Camera lens

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A camera lens (also known as a photographic lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body to capture images on a digital sensor, photographic film or on other media capable of storing an image.

A camera lens can be as basic as a single simple convex element, but most lenses consist of multiple single elements or specialized groups of elements such as doublets or triplets. These elements are typically made of ground glass, but sometime other materials such as plastic are used. The use of multiple elements allows for greater control of optical aberrations that accompany the bending of light. A lens designer must balance controlling these aberrations against increased size, weight, complexity, and target price that accompanies increasing the number and type of elements; thus no "perfect" camera lens exists.[1]

Aperture and focal length[edit | edit source]

Different apertures on the same lens.

The two fundamental parameters of an optical lens are its focal length and maximum aperture. Focal length determines the magnification of the image projected onto the image plane. For a given photographic system (i.e. sensor or film size) the focal length determines the angle of view. Shorter focal lengths provide a wider field of view, while longer focal length lenses provide greater magnification. Focal lengths are typically specified in millimeters (mm).

F-number[edit | edit source]

The diameter of the aperture (or entrance pupil of the lens) controls the intensity of the light that reaches the image plane. The "focal ratio" or "lens ratio"[2] expresses a quantitative relationship between the focal length and the aperture. This dimensionless number is commonly referred to as the f-number or f-stop, and is expressed using the f/ symbol. A wider aperture, identified by a smaller f-number, lets more light reach the focal plane, allowing use of a faster shutter speed for the same exposure.[3]

The maximum usable aperture of a lens is specified as the f-number of the lens. The lower the f-number, the greater the light intensity at the focal plane. Larger apertures (smaller f-numbers) provide a shallower depth of field than smaller apertures, other conditions being equal. Practical lens assemblies may also contain mechanisms to deal with measuring light and mechanisms to hold the aperture open until the instant of exposure to allow a brighter image with a shallower depth of field for the autofocus mechanism then stopping down the aperture to the chosen value before capturing the image. This design facilitates improved focus accuracy.

Focal length classifications[edit | edit source]

Focal lengths for a given system can be classified into a few general groups based on the types of images they can be used to produce. These classifications depend on the diagonal measurement of the given sensor or film plane. For M43, the sensor measures 21.6mm diagonally.

Ultra wide-angle lens[edit | edit source]

The lenses with the shortest focal lengths which provide the widest field of view are classified as ultra wide-angle lenses. The widest of these are typically fisheye lenses which produce an extremely wide angle or even hemispherical view with strong geometric distortion, particularly at the edges of the frame. Fisheye lenses typically capture a field of view of between 150º and 220º (yes, some lenses can "see" behind themselves).[4] Rectilinear ultra wide-angle lenses are ones with minimal distortion or stretching throughout the frame, even at the edges, where straight features appear with straight lines. Rectilinear lenses produce an image more consistent with normal human sight.

Image taken with a circular fisheye lens
Image taken with a rectangular fisheye lens
Circular fisheye lens

For M43, circular fisheye lenses (those which produce hemispherical projections) are available at focal lengths between 3.5mm and 7mm.

Rectangular fisheye lens

Fisheye lenses which fill the rectangular M43 frame are available in focal lengths from 7.5mm up to 12mm.

Rectilinear ultra-wide angle lens

With the M43 sensor, any focal length less than 12mm is classified as an ultra-wide angle lens. There are several lenses (both primes and zooms) which cover these focal lengths while producing rectilinear output.

