---

ABS: 'acrylonitrile-butadiene-styrene co-polymer' - a cross between plastic and rubber with excellent engineering properties, used to make underwater housings, cabinets for electronic equipment, solvent-weld plumbing fittings, etc.. ABS is attacked by many industrial cleaning agents, especially chlorinated solvents, ketones (e.g., MEK), and furans (e.g., THF). Cleaning with agents other than mild detergent and water is not recommended.

Aberration: Deviation, Wandering away, The non-convergence of rays of light to one focus.

AC: Alternating current. An electrical current that changes over time, usually in a sinusoidal fashion. Usually associated with mains electricity.
     When Thomas Edison an his co-workers developed electrical lighting into a commercially practical technology, the original power distribution system used direct current (DC). The problem with DC is that it cannot easily be transformed from one voltage to another, and so the generator has to be operated at something close to the voltage which will be made available to the end user, i.e., a lowish voltage with a correspondingly high current for a given amount of transmitted power. Low voltage distribution systems unfortunately suffer from the problem of resistance, or voltage drop, in the transmission cables, and it was said that if a skyscraper had been supplied with DC, the lights at the bottom of the building would have glowed a lot brighter than the lights at the top. The problem was solved by the adoption of a low-frequency AC system (largely attributable to Nicola Tesla), ie., an electrical current which changes from positive to negative and back again, sinusoidally, at a speed too low to cause significant radiation loss but fast enough that no-one can see the lights flickering. The beauty of AC power is that it can be transmitted as high voltage, low current, and then changed to low voltage, high current for the end user by means of a simple magnetic device called a transformer (ie., two coils of wire on a common iron core). In America, the finally adopted AC power-line frequency is 60 cycles per second (60Hz). In Europe, the power-line frequency is 50Hz. The process of changing AC back into DC, for equipment which can only run on DC (such as electronic circuitry and early 20th Century subway trains) is called "rectification" (ie., 'putting-right').

Achromat: a lens corrected for chromatic aberration (colour fringing) at two wavelengths in the visible spectrum. Most modern camera lenses are achromats.

Achromatic doublet: a lens in which achromatic correction is achieved by cementing together two lenses made from different types of glass.

Acrylic: Poly methyl methacrylate. A clear plastic with good optical properties and very high transparency. Refractive index: n_d = 1.492.  Trade names: Perspex, Plexiglass, Lucite. Acrylic is attacked by chlorinated organic solvents. Stressed (i.e., shaped or moulded) acrylic will shatter in contact with alcohols, so be very careful about what you put on it if you want to clean it (mild detergent and water is preferred). Wiping with 60-80 petrol (cigarette lighter fluid) can be used to remove severe non-water-soluble adhesions.  

AE: Automatic Exposure. A method of linking the camera exposure settings to the ambient or reflected light level.

AF: Automatic-Focusing. There are two basic types of automatic focusing system; range-finding systems, and contrast or image-sharpness sensing systems. Range-finding systems send out a signal; usually an infrared beam (but sometimes ultrasonic), and see how long it takes for the reflected signal from the subject to come back. Such systems can be fooled by windows, and they don't work underwater because the water absorbs or modifies the signal. Contrast sensing systems determine image sharpness in much the same way as the human eye, and are much more difficult to fool, but they can still be fooled by repeating patterns, and they don't work if the subject lacks detail. Range-finding systems are sometimes called 'active autofocus' because they send out a signal, whereas contrast sensitive systems are called 'passive autofocus' because they don't. This is one case where it is usually better to be passive.

Ah, Ampere hours: The capacity rating of an electrical cell or battery. One point to watch when interpreting ampere-hour ratings is that the capacity of a battery or cell varies depending on the rate of discharge. The figure is normally specified at the 10 hour rate. E.g., A 10Ah battery will deliver a current of 1 amp for 10 hours. It will however, deliver a current of 10 amps for somewhat less than 1 hour. I.e., the effective capacity is reduced at high discharge rates (and also, not greatly increased at very low discharge rates). In combination with the average battery terminal voltage throughout the discharge cycle, the battery capacity is also a measure of the available stored energy. E.g., a 6V 10Ah battery can deliver 6 Watts (6 Volts x 1 Amp) for 10 hours, i.e., 6 Watts for 36000 seconds = 216000 Joules (216KJ). That is a lot of energy. If you short-circuit a battery, nearly all of the stored energy will be dissipated in the battery itself, which should explain why batteries can pose a severe fire hazard if misused or mishandled. Some modern batteries have energy densities close to that of Dynamite.

Al, Aluminium, Aluminum: Humphrey Davy, the discoverer of this chemical element, originally named it "A l u m i u m". He then changed his mind and called it "A l u m i n u m", and this naming convention is the one adopted in North America. The British Chemical Society however, felt that element names should always end with "-ium", and so changed the name to "A l u m i n i u m". The Americans have the wishes of the discoverer on their side.

Amp, Ampere, A: The unit of electrical current, ie., quantity of electricity per unit time. Current flows through wires and conductors, whereas voltage (electrical pressure-difference) appears across things like generators, batteries and resistors.

Angle of Coverage (of a lens): The angle between rays of light entering a lens from the extreme corners of a scene.  See also angle of coverage article.

Angle of Coverage (of a flash unit): The angle of the light-cone or pyramid emanating from the light source. The quoted angle of coverage for a flash unit corresponds to the angle between two rays, on opposite sides of the beam axis, chosen so that the light intensity is half of that at the beam centre. I.e., it is assumed that the useful field of illumination ends at the point where the light intensity has fallen by 1 EV (1 stop) relative to the centre. Circular light sources are given a single value for angle of coverage, rectangular light sources are given two values (Horizontal and Vertical).

