1.
Nikkormat
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Nikkormat was a brand of cameras produced by the Japanese optics company Nippon Kogaku K. K. as a consumer version of the professional Nikon brand. Nikkormat cameras, produced from 1965 until 1978, were simpler and more affordable than Nikon-branded cameras, the Nikkormat FT was an all-metal, mechanically controlled, manual focus SLR with match-needle exposure control, manufactured in Japan from 1965 to 1967. It was available in two colors, black with chrome trim and all black, the unmetered version was designated Nikkormat FS. The FT had dimensions of 95 mm height,146 mm width,54 mm depth and 745 g weight, the FT used a metal-bladed, vertical travel, focal plane shutter with a speed range of 1 to 1/1000 second plus Bulb and flash X-sync of 1/125th second. The Nikkormat F-series had a shutter speed ring concentric with the lens mount, the photographer would adjust the shutter speed to freeze or blur motion and/or the lens aperture f-stop to control depth of field until the needle was centered between two pincer-like brackets. The needle array was duplicated in a window next the film rewind crank to allow exposure control without looking through the viewfinder. The meter was turned on by pulling the film wind lever out to the ready position. This was very advanced in 1965 and proved to be remarkably long-lived, Nippon Kogaku used it for all versions of the Nikkormat FT with incremental improvements. The Nikon FM, FM2 and FM2N of the succeeding Nikon compact F-series SLRs used an improved viewfinder only, the FTs viewfinder also had a fixed focusing screen with Nippon Kogakus then standard central 4 mm microprism focusing aid plus 12 mm matte focusing surface. The Nikkormat FT accepted all lenses with the Nikon F bayonet mount, the FT had a mirror lockup allowing its use with some specialised lenses for which an auxiliary viewfinder was provided. The FT was Nippon Kogakus first SLR with a built-in TTL light meter, as such, Nippon Kogaku could not find a way to automatically synchronize their Nikkor Auto lenses aperture information with the FT body. Therefore, mounting lenses required a special preparatory procedure, first, the lens maximum aperture must set against the film speed scale on the FTs shutter speed ring. Note that the maximum aperture had to be reset every time the lens was changed. This was very inconvenient compared to some other SLRs of the 1960s, although most AF Nikkor lenses will mount and manually focus on the FT, the combination cannot provide open aperture metering, only stop down metering. However, the FT did not have an accessory shoe to mount flash units. The Nikkormat accessory shoe must be screwed to the top of the pentaprism cover via the eyepiece first, note that this shoe only mounts the flash. A PC cord must still be plugged into the appropriate PC terminal and this was normal for most SLRs of the 1960s. The Nikkormat FS, manufactured from 1965 to 1971, was an FT stripped of the light meter with its exposure information system
2.
Photography
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Typically, a lens is used to focus the light reflected or emitted from objects into a real image on the light-sensitive surface inside a camera during a timed exposure. With an electronic sensor, this produces an electrical charge at each pixel. A negative image on film is used to photographically create a positive image on a paper base, known as a print. The word photography was created from the Greek roots φωτός, genitive of φῶς, light and γραφή representation by means of lines or drawing, several people may have coined the same new term from these roots independently. Johann von Maedler, a Berlin astronomer, is credited in a 1932 German history of photography as having used it in an article published on 25 February 1839 in the German newspaper Vossische Zeitung. Both of these claims are now widely reported but apparently neither has ever been confirmed as beyond reasonable doubt. Credit has traditionally given to Sir John Herschel both for coining the word and for introducing it to the public. Photography is the result of combining several technical discoveries, later Greek mathematicians Aristotle and Euclid also independently described a pinhole camera in the 5th and 4th centuries BCE. Daniele Barbaro described a diaphragm in 1566, wilhelm Homberg described how light darkened some chemicals in 1694. The fiction book Giphantie, published in 1760, by French author Tiphaigne de la Roche, the discovery of the camera obscura that provides an image of a scene dates back to ancient China. Leonardo da Vinci mentions natural camera obscura that are formed by dark caves on the edge of a sunlit valley, a hole in the cave wall will act as a pinhole camera and project a laterally reversed, upside down image on a piece of paper. So the birth of photography was primarily concerned with inventing means to capture, renaissance painters used the camera obscura which, in fact, gives the optical rendering in color that dominates Western Art. The camera obscura literally means dark chamber in Latin and it is a box with a hole in it which allows light to go through and create an image onto the piece of paper. Around the year 1800, British inventor Thomas Wedgwood made the first known attempt to capture the image in a camera obscura by means of a light-sensitive substance and he used paper or white leather treated with silver nitrate. The shadow images eventually darkened all over, the first permanent photoetching was an image produced in 1822 by the French inventor Nicéphore Niépce, but it was destroyed in a later attempt to make prints from it. Niépce was successful again in 1825, in 1826 or 1827, he made the View from the Window at Le Gras, the earliest surviving photograph from nature. Because Niépces camera photographs required a long exposure, he sought to greatly improve his bitumen process or replace it with one that was more practical. With an eye to eventual commercial exploitation, the partners opted for total secrecy, Daguerres efforts culminated in what would later be named the daguerreotype process
3.
