of view narrower than 25° and focal length longer than normal. These lenses are used for close-ups, e.g., for images of the same size as the object. They usually feature a flat field as well, which means that the subject plane is exactly parallel with the film plane.
  • Wide-angle lens: angle of view wider than 60° and focal length shorter than normal.
  • Telephoto lens or long-focus lens: angle of view narrower and focal length longer than normal. A distinction is sometimes made between a long-focus lens and a true telephoto lens: the telephoto lens uses a telephoto group to be physically shorter than its focal length.

The 35mm film format is so prevalent that a 90mm lens, for example, is sometimes assumed to be a moderate telephoto; but for the 7×5cm format it is normal, while on the large 5×4 inch format it is a wide-angle. In general, the smaller the film or sensor surface, the smaller the angle of view. This can be corrected with lenses with shorter focal lengths.

An example of how lens choice affects angle of view. The photos below were taken by a 35 mm camera at a constant distance from the subject.

A side effect of using lenses of different focal lengths is the different distances from which a subject can be framed, resulting in a different perspective. Photographs can be taken of a person stretching out a hand with a wideangle, a normal lens, and a telephoto, which contain exactly the same image size by changing the distance from the subject. But the perspective will be different. With the wideangle, the hands will be exaggeratedly large relative to the head. As the focal length increases, the emphasis on the outstretched hand decreases. However, if pictures are taken from the same distance, and enlarged and cropped to contain the same view, the pictures will have identical perspective. A moderate long-focus (telephoto) lens is often recommended for portraiture because the perspective corresponding to the longer shooting distance is considered to look more flattering.

Number of elements

The complexity of a lens—the number of elements and their degree of asphericity—depends upon the angle of view and the maximum aperture, among other variables including intended price point. An extreme wideangle lens of large aperture must be of very complex construction to correct for optical aberrations, which are worse at the edge of the field and when the edge of a large lens is used for image-forming. A long-focus lens of small aperture can be of very simple construction to attain comparable image quality; a doublet (with two elements) will often suffice. Some older cameras were fitted with "convertible" lenses of normal focal length; the front element could be unscrewed, leaving a lens of twice the focal length and angle of view, and half the aperture. The simpler half-lens was of adequate quality for the narrow angle of view and small relative aperture. Obviously the bellows had to extend to twice the normal length.

Good-quality lenses with maximum aperture no greater than f/2.8 and fixed, normal, focal length need at least three (triplet) or four elements (the trade name "Tessar" derives from the Greek tessera, meaning "four"). The widest-range zooms often have fifteen or more. The reflection of light at each of the many interfaces between different optical media (air, glass, plastic) seriously degraded the contrast and color saturation of early lenses, zoom lenses in particular, especially where the lens was directly illuminated by a light source. The introduction many years ago of optical coatings, and advances in coating technology over the years, have resulted in major improvements, and modern high-quality zoom lenses give images of quite acceptable contrast, although zoom lenses with many elements will transmit less light than lenses made with fewer elements (all other factors such as aperture, focal length, and coatings being equal)

Zoom lenses

Some lenses, called zoom lenses, have a focal length that varies as internal elements are moved, typically by rotating the barrel or pressing a button which activates an electric motor. Commonly, the lens may zoom from moderate wide-angle, through normal, to moderate telephoto; or from normal to extreme telephoto. The zoom range is limited by manufacturing constraints; the ideal of a lens of large maximum aperture which will zoom from extreme wideangle to extreme telephoto is not attainable. Zoom lenses are widely used for small-format cameras of all types: still and cine cameras with fixed or interchangeable lenses. Bulk and price limit their use for larger film sizes.

Lens mounts

Many Single-lens reflex cameras, and some rangefinder cameras have detachable lenses. A few other types do as well, notably the Mamiya TLR cameras. The lenses attach to the camera using a lens mount, which often also contains mechanical or electrical linkages between the lens and camera body. The lens mount is an important issue for compatibility between cameras and lenses; each major camera manufacturer typically has their own lens mount which is incompatible with others; notable exceptions are the Leica M39 lens mount for rangefinders, M42 lens mount for early SLRS, the later Pentax k mount, and the Four Thirds System mount for dSLRs, all of which are used by multiple camera brands. Most large-format cameras take interchangeable lenses as well, which are usually mounted in a lensboard or on the front standard.

Special-purpose photographic lenses

Process and apochromat lenses are normally of small aperture, and are used for extremely accurate photographs of static objects. Generally their performance is optimized for subjects a few inches from the front of the lens, and suffers outside this narrow range.

  • Enlarger lenses are made to be used with photographic enlargers (specialised projectors), rather than cameras.
  • Lenses for aerial photography.
  • Fisheye lenses: extreme wide-angle lenses with an angle of view of up to 180 degrees or more, with very noticeable (and intended) distortion.
  • Stereoscopic lenses, to produce pairs of photographs which give a 3-dimensional effect when viewed with an appropriate viewer.
  • Soft-focus lenses which give a soft, but not out-of-focus, image and have an imperfection-removing effect popular among portrait and fashion photographers.
  • Infrared lenses .
  • Ultraviolet lenses.
  • Swivel lenses rotate while attached to a camera body to give unique perspectives and camera angles.
  • Perspective control lenses (PC lenses), used in architectural photography to mitigate the effects of perspective. These lenses are very popular with landscape and macro photographers as well. These lenses have a larger image circle than standard lenses for the format, and shifting the lens will provide the same effect as moving the entire camera in the direction of the shift. Only a few perspective control lenses for 35mm and medium format include the tilt function, usually reserved for large format cameras. These are called tilt-shift lenses.

Notable photographic optical lens designs

Some notable photographic optical lens designs are:

Some lens manufacturers (2006):

References

See also

External links

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