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Additional Lens Types

lenses focal length image

MICHAEL R. PERES
Rochester Institute of Technology

At the time of purchase, many basic cameras come with a standard lens. Cameras that do not have a removable lens are the most common types of cameras sold and represent principally amateur and some prosumer camera models. Many special camera types may also have an attached lens. Single-use and some point-and-shoot cameras are examples of cameras with a fixed lens. Lenses that are attached at manufacture will vary in their focal length from wide angle to normal view. Many of these cameras might also have a zoom lens instead of a fixed focal lens, which is not removable. The standard lens for such cameras would be calculated by the angle of view requirement that is needed for the illumination to cover the film/sensor diagonal, which is a view similar to that of the human eye or 53 degrees. For more about lenses, please refer to the Photographic Optics essay. In the professional and advanced amateur markets, interchangeable lenses are desirable and offer a wide choice of alternative focal lengths. Many specialized lens types were designed and sold during the 20th century. Their features were influenced by the application/purpose of their design.

Afocal Lens — A lens with two elements that are separated by the total of their focal length together. An afocal lens produces magnification and is found in astronomical telescopes. Afocal lenses may also have variable-power attachments. Anamorphic Lens — A lens that produces images having different scales of reproduction in the height and width of the image dimensions. Most anamorphic lenses compress only one direction. The anamorphic lens was first demonstrated in 1927 by H. Chrétien, but did not become popular until the introduction of wide screen projection, primarily in cinematography, became popular. Aspheric Lens — A lens that has one or more of its surfaces that are not spherical. A lens’ refractive error, which might result in spherical aberration, is reduced with this feature so fewer elements may be needed. Aspheric single-element lenses may be used in simple cameras or for specialized work. Aspheric lenses are difficult and costly to make, so the price of these lenses is high. For illumination systems, however, condenser lenses with molded, rather than ground aspheric surfaces, are cost-effective to produce. Catadioptric Lens — Special lenses that are used where long focal distances are required. Telephoto lenses were invented to minimize the long lens to detector distances and catadioptric (cat) lenses further this compacting by using lenses that are characterized as folded or shortened. Cat lenses use both lenses and mirrors to form images. The name is derived from the dioptrics (reflecting lens surface and catoptrics) refracting lens. The image is formed as the light from the scene passes through a glass element except for the central region where there is an opaque disc. It is there that a concave mirror and then another smaller mirror reflects and focuses the image on its way to the detector. The glass lens helps to control the aberrations that are typical of mirror lenses. Location of the lens focal length is determined using traditional methods. Catadioptric lenses are capable of forming images of excellent definition, but as a consequence of the lens focal length, which is very long, the lens does not have a variable aperture. Condenser Lens — A lens system that controls the light from a source, which forms an evenly illuminated image field of the light source. Condenser lenses form illumination that may be directed into the entrance pupil of another lens. Condenser systems were and are used in enlargers, projectors, camera viewfinders, and microscopes. Since the illumination is forming but is not an image, the optical quality of these lenses can be inferior. The subject would be located between the condenser and the image-forming lens, which is nearer to the condenser. In a condenser enlarger, various lenses are used for the different formats. In a microscope, the substage condenser is rated by its numerical aperture and is usually color corrected. Convertible Lens — Designed so that one or more of the elements can be removed, which subsequently changes the lens’ focal length. Generally removing one of the positive elements will increase the lens’ focal length, Since the focal length and lens-to-detector distance is changed, the marked f-stop scale is not accurate and most often will have an additional set of information provided for use in both situations. The image quality will be different with the two focal lengths. Stopping the lens down will minimize any aberrations and other image defects that may surface from this situation. Enlarger Lens — Used to produce images for projection printing applications. The lens aberration corrections are for short object distances and produce a plan or a flat image field relatively free from distortions. Performance is optimized for a given magnification. The lenses may successfully be used on cameras for magnification work and copying when used in reverse. Field Lens — Usually a simple lens that is located at or near the focal plane of another lens. Field lenses may flatten the image from the primary lens and is used to avoid the darkening in the corners of illumination and imaging systems such as in viewfinders and in microscopes and endoscopic imaging systems. Fisheye Lens — A lens that has an extreme angle of view. The limits of physics of such