Image sensor
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An image sensor or imaging sensor is a sensor that detects and conveys the information that constitutes an image. It does so by converting the variable attenuation of light waves into signals, the waves can be light or other electromagnetic radiation. As technology changes, digital imaging tends to replace analog imaging, early analog sensors for visible light were video camera tubes. Currently, used types are semiconductor charge-coupled devices or active pixel sensors in complementary metal–oxide–semiconductor or N-type metal-oxide-semiconductor technologies, analog sensors for invisible radiation tend to involve vacuum tubes of various kinds. Digital sensors include flat panel detectors, today, most digital cameras use a CMOS sensor, because CMOS sensors perform better than CCDs. An example is the fact that they incorporate an integrated circuit, CCD is still in use for cheap low entry cameras, but weak in burst mode. Both types of sensor accomplish the task of capturing light. Each cell of a CCD image sensor is an analog device, when light strikes the chip it is held as a small electrical charge in each photo sensor. This process is repeated until all the lines of pixels have had their charge amplified. A CMOS image sensor has an amplifier for each compared to the few amplifiers of a CCD. Some CMOS imaging sensors also use Back-side illumination to increase the number of photons that hit the photodiode, CMOS sensors can potentially be implemented with fewer components, use less power, and/or provide faster readout than CCD sensors. They are also vulnerable to static electricity discharges. Another approach is to utilize the very fine dimensions available in modern CMOS technology to implement a CCD like structure entirely in CMOS technology and this can be achieved by separating individual poly-silicon gates by a very small gap. These hybrid sensors are still in the phase and can potentially harness the benefits of both CCD and CMOS imagers. There are many parameters that can be used to evaluate the performance of a sensor, including dynamic range, signal-to-noise ratio. For sensors of comparable types, the ratio and dynamic range improve as the size increases. In order to avoid interpolated color information, techniques like color co-site sampling use a mechanism to shift the color sensor in pixel steps. 3CCD, using three discrete image sensors, with the color separation done by a dichroic prism, while in general digital cameras use a flat sensor, Sony prototyped a curved sensor in 2014 to reduce/eliminate Petzval field curvature that occurs with a flat sensor
4.
F-number
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The f-number of an optical system such as a camera lens is the ratio of the systems focal length to the diameter of the entrance pupil. It is a number that is a quantitative measure of lens speed. It is also known as the ratio, f-ratio, f-stop. The f-number is commonly indicated using a hooked f with the format f/N, the f-number N or f# is given by, N = f D where f is the focal length, and D is the diameter of the entrance pupil. It is customary to write f-numbers preceded by f/, which forms a mathematical expression of the pupil diameter in terms of f and N. Ignoring differences in light transmission efficiency, a lens with a greater f-number projects darker images, the brightness of the projected image relative to the brightness of the scene in the lenss field of view decreases with the square of the f-number. Doubling the f-number decreases the brightness by a factor of four. To maintain the same photographic exposure when doubling the f-number, the time would need to be four times as long. Most lenses have a diaphragm, which changes the size of the aperture stop. The entrance pupil diameter is not necessarily equal to the aperture stop diameter, a 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A200 mm focal length f/4 lens has a pupil diameter of 50 mm. The 200 mm lenss entrance pupil has four times the area of the 100 mm lenss entrance pupil, a T-stop is an f-number adjusted to account for light transmission efficiency. The word stop is sometimes confusing due to its multiple meanings, a stop can be a physical object, an opaque part of an optical system that blocks certain rays. In photography, stops are also a used to quantify ratios of light or exposure. The one-stop unit is known as the EV unit. On a camera, the setting is traditionally adjusted in discrete steps. Each stop is marked with its corresponding f-number, and represents a halving of the light intensity from the previous stop. This corresponds to a decrease of the pupil and aperture diameters by a factor of 1/2 or about 0.7071, each element in the sequence is one stop lower than the element to its left, and one stop higher than the element to its right
5.