Ultra wide angle lenses are often used to capture vast landscapes. They can also be useful to exaggerate the subject by use of perspective. Finally, they're employed regularly in architectural photography, particularly for interior images.[5]

Image taken with Olympus 12mm wide-angle lens

Wide-angle lens[edit | edit source]

Wide-angle lenses are those that produce a field of view which is only a bit wider than what a person typically perceives as in focus in normal vision. This includes any lenses delivering an angle of view wider than about 60º. In the M43 system lenses with focal lengths from 12mm up to about 18mm are considered to be wide-angle lenses. Wide angle lenses are useful for shooting in tight spaces where the photographer would have difficulty increasing the distance between herself and her subject. They are also good choices for landscape and architectural photography as well as for capturing full-length or environmental portraits.[5] Be aware, though, that close-up images of people taken with wide-angle lenses can be perceived as unflattering since the effects of perspective can cause the sizes of features (particularly facial features) to appear exaggerated.[6]

Normal lens[edit | edit source]

Normal, or standard, lenses are those delivering a diagonal angle of view of around 40° to 60º. These are lenses with a focal length similar to the sensor diagonal. M43 lenses with focal lengths from around 18mm to 35mm are considered normal lenses. These lenses are ideal for photojournalism and street photography since they capture a view that closely matches human vision.[5]

Telephoto lens[edit | edit source]

Any lens with a focal length significantly longer than the diagonal measure of the film or sensor is a long-focus lens.[7] The angle of view of a such lenses is narrower than the viewer's typical field of view. The most common type of long-focus lens is the telephoto lens, a design that uses special optical configurations to make the lens shorter than its focal length; although this term is used in common parlance to indicate all long-focus lenses. In the M43 system lenses over around 35mm are considered long-focus or telephoto options.

A short-to-moderate telephoto lens (from 35–75mm on M43) is often recommended for portraiture because the perspective corresponding to the longer shooting distance is considered to look more flattering. Another benefit is that these lenses will have shallower depth of field causing backgrounds to go out of focus and thus increasing subject separation.[6]

Super-telephoto lens[edit | edit source]

Lenses with the longest focal lengths are classified as super-telephoto lenses. On M43, these are typically defined as any lenses with focal lengths longer than about 200mm.[5] These lenses are useful for capturing wildlife and sports or any application where the photographer is limited in how close he can get to the subject.[6]

Optical properties[edit | edit source]

All lenses are constrained by certain optical properties which are determined by the focal length, aperture, and other elements of the lens' design.

Depth of field[edit | edit source]

Depth of field (DoF) is the area in front of and behind a focused subject in which the photographed image appears acceptably sharp. This can be calculated based on focal length, aperture, distance to subject, and the acceptable size of the circle of confusion. A particular DoF might be chosen for artistic (e.g. to isolate the subject in the image or to insure as much of the image as possible is in focue) or technical reasons (e.g. to maintain a particular exposure at a certain shutter speed).

For a given subject framing and camera position, the DoF is controlled by the lens aperture diameter or f/number. Reducing the size of the aperture (i.e. increasing the f/number) increases the DoF because only the light travelling at shallower angles passes through the aperture. Because the angles are shallow, the light rays are within the acceptable circle of confusion for a greater distance.[8]

For a given size of subject on the focal plane, the same f-number on any focal length lens will give the same depth of field.[9] This is evident from the DOF equation by noting that the ratio u/f is constant for constant image size. For example, if the focal length is doubled, the subject distance is also doubled to keep the subject image size the same.

Motion pictures make only limited use of aperture control; to produce a consistent image quality from shot to shot, cinematographers usually choose a single aperture setting for interiors and another for exteriors, and adjust exposure through the use of lens filters or light levels. Aperture settings are adjusted more frequently in still photography, where variations in depth of field are used to produce a variety of special effects.