lenses define the angle of coverage of distortion-free images to about 120 degrees. The design of retrofocus configurations allows for easy use in a single-lens reflex (SLR) camera. There are two versions of such fisheye lenses. The quasi-fisheye has a circle of illumination that circumscribes the film format and gives a 180 degree angle of view across the diagonal while a true fisheye lens produces a circular image. Flat-Field Lens — A lens that is highly corrected for curvature and produces a flat image surface to accurately match the focal plane. Copying, enlarger, microscope, close-up, and process lenses all have flat fields. Fluorite Lens — A lens using one or more elements of calcium fluoride (CaF 2 ) made from synthetic crystals. This lens has a very high color correction. Fresnel Lens — A converging lens found in the form of a thin panel of plastic or glass with a series of concentric stepped grooves or rings. Each groove’s face has a different angle. Fresnel lenses are used in theater focusing spotlights, field lenses for focusing screens, the overhead projectors, and in electronic flash units. Long-Focus Lens — A lens with a focal length markedly longer than the diagonal of the format in use. The image size is always proportional to lens focal length, and an increase in focal length will also produce a larger image. Telephoto and catadioptric designs are smaller and can possess similar focal lengths. Macro Lens — A term used to describe a lens designed for close-up photography capable of good results up to an image magnification of 1:1. A macro lens has been designed to control aberrations at close working distances and requires the object distance to be pre-set. Macro lenses are optimized for shorter conjugates. As a consequence of these corrections, most lenses of this type have a maximum aperture of approximately f/4. For magnifications greater than ×1.0, when using a bellows or extension tubes, the lens should be reverse mounted to improve performance. Most automatic features will be lost when reverse mounting. Macro-Zoom Lens — A type of zoom or variable focal length lens that also allows close focusing. This feature will rarely create images that are possible using a fixed focal length macro lens. Macro zoom lenses will often create images with some resolution change as well as other performance loss when used in the macro mode. It is, however, a very convenient feature nonetheless and was first demonstrated in 1967. Meniscus Lens — A single element thin lens that has both of its surfaces curved in the same direction. Meniscus lenses can be either positive or negative. If the convex surface radius is smaller, the lens is thicker in the center and is positive. Meniscus lenses are commonly found in supplementary lenses used in close-up photography and simple camera lenses. Micro-Imaging Lens — A lens of the highest quality whose performance is limited only by diffraction, which approaches the theoretical limits of resolving power. This lens is used to produce micro images. Performance is achieved by having a fixed reproduction ratio, a fixed large aperture, and a very small field. Mirror Lens — An image-forming lens that uses flat and curved mirrors instead of glass elements. A mirror lens has minimal chromatic aberration problems and visual focusing when used in the infrared and ultraviolet regions. Many astronomical telescopes use mirror lenses. Negative Lens — Another term for a diverging lens, which is a lens of negative refracting power. In a system where an aerial image is observed, this lens would be indicated. In a simple negative lens, the middle will be thinner than the periphery. Negative lenses produce virtual images. Orthoscopic Lens — A term describing a lens that produces an image with no curvilinear distortion, either pincushion or barrel. Orthoscopic lenses were considered essential in applications such as copying, architectural photography, aerial survey, and photogrammetry Perspective-Control/PC Lens — A wide angle lens that has extra covering power. The image circle exceeds the diameter of the imaging area and these lenses are designed to allow movement of the lens to control image shape without vignetting the image. PC lenses are used on small and medium format cameras that do not have camera movements. Both a vertical and horizontal shift is possible. Contemporary image-processing software such as Adobe Photoshop can correct for this same perspective control. Portrait Lens — A general term used to describe lenses desirable for portraiture that typically have a long focal distance. These lenses are minimally corrected for aberrations, which are often preferred by creating images with softness and lower contrast or soft-focus lenses directly. Positive Lens — Another name for a converging lens or a simple lens that is thicker in the middle than on the periphery. For distant subjects, the light image is focused as a real inverted image on the opposite side of the lens on a screen. Process Lens — A lens found on process cameras that is used for precision copying and color separation work used for photomechanical reproduction application before scanners. Process lenses were corrected for near unit magnification and were apochromatic and essentially distortion free. Long focal lengths were needed to cover large size film formats such as 16 × 20 Kodak Kodalith type 2556. Projector Lens — A lens used for the projection of magnified images on a screen for general viewing as in the cinema or for viewing small format color transparencies or currently found in video projectors. A