Film speed
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Film speed is the measure of a photographic films sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. A closely related ISO system is used to measure the sensitivity of digital imaging systems, highly sensitive films are correspondingly termed fast films. In both digital and film photography, the reduction of exposure corresponding to use of higher sensitivities generally leads to reduced image quality, in short, the higher the sensitivity, the grainier the image will be. Ultimately sensitivity is limited by the efficiency of the film or sensor. The speed of the emulsion was then expressed in degrees Warnerke corresponding with the last number visible on the plate after development. Each number represented an increase of 1/3 in speed, typical speeds were between 10° and 25° Warnerke at the time. The concept, however, was built upon in 1900 by Henry Chapman Jones in the development of his plate tester. In their system, speed numbers were inversely proportional to the exposure required, for example, an emulsion rated at 250 H&D would require ten times the exposure of an emulsion rated at 2500 H&D. The methods to determine the sensitivity were later modified in 1925, the H&D system was officially accepted as a standard in the former Soviet Union from 1928 until September 1951, when it was superseded by GOST 2817-50. The Scheinergrade system was devised by the German astronomer Julius Scheiner in 1894 originally as a method of comparing the speeds of plates used for astronomical photography, Scheiners system rated the speed of a plate by the least exposure to produce a visible darkening upon development. ≈2 The system was extended to cover larger ranges and some of its practical shortcomings were addressed by the Austrian scientist Josef Maria Eder. Scheiners system was abandoned in Germany, when the standardized DIN system was introduced in 1934. In various forms, it continued to be in use in other countries for some time. The DIN system, officially DIN standard 4512 by Deutsches Institut für Normung, was published in January 1934, International Congress of Photography held in Dresden from August 3 to 8,1931. The DIN system was inspired by Scheiners system, but the sensitivities were represented as the base 10 logarithm of the sensitivity multiplied by 10, similar to decibels. Thus an increase of 20° represented an increase in sensitivity. ≈3 /10 As in the Scheiner system, speeds were expressed in degrees, originally the sensitivity was written as a fraction with tenths, where the resultant value 1.8 represented the relative base 10 logarithm of the speed. Tenths were later abandoned with DIN4512, 1957-11, and the example above would be written as 18° DIN, the degree symbol was finally dropped with DIN4512, 1961-10
6.
Exposure (photography)
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In photography, exposure is the amount of light per unit area reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance. Exposure is measured in lux seconds, and can be computed from exposure value, in photographic jargon, an exposure generally refers to a single shutter cycle. For the same speed, the accumulated photometric exposure should be similar in both cases. A photograph may be described as underexposed when it has a loss of detail, that is. As the adjacent image shows, these terms are technical ones, there are three types of settings, manual, automatic and exposure compensation. Radiant exposure of a surface, denoted He and measured in J/m2, is given by H e = E e t, where Ee is the irradiance of the surface, measured in W/m2, t is the exposure duration, measured in s. Luminous exposure of a surface, denoted Hv and measured in lx⋅s, is given by H v = E v t, many photographic materials are also sensitive to invisible light, which can be a nuisance, or a benefit. The use of units is appropriate to characterize such sensitivity to invisible light. In sensitometric data, such as curves, the log exposure is conventionally expressed as log10. Photographers more familiar with base-2 logarithmic scales can convert using log2 ≈3.32 log10, correct exposure may be defined as an exposure that achieves the effect the photographer intended. A more technical approach recognises that a film has a physically limited useful exposure range. If, for any part of the photograph, the exposure is outside this range. In a very simple model, for example, out-of-range values would be recorded as black or white rather than the precisely graduated shades of colour and this ensures that no significant information is lost during capture. However, it is much easier to discard recorded information during post processing than to try to re-create unrecorded information. In a scene with strong or harsh lighting, the ratio between highlight and shadow luminance values may well be larger than the ratio between the maximum and minimum useful exposure values. A photograph may be described as underexposed when it has a loss of detail, that is. In manual mode, the photographer adjusts the lens aperture and/or shutter speed to achieve the desired exposure, manual exposure calculations may be based on some method of light metering with a working knowledge of exposure values, the APEX system and/or the Zone System. A camera in automatic exposure mode automatically calculates and adjusts exposure settings to match the subjects mid-tone to the mid-tone of the photograph, for most cameras this means using an on-board TTL exposure meter
7.
