# Modern Photography/Lenses

There are many types of photographic lenses, the most common categories of which are outlined below. (For a more technical treatment of the subject you may wish to refer to Optics.)

## Construction

The camera lens controls how much of the shooting scene is visible. A lens comprises a number of lens elements, a mechanism for controlling focus, an aperture diaphragm, a separate mechanism for controlling focal length (on zoom lenses only), all within a light-proof barrel. The lens may receive filter attachments for special purposes.

### Lens elements

Lens elements are the individual pieces of glass that bend light in a controlled manner. Each element serves a purpose, whether it be converging or diverging incoming light, adjusting focus, or reducing errors.
Each additional lens element slightly reduces the total amount and clarity of light transmitted.

### Focal length

The camera lens directly controls how much of the scene is visible—the field of view—while the shooter chooses the location and direction of the shot.
The field of view correlates to the focal length of the lens—the distance between where the lens is focused (at infinity) and the "optical center". The focal length (f) is given in millimeters (e.g. f=50mm). A larger focal length produces a narrow field of view, while a shorter focal length produces a wider angle.
The relation between focal length and image sensor size determines whether the lens will produce a wider or narrower field of view. An 18mm lens may be considered wide angle on a full-frame DSLR, but may produce a narrow field of view on a subcompact camera.

### Focus

A lens allows the user to focus on a specific part of the scene. The camera body may provide autofocus for automatically setting focus on a target within the scene. Focus can be manually controlled by the shooter, or automatically controlled by the camera, selected by an M/A switch on the lens and/or the camera body.
By design, a camera lens most sharply focuses along a "focal plane"—a region of space parallel to the film or image sensor. (This design is challenged by tilt/shift lenses.) The length from the optical center to the focal plane is the "focal distance". On better lenses, there are markings for focal distance on the barrel. Focal distance is not bound to focal length—focal distance occurs in front of the lens' optical center, and focal length behind.
The range of focus often starts a specific distance away from the lens (as little as a few inches for wide angle lenses, to whole feet for telephoto lenses), progresses slowly at close range, then greatly accelerates to "infinity"—the setting at which distant objects will be in perfect focus. Some lenses allow for slight adjustment beyond infinity for purposes of reducing depth of field at distance.

### Aperture

The lens directly controls aperture, one of the three main exposure controls. (Sensitivity is controlled by the image sensor, and shutter speed is controlled by the camera body. The shooter may also have control over how much light is present in the scene.)
The aperture is a hole in the lens "diaphragm" or "iris", centered along the optical axis and of a specific diameter. Several aperture "blades" are opened or closed as needed to admit more or less light through the lens. The number and shape of the blades can sometimes be seen in off-focus highlights, with pleasing shapes termed as "bokeh".
The size of the aperture is given as a fraction of the focal length (e.g. f/4.0). For a lens of 50mm focal length, f/4.0 is an aperture of 12.5mm; for 200mm, f/4.0 is 50mm. For calculating exposure, this ratio—better known as an "f-number" or "f-stop"—is more important than the absolute size, because for any focal length the same ratio admits the same amount of light from the scene. f-numbers grow by a geometric scale based on the square root of 2, as illumination varies with area (a two-dimensional value), but diameter is a single-dimension value. While divisions were originally set to a whole stop—a factor of 1/2—modern cameras allow setting aperture in half- or third-stops.
A wider maximum aperture—occasionally referred to as the "speed" of the lens—requires more expensive design and higher-quality lens elements. Telephoto lenses nay be seen with a max aperture of f/2.0. Normal and wide-angle lenses may approach f/1.0. While lenses faster than f/1.0 are possible, optical and design constraints may result in less-favorable images.
On strictly mechanical lenses, the user sets aperture by rotating an adjustment ring on the lens. While the shutter is released, a lever in the camera body pushes against a lever in the lens mount, causing the diaphragm blades to close at the chosen aperture setting. Lenses that offer electronic operation will pass control to the camera body, by means of a second lever.

#### Depth of field

Adjusting the aperture controls how much off-angle light may enter the camera. This has a side effect of controlling how much of the scene before and behind the focal plane is in focus—this is called depth of field. The amount of depth of field is determined by the absolute size of the aperture, with a smaller aperture resulting in wider depth of field, and a wider aperture giving shallower depth of field. Because telephoto lenses generally have a larger absolute aperture, they often produce thin depth of field. Wide angle lenses may allow for depth of field to reach infinity.
Depth of field is proportional to focal distance. Focusing closer will reduce depth of field. With the same aperture size, focusing farther away greatly increases depth of field. Depth of field can extend to infinity without necessarily focusing at infinity. Some lenses have barrel markings indicating depth of field at a given aperture.
While composing the shot, the lens will hold aperture wide open, allowing the scene to be clearly viewed. Some camera models allow the shooter to preview depth of field; holding down the "DoF preview" button temporarily sets the aperture while keeping the viewfinder active (instead of exposing the sensor).
Depth of field is also indirectly affected by the image format. Larger format require lenses with longer focal lengths to achieve usable field of view. Smaller formats can take advantage of very short focal lengths, and obtain greater magnification from longer focal lengths, but generally have a smaller range of available apertures. Thus, a smartphone or compact camera is less able to capture shallow depth of field than a full-frame camera.