Focus stacking is a technique that allows a photographer to expand the depth of field in an image. This is done by capturing a series of images at slightly different focal distances and then using software to combine them into a single image with a deeper depth of field. This is particularly useful in macro photography since depth of field is smaller at greater subject magnification.[11] This is most often achieved during post-processing, but there are also some cameras in the system which can achieve this effect in camera.[12]

Bokeh[edit | edit source]

Bokeh (or boke) has to do with the quality of the rendering of the out of focus areas of an image. The term comes from the Japanese boke-aji, or just boke, which translates literally as blur, blurred or haze.[13][14] Some misuse the term to mean the out of focus area itself or even the quantity or depth of the such, but these definitions are incorrect.[15]

Since bokeh is a qualitative judgement, opinions on the subject are subjective. That said, image viewers generally agree that some bokeh is more pleasing than other, with most preferring smooth, "creamy" bokeh over discordant, jarring bokeh. Beyond the subjective, discussions of bokeh can also involve the shape of "bokeh balls" which are caused by discreet light sources in the out of focus areas of an image. These can be round (which is typically favored), they can take on a polygon shape mirroring that of the aperture, or they can be distorted into "cats eye" or even swirly shapes.[15] Bokeh balls can also be judged by the severity of their edges; those which exhibit particularly harsh edges being described as "onion rings."[13]

Visual perspective[edit | edit source]

Using lenses of different focal lengths to capture an image with a different camera to subject distance will result in images with different visual perspectives.[5] Imagine a set of photographs taken of a person stretching out a hand with a variety of lenses where the distance from the subject is altered to allow the subject to appear as a consistent size. In this set of images the perspective will be different. Using a wide-angle lens, the size of the hands in the foreground will be exaggerated relative to the head. As the focal length increases, the amount of this exaggeration will decrease. However, if the distance from the camera to subject remains constant, and the images taken at the various focal lengths were enlarged and cropped to contain the same view, the pictures will have identical perspective.

Diffraction limit[edit | edit source]

Diffraction is a physical phenomenon which causes a reduction in image sharpness at small apertures. The impact of diffraction depends on the pixel pitch of the image sensor, that is, the density of image capturing cells on the sensor.[16] Since the M43 sensor is smaller than that of "full frame" or APS-C it reaches the diffraction limit sooner for a particular megapixel count. One can begin noticing the impact of diffraction on image resolution on denser (higher megapixel) sensors when the lens is stopped down to f/8. For the earlier 12MP sensors the diffraction limit isn't a factor until apertures beyond f/11.[16] That's not to say that smaller apertures are unusable; they will still provide a deeper depth of field in the image, but there will be a trade off in decreased resolution when using smaller apertures.

Types of lenses[edit | edit source]

Beyond focal length and aperture, lenses can be categorized in other ways as well. The most obvious would be whether the lens has a fixed or variable focal length. Lenses with a single, fixed focal length are referred to as prime lenses, while those with variable focal lengths are known as zoom lenses. Lenses can further be categorized by the amount of magnification they're able to capture, by their physical dimensions or by special features they employ.

Prime lens[edit | edit source]

As noted above, prime lenses have a single focal length that doesn't change. While these lenses impose the restriction of a single angle of view, the trade-off is that they typically are physically smaller than zoom lenses and often deliver superior optical quality and a wider maximum aperture than can be found in a zoom covering the same focal length. These benefits are due to the need for less complex lens formulas and often fewer lens elements than zooms.

Prime users often say they "zoom with their feet" since moving the camera position is the only way to change perspective when using such a lens.

Zoom lens[edit | edit source]

Zoom lenses offer the user the ability to select from a variety of focal lengths. While these lenses offer convenience, this comes at the cost of complexity. Zoom lenses typically have more intricate optical designs with more elements than primes. This leads them to typically be larger and heavier. Zoom lenses also generally deliver lower resolution and gather less light compared to primes. The complexity of their optical designs can also increase the amount and severity of aberrations they may exhibit.