Exposure value
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In photography, exposure value is a number that represents a combination of a cameras shutter speed and f-number, such that all combinations that yield the same exposure have the same EV. Exposure value is used to indicate an interval on the photographic exposure scale, with a difference of 1 EV corresponding to a standard power-of-2 exposure step. The EV concept was developed by the German shutter manufacturer Friedrich Deckel in the 1950s and its intent was to simplify choosing among equivalent camera exposure settings by replacing combinations of shutter speed and f-number with a single number. This was especially helpful to beginners with limited understanding of the shutter speed and aperture. The concept became known as the Light Value System in Europe, the proper EV was determined by the scene luminance and film speed, it was intended that the system also include adjustment for filters, exposure compensation, and other variables. With all of elements included, the camera would be set by transferring the single number thus determined. Exposure value has been indicated in various ways, although all camera settings with the same EV nominally give the same exposure, they do not necessarily give the same picture. The f-number determines the depth of field, and the shutter speed determines the amount of motion blur, if the EV is known, it can be used to select combinations of exposure time and f-number, as shown in Table 1. Greater exposure values are appropriate for photography in more brightly lit situations, to avoid confusion, some authors have used camera exposure to refer to combinations of camera settings. The 1964 ASA standard for automatic exposure controls for cameras, ASA PH2. 15-1964, took the same approach, common practice among photographers is nonetheless to use “exposure” to refer to camera settings as well as to photometric exposure. If, for example, the f-number is changed, an equivalent exposure time can be determined from t 2 t 1 = N22 N12. Performing this calculation mentally is tedious for most photographers, but the equation is solved with a calculator dial on an exposure meter or a similar dial on a standalone calculator. If the camera controls have detents, constant exposure can be maintained by counting the steps as one control is adjusted, the ratio t/N2 could be used to represent equivalent combinations of exposure time and f-number in a single value. But for many such combinations used in photography, the ratio gives a fractional value with a large denominator. For example, beginning with 1 s and f/1, decreasing exposure gives the simple sequence 0,1,2,3, the last two values shown frequently apply when using ISO100 speed imaging media in outdoor photography. Current cameras do not allow direct setting of EV, and cameras with automatic exposure control generally obviate the need for it, EV can nonetheless be helpful when used to transfer recommended exposure settings from an exposure meter to an exposure calculator. Used as an indicator of camera settings, EV corresponds to actual combinations of shutter speed, when the actual EV matches that recommended by the light level and the ISO speed, these settings should result in the “correct” exposure. If the common value of K =12.5 is used, at EV =15 the luminance is 380 cd/ft2 or 4096 cd/m2
8.
Aperture
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In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of a system determine the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are, which is of importance for the appearance at the image plane. If an aperture is narrow, then highly collimated rays are admitted, a wide aperture admits uncollimated rays, resulting in a sharp focus only for rays coming from a certain distance. This means that a wide aperture results in an image that is sharp for things at the correct distance, the aperture also determines how many of the incoming rays are actually admitted and thus how much light reaches the image plane. In the human eye, the pupil is the aperture, an optical system typically has many openings or structures that limit the ray bundles. In general, these structures are called stops, and the stop is the stop that primarily determines the ray cone angle. In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the stop or the opening itself. For example, in a telescope, the stop is typically the edges of the objective lens or mirror. One then speaks of a telescope as having, for example, note that the aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a large stop to be the aperture stop for the system. In astrophotography, the aperture may be given as a measure or as the dimensionless ratio between that measure and the focal length. In other photography, it is given as a ratio. Sometimes stops and diaphragms are called apertures, even when they are not the stop of the system. The word aperture is used in other contexts to indicate a system which blocks off light outside a certain region. In astronomy, for example, a photometric aperture around a star usually corresponds to a window around the image of a star within which the light intensity is assumed. The aperture stop is an important element in most optical designs and its most obvious feature is that it limits the amount of light that can reach the image/film plane. This can be unavoidable, as in a telescope where one wants to collect as much light as possible, or deliberate
9.