### Lens mount

Enthusiast and professional cameras often offer a range of interchangeable lenses. The lens mount allows a lens to be securely attached to a camera and connected to mechanical and/or electronic controls. The lens mount thus determines what lenses are compatible for use with a certain camera body.
When the camera is not in use, the lens mount cover should be attached to protect the mounting surfaces and the inner chamber of the camera. Though it is often the strongest part of a camera, the lens mount can still be vulnerable to damage from drops, or from excessive force against an attached lens.

### Attachments
#### Lens hood

A lens hood gives some protection to the front of the lens. The hood also shields against light entering the lens from outside the field of view, and potentially causing glare.

#### Filters

By means of a filter mount on the front of the lens, various filters may be attached for special purposes.

##### UV/Clear

    A clear filter that may also filter out ultraviolet light. The film or sensor may capture ultraviolet light as if it were visible light, causing haze in the image.
    While the additional glass may increase the potential for image defects, the filter can also protect against accidental drops and collisions. The shattering glass absorbs energy that might otherwise damage the much more expensive lens or camera body.

##### Polarizer

    A filter that limits the transmission of light to a specific plane of polarization, the angle of which is adjusted by the user. This can increase contrast and reduce the intensity of reflections (which tend to polarize incoming light). The filter may also interact with polarized objects such as digital screens and sunglasses. Digital cameras may require a "circular polarizer" (CP) in order to preserve the use of autoexposure and autofocus sensors.

##### Neutral density (ND)

    A filter that reduces incoming light by a specific amount. This can serve as a backup means of controlling exposure, especially when a longer shutter speed is desired. The filter is neutral, not favoring any specific color over any other. This class of filters is often available in different densities, given as the number of stops that exposure is reduced. Neutral density filters may be required for lenses that do not contain an adjustable aperture.
    Some ND filters may be "split", with two different regions of density, or "graduated" with a gradient of increasing density. Some ND filters may be composed of two polarizing filters, and can be adjusted in strength by changing the angle of rotation between them.

##### Color

    Permanently colored filters that can be used for adjusting white balance, adding a weak or strong color cast, or controlling shots taken on monochrome film.

##### Close focus

    Magnifying lenses that can be attached to a lens to allow for focusing at a closer distance than originally designed. These are typically less expensive than a dedicated close focus lens.

##### Special effect

    Filters can create special effects such as patterned highlights or soft focus.
Though filters can often be stacked upon each other, it is unwise to attach more than one or two, as this may cause vignetting. Wide-angle lenses may require a special mount for a wider filter to prevent vignetting.

#### Couplers and adapters

Various attachments alter how the lens performs:

##### Coupler

    In place of a macro lens, a coupler joins two lenses together at their filter mounts. The magnification provided by the coupled lenses is measured by dividing the focal length of the camera-mounted lens by that of the coupled lens. The camera retains control over the normally-mounted lens, and autofocus if available. Transmission is reduced by the sum of glass in both lenses.

##### Extension tube/ring

    Provides close focus ability by increasing the distance between the lens and the image plane, at a cost to exposure and the ability to focus at infinity. The effective reduction in aperture also collapses depth of field. Extensions do not otherwise affect transmission since there is no additional glass. Better extensions preserve connections to focus and aperture.

##### Adapter

    Allows mounting of lenses designed for other lens mounts.

### Quality

Lenses come in a variety of build quality, with more expensive lenses typically offering better performance and more durable construction.
At additional cost, some lens elements are made of exotic materials such as borosilicate or fluorite, or with non-spherical geometry that require specialized production. These serve to reduce or eliminate defects caused by conventional optical glasses.
When adjusting focus, the focal length may shift slightly, even on a prime/fixed lens where focal length is expected to remain constant.
Lenses may either use less-expensive "external focusing" with parts visibly projecting from the barrel, or "internal focusing" where barrel length remains unchanged and heavier attachments can be mounted.
Some less-expensive zoom lenses suffer from inconsistent speed when adjusting focal length. For instance, a 70–210mm f/4–5.6 lens may offer f/4 at 70mm, but drops down to f/5.6 at 210mm.
To achieve faster speed, it may be necessary to increase the barrel diameter, requiring larger lens elements and sturdier build.