Most zoom lenses have a variable maximum aperture. That is, the maximum aperture at the wide end of the zoom range is wider than that at the opposite end of the range. Other zooms offer a constant maximum aperture across their full focal length range. These can be useful in allowing the user to adjust the focal length without the concern that doing so will change the exposure; this is particularly valued when shooting with controlled lighting or when shooting video. Constant aperture zooms utilize more sophisticated optical designs than variable aperture lenses. As such, these lenses are generally larger, heavier and more expensive than variable aperture zooms covering the same range.[17]

Zoom lenses are sometimes referred to by the range of focal lengths they offer. For example, a lens that covers 14-42mm would be referred to as a 3x zoom, while one that covers 14-140 would be a 10x zoom. The trade-offs in terms of image quality are more apparent the greater the range of focal lengths the lens covers. It is generally understood that a 3–4x zoom range is about the limit for a constant wide aperture zoom to offer performance approaching that of a prime lens. Lenses with zoom ranges longer than about 5x are sometimes referred to as superzooms.

A zoom lens that maintains focus when the focal length is changed is considered parfocal. Such lenses are particularly prized for use when shooting video as one can change the focal length without having to refocus. While most consumer lenses for the M43 system are not parfocal, many are close enough to it to be usable. True parfocal lenses for video are available for the system, but such lenses typically come with large price tags.

Macro lens[edit | edit source]

A macro lens is one that is used for close-up photography. These lenses are defined as any which produce a representation of an object on the sensor which is at least one quarter of life size (represented as 1:4 magnification). Some will further define a "true" macro lens as one which produces a 1:1 ratio of object to image size.

Most macro lenses are primes, but zoom lenses can also be used to capture macro images. The Panasonic Leica 45mm f/2.8 and Olympus 60mm f/2.8 are examples of M43 macro options.

Panasonic 20mm pancake lens mounted on a GX1

Pancake lens[edit | edit source]

Pancake is a term for any lens that has a short barrel length. While there's no solid, agreed upon definition, a pancake lens will generally have a length that is notably shorter than its diameter.

The M43 system includes both prime and zoom pancake options including the Olympus 14–42mm EZ zoom and the Panasonic 20mm prime. A body cap lens is an example of a pancake lens taken to the extreme.

Kit lens[edit | edit source]

The term kit lens literally applies to any lens bundled for sale with a camera. Typically such camera and lens kits are marketed toward entry-level consumers, so often kit lenses are among the more affordable lenses available. As such the term "kit lens" can have a pejorative connotation.

Within the M43 system the term is typically used to describe any of the standard, variable maximum aperture, zoom lenses that cover focal lengths from 12mm to 50mm, specifically the Olympus 14–42 (I), 14–42 L, 14–42 II, 14–42 II R, 14–42 EZ, and 12–50 EZ as well as the Panasonic 14–45, 14–42 (I), 14–42 II, 14–42 PZ, and 12–32. Beyond these zooms, the Panasonic 14mm f/2.5 and Olympus 17mm f/2.8 pancakes could be considered as kit lenses. A few other lenses were included in bundles, such as the Panasonic 14–140 (I) and the Panasonic 20mm pancake, but these are not widely considered to be kit lenses.

Tilt-shift lens[edit | edit source]

Tilt or shift lenses fall under the category of perspective correction lenses. A tilt lens allows the user to alter (or tilt) the angle of the lens plane so it is no longer parallel to the sensor plane. A shift lens is one in which the lens' optics can be repositioned (or shifted) in relation to the image plane while remaining parallel.[18] While lenses are available with either of these two functions independently, often they are combined into a tilt-shift lens.

Tilt lenses take advantage of the Scheimpflug principle to change the plane of focus in an image. The plane of focus will be parallel to the lens plane when using a tilt lens, rather than parallel to the sensor. This means that you can selectively place one area of the image in focus while throwing other areas out of focus. This can be useful to isolate a subject in a unique way. It can also be used to produce a "miniature effect" where one can cause, for example, a cityscape rendered from an elevated position to appear as though it's a scale model.[18]

Shift lenses can be used to simulate capturing an image from a different camera position. One common use of this is in architectural photography whereby one can prevent the edges of a building from converging within the image. It can also be a useful trick when photographing mirrors where you want to keep the camera out of the shot. A third use case for shifting a lens is to create stitched panoramas captured using a series of images from a static camera position with only the shift of the lens changing, which eliminates the effect of parallax that is inherent to other stitched panoramic techniques.[18]

Both Samyang and Arax have produced manual focus tilt-shift lenses with M43 mounts. Lensbaby produces a number of lenses with tilt capability. In addition, tilt-shift lenses from other systems (such as Canon or Nikon) can be adapted for use on M43 cameras. Finally, tilt and tilt-shift adapters have been produced for use with a wide array of traditional legacy manual focus lenses from other systems.