135 film
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135 is photographic film in a film format used for still photography. It is a film with a film gauge of 35 mm. Its engineering standard for the film is controlled by ISO1007, the term 135 was introduced by Kodak in 1934 as a designation for the cassette for 35 mm film, specifically for still photography. It quickly grew in popularity, surpassing 120 film by the late 1960s to become the most popular photographic film size, despite competition from formats such as 828,126,110, and APS, it remains so today. 135 camera film always comes perforated with Kodak Standard perforations, the size of the 135 film frame has been adopted by many high-end digital single-lens reflex and digital mirrorless cameras, commonly referred to as full frame. Even though the format is smaller than historical medium format and large format film, it is much larger than image sensors in most compact cameras. Individual rolls of 135 film are enclosed in single-spool, light-tight, the film is clipped or taped to a spool and exits via a slot lined with flocking. The end of the film is cut on one side to form a leader and it has the same dimensions and perforation pitch as 35 mm movie print film. Most cameras require the film to be rewound before the camera is opened, disposable cameras use the same technique so that the user does not have to rewind. Since the 1980s film cassettes have been marked with a DX encoding pattern, conforming cameras detect this, different films are sensitive to light at different degrees, this film speed is standardised by ISO.135 film has been made in several emulsion types and sensitivities. Since the introduction of digital cameras the most usual films have colour emulsions of ISO 100/21° to ISO 800/30°, films of lower sensitivity and higher sensitivity are for more specialist purposes. There are colour and monochrome films, negative and positive, monochrome film is usually panchromatic, orthochromatic has fallen out of use. Film designed to be sensitive to infrared radiation can be obtained, more exotic emulsions have been available in 135 than other roll-film sizes. The term 135 format usually refers to a 36×24 mm film format, the 36×24 mm format is common to digital image sensors, where it is typically referred to as full frame format. On 135 film, the dimension of the 36×24 mm frame runs parallel to the length of the film. The perforation size and pitch are according to the standard specification KS-1870, for each frame, the film advances 8 perforations. This is specified as 38.00 mm and this allows for 2 mm gaps between frames. Each camera model has a different location for the sprocket which advances the film, therefore, each camera model’s frame will vary in position relative to the perforations
10.
Tripod (photography)
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In photography, a tripod is used to stabilize and elevate a camera, a flash unit, or other photographic equipment. All photographic tripods have three legs and a head to couple with a camera. Tripod legs are made to telescope, in order to save space when not in use. Tripods are usually made from aluminum, carbon fiber, steel, tripods are used for both still and motion photography to prevent camera movement. They are necessary when slow-speed exposures are being made, or when lenses of extreme focal length are used, in the same vein, they reduce camera shake, and thus are instrumental in achieving maximum sharpness. A tripod is also helpful in achieving precise framing of the image, or when more than one image is being made of the same scene, use of a tripod may also allow for a more thoughtful approach to photography. For all of reasons, a tripod of some sort is often necessary for professional photography as well as certain video uses. Tripods are also used as an alternative to C-Stands to photographic accessories, for maximum strength and stability, most photographic tripods are braced around a center post, with collapsible telescoping legs and a telescoping section at the top that can be raised or lowered. Some tripods also feature integrated remote controls to control a camcorder or camera, materials used in the construction of tripod or monopod legs include metal, wood and carbon fiber-reinforced plastics, among others. Per ISO1222,2010, the current tripod screw thread standard for attaching the camera calls for a 1/4-20 UNC or 3/8-16 UNC thread, most consumer cameras are fitted with 1/4-20 UNC threads. Larger, professional cameras and lenses may be fitted with 3/8-16 UNC threads, plus a removable 1/4-20 UNC adapter, in this application, the BSW and UNC thread profiles are similar enough that one can mount a modern camera on a legacy tripod and vice versa. The UNC threads are a 60-degree angle and flattened, whereas the BSW are a 55-degree angle, however, at least one English manufacturer uses No.1 B. A. for its tripod mount thread. There are several types of tripods, the least expensive, generally made of aluminum tubing and costing less than US$50, is used primarily for consumer still and video cameras, these generally come with an attached head and rubber feet. The head is very basic, and often not entirely suitable for smooth panning of a camcorder, a common feature, mostly designed for still cameras, allows the head to flip sideways 90 degrees to allow the camera to take pictures in portrait format rather than landscape. Often included is a pin on the front of the mounting screw that is used to stabilize camcorders. This is not found on the more expensive photographic tripods, more expensive tripods are sturdier, stronger, and usually come with no integrated head. The separate heads allow a combination to be customized to the photographers needs. There are expensive carbon fiber tripods, used for applications where the needs to be lightweight
11.