## Phenomena

Lenses do not perfectly transmit light. They may capture certain effects that are not present in the scene:
* Bloom: Glow around intense light sources or reflections.
* Flare: Shapes appearing elsewhere in the image due to intense lighting in the visible scene
* Haze: Reduction in contrast, sometimes due to fogging of the lens, excess heat, or ultraviolet light. Haze may also be caused by the breakdown of optical cement in between adjoining lens elements, requiring expensive repair.
* Leak: Light entering the camera through defects in construction, or improper lens mounting.
* Smudges: Blemishes caused by fingerprints and stains on the front or rear surface, scratches to same.
The design and choice of lens elements may also introduce various errors:
* Distortion: Imperfections in the grinding of a lens, as well as deviations due to spherical lens geometry.
* Chromatic aberration: The splitting of colors, same as observed on an optical prism.
* Internal reflection: Undesired bounding of light across multiple surfaces, often due to insufficient coating.
* Vignetting: Reduced transmission at the edge of the field of view, causing corners of the image to be darkened. These may also be caused by obstructing filters.

## Lenses by focal length
### Normal

A so-called normal lens bends light roughly the same way our eyes do, thus providing an image with the proper proportions. It has a focal length close to the diagonal measurement of the image frame. That is, with a standard 35mm format full frame camera, the image on the film or digital image sensor measures 24x36mm. The diagonal measures 43mm (sqrt(36$^2$+24$^2$)). The closest lens most manufacturers produce is the 50mm lens. Cameras with formats other than 35mm have 'normal' lenses with different focal lengths — longer for larger formats, and shorter for smaller formats.
    35mm - 50mm (eg. Nikon Nikkor 50mm f/1.4, Canon EF 50mm f/1.4, Pentax smc P-FA 50mm f/1.4, Minolta AF 50mm f/1.4, et al)
    645 - 75mm (eg. Pentax smcp-FA 645 75mm F2.8, et al)
    6x7 - 105mm (eg. Pentax smcp 67 105mm F2.4, et al)
    APS-C - 30mm (eg. Sigma 30mm f/1.4)
You can compare lens focal lengths across various film formats using this external chart.

### Wide-angle

Wide-angle lenses have a larger field of view than normal lenses. In other words, they capture more of the scene in front of the camera by capturing more of the periphery. However, as the size of the film or sensor in the camera is still the same, the wider scene appears on the film or sensor as being slightly distorted: the bigger scene is "squeezed" onto the same area of film or sensor, so the typical effect is that each object is smaller (and therefore looks farther away) because the image now "includes" more objects. With each object smaller, the typical effect is that objects seem farther away. What you see in the passenger side wing mirrors in cars is the same. Those mirrors give a wide-angle effect and allows you to see more of the scene behind the car by making each object smaller, hence the common warning printed there: "Objects in mirror are closer than they appear."
There are various wide-angle lenses, measured by the focal length. The "normal" lens for a 35mm camera is about 50mm focal length, and a wide angle lens has a shorter focal length, such as 35mm or 28mm. The shorter the focal length, the greater the perspective distortion. At the extreme end, there are wide-angle lenses with a focal length of 10mm or so using a "fish eye" projection: up to a 180 degree field of vision can be captured. However, the final photograph looks highly distorted and looks like a photograph of the scene as if reflected on a silvered ball. It may be that the name of this type of lens comes from what one imagines a fish sees under water, or that the image is distorted around a central point much like what a fish eye looks like.

### Long-focus

A long focus lens is any fixed focal length lens that is longer than a normal lens (focal length is longer than the diagonal measure of the film or sensor). This includes the common sub-type, the telephoto lens, which uses special optics to compress the length of the lens.
These lenses bring the subject in by magnifying the subject and isolating it in the viewfinder. Lenses of this type are very useful for sports and wildlife photography for their ability to isolate a subject. However, these lenses have a drawback in that they typically are poor for low light use. Most long-focus lenses have a maximum aperture of only f/4, thus not permitting much light to reach the film and causing the use of slow shutter speeds to get correct exposure. In doing so, the photographer runs the risk of blurring the image due to his or her own movement.
One thing to bear in mind, try not to handhold your camera while using a shutter speed lower than that of the focal length of the lens being employed. This will help to assure sharper images. Using a sturdy tripod and a remote release will help a lot in low light photography.