The aperture of an anamorphic lens appears as an oval.

Anamorphic lens[edit | edit source]

Anamorphic lenses are a tool that can be used to capture a wider aspect ratio than what the chosen sensor offers. They do this by squeezing the image horizontally. This image must then be stretched in post-processing to eliminate the distortion caused by this compression. This results in a final image with a more panoramic aspect ratio.[19]

These lenses are typically used by filmmakers, although there's no reason they cannot be employed for still photography as well. In addition to the aspect ratio, images from such lenses are characterized by streaking horizontal lens flares.[19] Companies such as SLR Magic, Vazen and Sirui offer M43 anamorphic lenses with manual focus.

Optical aberrations[edit | edit source]

Diagram illustrating CA with a simple lens

In practice, camera lenses exhibit a number of optical aberrations or defects that cause the image to be distorted, blurred or otherwise flawed. These can be caused or exacerbated by a variety of factors including lens design, environmental conditions, or flaws within a lens element or with the lens' construction. Some of the more common aberrations photographers encounter are discussed here.

Chromatic aberration[edit | edit source]

Chromatic distortion or chromatic aberration (often abbreviated as CA) is caused when the lens fails to refract all of the wavelengths of light into a single point. The effect of this is color fringing (sometimes referred to a "purple fringing" or "green fringing"). This fringing appears in areas of an image with high-contrast edges. CA is most apparent in scenes shot at wide apertures; stopping down the lens (reducing the size of the aperture) can help control it.[20]

In lens design an achromatic doublet, a pair of two special lens elements, is often employed to reduce the impact of CA.[21]

Diagram illustrating coma in a single lens

Coma[edit | edit source]

Comatic aberration, or coma, is a defect which causes a point light source to flare out into a shape resembling a comet with a tail rather than refocusing on a single point on the image sensor. This is more likely to occur and be more severe away from the center of the frame.[22] While coma is inherent to some particular lens designs, it can also be caused by imperfections in a lens or element.

The presence of coma is of particular concern in astrophotography due to the prevalence of single-point light sources in that discipline. Like CA, the effect of coma can be reduced by stopping down the lens aperture.[22]

Diagram illustrating (L-to-R) a non-distorted image, an image suffering from pincushion distortion and an image with barrel distortion

Geometric distortion[edit | edit source]

Deviations from the rectilinear in an image are considered geometric distortion. While this distortion is expected in certain types of lenses, such as fisheyes, it should not present conspicuously in a well-designed rectilinear lens. The two most common type of lens distortion are barrel distortion, wherein straight edges appear to bow outward from the center, particularly near the edge of the frame, and pincushion distortion, which is essentially the opposite with lines curving inward.[22]

Distortion is more prevalent in zoom lenses, especially those with a long zoom range. One way it can be prevented through lens design is to place the aperture as close as possible to the lens.[22]

Extreme vignetting as seen in a photograph produced a Holga "toy camera".

Vignetting[edit | edit source]

Lens vignetting is the falloff in saturation, contrast or light intensity near the periphery of an image. It presents as an image with dark corners. Like other aberrations it can have a number of possible causes, from lens design to imperfections to focal length. Vignetting can be more acute with zoom, telephoto and wide-angle lenses. It can also be alleviated to some degree by reducing the aperture of the lens.[23]

While vignetting can be a negative, it can also have the effect of drawing the viewer's eye toward the center of the image.

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