common focal length for projectors would be twice the diagonal of the format used, but various focal length versions are used also, depending on the projector-to-screen distance. Zoom or varifocal lenses are popular because of the convenience in setting up, but some curvilinear distortion is inevitable. Projection lens systems are prone to a keystoning effect, a condition where an image’s magnification is larger at the base and smaller near the top. Reduction Lens — A lens used in a system such as an optical printer to produce an image in a smaller format by optical reduction. Relay Lens — A lens or group of lenses that is used to transfer an intermediate real image formed by one lens group of an optical system to another part by optical projection. In a microscope, a change of magnification will occur—the image will be inverted or lengthened in the optical path in an imaging system or viewfinder. Soft-Focus Lens — A lens designed primarily for portrait photography by the deliberate introduction of spherical aberrations that produce images diffused and unsharp with correct color. Some are characterized by a sharp core focus with halo as the outer area of the lens is involved. Stopping the lens down will reduce the aberrations and halo effect. Split-Diopter Lens — A supplementary close-up lens in the form of a half-lens or semi-circle to cover one half of the front element of the prime lens. It is used to enable two subjects at different distances to be recorded in sharp focus. Supplementary Lens — A lens that is added to the front of the prime camera lens to change that lens’ focal length and frequently allowing shorter working distances. The term supplementary is typically used to describe positive and negative simple lenses. A negative meniscus lens will increase a lens’ focal length; however, the lens’ focusing system will require additional separation. Teleconverter Lens — An optical attachment that is inserted between the camera lens and body to increase the focal length of the camera’s prime lens. Teleconverters are not strictly a simple lens but rather a group of lens elements. They will often double or triple the focal length of the prime lens acting as a telephoto of short physical length. The entrance pupil of a lens with a teleconverter is unaffected. However, the doubling or tripling of the focal length will mean that maximum aperture is decreased by two or three stops, respectively, e.g., a 200mm f/4.0 lens with a x2 converter becomes a 400mm f/8.0 telephoto. The minimum focusing distance will not change. Telephoto Lens — A long focus lens distinguished by a different optical construction providing a more compact design. Telephoto lenses and microscopes were invented to see more distant objects more clearly. Long focal length lenses will achieve image magnification; however, their focus distance is impractical based on the camera limitations. A negative lens group, which is located behind a positive lens of longer focal length, displaces the nodal planes in front of the lens and shortens the image focusing distance requirements. Tele-photo lenses are well corrected, possibly with some residual pincushion. The use of ED glass and internal focusing have given rise to a class of lenses called super telephotos, which have focal lengths from 200 to 800mm and maximum apertures f/2 to f/5.6, depending on focal length. Use of a teleconverter increases their focal length. Variable Focal Length Lens — A term used to describe a zoom lens. Wide Angle Lens — A lens having significantly greater covering power than a normal lens of equal focal length. Early versions suffered from poor covering power and fall off. A wide angle lens may be defined as a lens having a focal length of less than 53 degrees, the angle of coverage of a normal lens. A fisheye lens, although quite unique, is a wide angle lens. Common wide angle lenses might include a 20, 24, or 35 mm lens. Contemporary wide angle lenses have a retrofocus design.

Retrofocus produces a short focal length with a long back focal distance by shifting the nodal planes. The asymmetry can cause some residual barrel distortion, but reflex mirrors or beam splitters can be inserted between lens and film. Such lenses have increased complexity, bulk, and cost, but they provide less fall off of light toward the corners of the film than conventional wide angle lenses of the same focal length.


Zoom Lens — A lens with a variable focal length that can be changed continuously between fixed limits while the image stays in acceptably sharp focus regardless of focal length. The visual effect in the viewfinder is that of a smaller or larger image as the focal length is decreased or increased. Zoom ratios of 2:1 to 8:1 are available for still photography cameras and 10:1 to 20:1 might be found in cine and video.

For a working photographer, the ideal solution to picture making is having the right lens for all situations. There are two basic methods for achieving movement of the lens elements in zoom lenses to accomplish focal length changes. One method is to link the elements so everything moves together at the same distances. This method is an optical design. The other method to achieve variable focal lengths is referred to as mechanical where various elements move separately. This type of design is very complicated to manufacture.

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almost 7 years ago

the type of lenses could be placed to one side while the definition on the other, this makes it easier for people who are looking for a specific type of lens