## Focal length range
### Fixed/Prime

A fixed lens or prime lens) is designed for a single focal length, and has one focal length descriptor (e.g. 55mm). Fixed lenses can be less expensive than zoom lenses, with simple construction allowing for reduced cost and size. Fixed lenses may also be constructed with higher-quality materials and improved capability, such as a wider aperture or near-elimination of distortion.

### Zoom

Zoom lenses are multi-focal length lenses. Zoom lenses cover a range of focal lengths (e.g. 35–105mm), and are continually adjustable within this range. The difference between the widest and narrowest focal length may sometimes be given as a multiple (e.g. 3× zoom), especially on consumer cameras.
Zoom lenses may either have separate control rings for focus and focal length, or may combine the two with a single control sleeve. Zoom lenses may be confined to a small range of focal lengths, or may be capable of wide angle and telephoto shooting in a single package. The latter may be termed a "superzoom" when the focal range is 10× or greater.
One disadvantage of zoom lenses is that the focal complexity and number of lens elements required to achieve a range of focal lengths is much greater than for prime lenses. Again, each lens element slightly reduces transmission and clarity.
Another disadvantage of zoom lenses is that the maximum aperture of the lens is usually lower, since the same aperture must accommodate the full range of focal lengths. This makes inexpensive zoom lenses hard to use in low-light conditions without a flash. Some lenses may have a varying maximum aperture along the focal range due to design constraints.

## Lenses of special capability
### Macro and close focus

A macro lens allows a subject to be captured onto film at 1× magnification or greater. This meaning is challenged by digital photography that allows sensors to be made ever smaller, and image files to be zoomed at will.
While a regular lens can be converted for macrophotography by means of extension tubes, bellows, or reversing rings, macro lenses offer better optical properties and integration with on-camera controls.
Macro lenses are often confused with close-focus lenses that allow photographs of subjects much closer than standard lenses allow.
Again, regular lenses may be converted to close-focus duty with a close-up filter, but a purpose-built lens may prove favorable.

### Tilt/Shift (TS)

Also known as Perspective-control (PC) lenses, these lenses allow the correction of image geometry. This is most useful when the film plane is not parallel to the surface of a subject, and the subject would otherwise be rendered with converging lines (lines parallel in reality are rendered converging). The classic example is when the camera is tilted upwards to photograph a building. The effect of converging lines is often unwanted and can be avoided by using a perspective control (PC) lens. It provides a function that is usually only available in view cameras: the lens can be shifted out of the optical axis (in the above example: upwards) and thus the recording media can be positioned parallel to the subject (the camera points orthogonally towards the building) and the subject is rendered undistorted.
Shift lenses are mechanically and optically more complex than ordinary lenses, don't provide autofocus and are comparatively expensive. They are often wide angle lenses and in this case frequently used in architecture photography. Longer lenses are often used in product or advertising type studio photography.

### Catadioptric

A catadioptric, or mirror lens, makes use of mirrors to reflect light back and forth though the glass elements with the second convex mirror element acting as a negative lens, further extending the light cone. The result is a dramatic decrease in the length of a lens whilst still maintaining a larger focal length. Mirror lenses create tell-tale doughnut-shaped highlights when a light is located in an area of the photograph that is out of focus, as the center is obscured by the mirror elements. No such holes appear on the subject or the scene as a whole, since the light rays otherwise converge in the same manner as regular lenses.

### Soft-focus

A lens that renders the image a little softer (i.e. less sharp). This is sometimes used in portrait photography to conceal minor defects in the skin of the person. To suit this purpose soft focus lenses usually have focal lengths around 80-100mm (for 35mm cameras) most popular for portrait work.
Unlike soft-focus filters that disrupt light transmission, soft-focus lenses are optically constructed to achieve blurring around the edges and very short depth of field.

### Fisheye

The term fisheye was coined in 1906 by American physicist and inventor Robert W. Wood based on how a fish would see an ultra-wide hemispherical view from beneath the water (a phenomenon known as Snell's window). Their first practical use was in the 1920s for use in meteorology to study cloud formation giving them the name "whole-sky lenses". The angle of view of a fisheye lens is usually between 100 and 180 degrees while the focal lengths depend on the film format they are designed for. As it is impossible to preserve linear geometry at such wide angles, fisheye images often capture curved geometry and exaggerated forms.
Mass-produced fisheye lenses for photography first appeared in the early 1960s and are generally used for their unique, distorted appearance. For the popular 35 mm film format, typical focal lengths of fisheye lenses are between 8 mm and 10 mm for circular images, and 15–16 mm for full-frame images. For digital cameras using smaller electronic imagers such as 1/4" and 1/3" format CCD or CMOS sensors, the focal length of "miniature" fisheye lenses can be as short as 1 to 2mm.