Motion blur
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Motion blur is the apparent streaking of rapidly moving objects in a still image or a sequence of images such as a movie or animation. It results when the image being recorded changes during the recording of a single exposure, when a camera creates an image, that image does not represent a single instant of time. Because of technological constraints or artistic requirements, the image may represent the scene over a period of time, in such an image, any object moving with respect to the camera will look blurred or smeared along the direction of relative motion. This smearing may occur on an object that is moving or on a static background if the camera is moving, in a film or television image, this looks natural because the human eye behaves in much the same way. Because the effect is caused by the motion between the camera, and the objects and scene, motion blur may be avoided by panning the camera to track those moving objects. In this case, even with long exposure times, the objects will appear sharper, in computer animation this effect must be simulated as a virtual camera actually does capture a discrete moment in time. This simulated motion blur is typically applied when either the camera or objects in the scene move rapidly, without this simulated effect each frame shows a perfect instant in time, with zero motion blur. This is why a game with a frame rate of 25-30 frames per second will seem staggered. Many modern video games feature motion blur, especially vehicle simulation games, some of the better-known games that utilise this are the recent Need for Speed titles, Unreal Tournament III, The Legend of Zelda, Majoras Mask, among many others. In pre-rendered computer animation, such as CGI movies, realistic motion blur can be drawn because the renderer has more time to each frame. Temporal anti-aliasing produces frames as a composite of many instants, frames are not points in time, they are periods of time. Most computer animations systems make the classic fence-post error in the way they handle time, thus most computer animation systems will incorrectly place an object on a four frame trip along a path at 0%,0. 33%,0. 66%, and 1. Motion lines in cel animation are drawn in the direction as motion blur. Go motion is a variant of stop motion animation that moves the models during the exposure to create a less staggered effect, in 2D computer graphics, motion blur is an artistic filter that converts the digital image/bitmap/raster image in order to simulate the effect. Many graphical software products offer simple motion blur filters, however, for advanced motion blur filtering including curves or non-uniform speed adjustment, specialized software products are necessary. When an animals eye is in motion, the image will suffer from motion blur, to cope with this, humans generally alternate between saccades and fixation. Saccadic masking makes motion blur during a saccade invisible, similarly, smooth pursuit allows the eye to track a target in rapid motion, eliminating motion blur of that target instead of the scene. Although this gives sharper slow motion replays it can look strange at normal speed because the eye expects to see motion blurring and is not provided with blurred images, conversely, extra motion blur can unavoidably occur on displays when it is not desired
12.
Fog
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Fog consists of visible cloud water droplets or ice crystals suspended in the air at or near the Earths surface. Fog can be considered a type of low-lying cloud and is influenced by nearby bodies of water, topography. In turn, fog has affected many human activities, such as shipping, travel, the term fog is typically distinguished from the more generic term cloud in that fog is low-lying, and the moisture in the fog is often generated locally. By definition, fog reduces visibility to less than 1 kilometre, for aviation purposes in the UK, a visibility of less than 5 kilometres but greater than 999 metres is considered to be mist if the relative humidity is 70% or greater, below 70%, haze is reported. Fog forms when the difference between air temperature and dew point is less than 2.5 °C or 4 °F, Fog begins to form when water vapor condenses into tiny liquid water droplets suspended in the air. Water vapor normally begins to condense on condensation nuclei such as dust, ice, Fog, like its elevated cousin stratus, is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. Fog normally occurs at a relative humidity near 100% and this occurs from either added moisture in the air, or falling ambient air temperature. However, fog can form at lower humidities, and can fail to form with relative humidity at 100%. At 100% relative humidity, the air cannot hold additional moisture, thus, Fog can form suddenly and can dissipate just as rapidly. The sudden formation of fog is known as flash fog, Fog commonly produces precipitation in the form of drizzle or very light snow. Drizzle occurs when the humidity of fog attains 100% and the cloud droplets begin to coalesce into larger droplets. This can occur when the fog layer is lifted and cooled sufficiently, drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point. The inversion boundary varies its altitude primarily in response to the weight of the air above it, the marine layer, and any fogbank it may contain, will be squashed when the pressure is high, and conversely, may expand upwards when the pressure above it is lowering. Fog can form in a number of ways, depending on how the cooling that caused the condensation occurred, radiation fog is formed by the cooling of land after sunset by thermal radiation in calm conditions with clear sky. The warm ground produces condensation in the air by heat conduction. In perfect calm the fog layer can be less than a meter deep, radiation fogs occur at night, and usually do not last long after sunrise, but they can persist all day in the winter months especially in areas bounded by high ground. Radiation fog is most common in autumn and early winter, examples of this phenomenon include the Tule fog. Ground fog is fog that obscures less than 60% of the sky, advection fog occurs when moist air passes over a cool surface by advection and is cooled
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