Anodisation (Anodization): An electrolytic process for forming a stable film of aluminium (aluminum) oxide on the surface of aluminium alloys. Aluminium will normally grow an oxide layer on contact with air; but the layer formed in the anodisation process is harder and less porous, giving improved corrosion and abrasion resistance, and can be made to incorporate pigments as an alternative to painting the finished product.

Anti-reflection coating: Reflections that occur at optical surfaces are a general source of lens flare (i.e., image artifacts due to bright light sources outside of the field-of-view).  Reflection is reduced by reducing the abruptness of the change in refractive index at an optical surface.  This can be achieved by coating the surface with a very thin layer of some material that has a refractive index intermediate between the refractive indeces of the two media.

Aperture: The variable diameter hole used to control the amount of light passing through a lens. See f-stop.

Apochromat: a lens corrected for chromatic aberration (colour fringing) at three wavelengths in the visible spectrum. Traditional apochromatic lenses using spherically-ground glass elements are complicated and expensive. Apochromatic lenses can also be produced by using aspherical moulded lens elements, but moulding is generally an inferior process to grinding, and so, while a lens with moulded elements may be strictly described as apochromatic, that doesn't always mean that it will be superior to a traditional achromat (and if the elements are pressed out of plastic it may well be awful - caveat emptor). This is not to say that modern moulding methods cannot produce results equal to grinding, they can, but high quality optical mouldings are not to be expected in cheap lenses.

APS: Advanced Photographic System (now obsolete).  Photographic film with integral magnetic recording strip for data.  25.1 × 16.7mm image format.
APS-C:  Term used for camera sensors of format dimensions similar to those of APS.

AR: Anti-reflection (coating)

---

B, Bulb sync.: In old-fashioned flash-bulb photography, the bulb must be ignited a few milliseconds before the shutter is opened, to give it a chance to get going. Alternatively, the shutter must be opened for a relatively long time, to make sure that the bulb gives out its light while the shutter is open. Nowadays, the camera 'B' position is used for time exposures, it being a setting in which the shutter will remain open for as long as the shutter button is held pressed.

BA: Screw-thread system for small instruments proposed in the 19^th century by the British Association for the Advancement of Science. BA screws are based on metric dimensions with a constant pitch gradient. The pitch angle is 47.5°. Some BA sizes will fit with modern metric parts, e.g. 0BA fits M6×1, 13BA fits M1×0.25. Now largely obsolete.

Backscatter: Light reflected from suspended particles in the water, a problem in underwater flash photography caused by having the flash too close to the camera lens. It can manifest itself as anything from a slight fog, to a haze of bright specks obscuring the picture. In extreme cases, particles close to the camera shine so brightly that they cause lens flare, in which case some of the specks will appear to have the same shape as the lens iris. The problem is minimised by using an external flash, either mounted on a long adjustable arm, or un-mounted and positioned by hand.

bar: Metric unit of pressure (now deprecated in favour of the Pascal, but considerably more intuitive). Standard atmospheric pressure is 1.01325 bar. 1 bar = 100 000 Newtons / metre squared (100 kilo Pascals).  As you dive underwater, due to the weight of the water above you; the pressure increases by 1 bar for every 9.806 metres of depth (assuming fresh water).  For sea water (which is slightly denser than fresh water), we say, to a good approximation, that the pressure increases by 1 bar for every 10m of depth.

Bayer mosaic: The pattern of red green and blue filters used to separate colour in a conventional CCD or CMOS camera sensor. US Patent No. 3971065, 1976. See pixels and resolution article for more information.

bits-per-pixel (bpp): A measure of tonal rendering capability in digital pictures. A good impression of continuous tonal rendering can be created by having 256 possible levels of brightness for each of the colours red, green, and blue. 256 levels can be represented by an 8-bit (1 byte) binary number, so it takes 3 x 8 = 24 bits to create the impression of continuous tonal rendering. 24 bits corresponds to about 16.7 million possible unique values of hue and brightness. It is better to start with considerably more than 8 bits per colour if any image adjustment is to be carried out, so that there will be enough information left to give 8 bits per colour in the final result.

BK7 (Borosilicate-crown 7): A widely used high quality optical crown glass, relatively hard with good scratch resistance.
n_d = 1.5168 @ 587.6nm.  More information.

BNC connector: Bayonet Neill-Concelman (named after the designers, Paul Neill and Carl Concelman). Coaxial connector used for video, ethernet, and radio frequency signals. [Rad Com (RSGB publ), October 1995, p67. QEX (ARRL publ), May 1985, p2.]

Buccaneer connector: Multi-pole (multi-pin) waterproof connector, manufactured by Bulgin, with an ingress protection rating of IP68 1 bar (ie., submersible to a depth of at least 10m, and in practice, a lot more).

Burn-time: The time for which a lamp will operate after starting with a fully charged battery.

Bulkhead: A wall or partition in maritime parlance.
Bulkhead connector: An electrical connector which mounts on a panel or bulkhead.

Buoyancy: The propensity of an object to float (positive buoyancy) or sink (negative buoyancy) or to do neither (neutral buoyancy).

---

Camcorder: Sony always called its tape recorders 'Tapecorders', thereby avoiding a pronunciation difficulty for native Japanese speakers. A combined camera and video recorder therefore naturally became a 'Camcorder'.

Camera: Latin and Italian: a small room or bedroom. English equivalent: Chamber. The word Camera came into the English language as a shortened form of the phrase 'camera obscura', a darkened room. The invention of the camera arises from the observation that, if a house is completely shuttered, light coming through a chink or keyhole will project an upside-down laterally-reversed image of what lies outside on to the opposite wall. Artists developed the secret practice of setting-up such a room (or tent, or box) and placing an easel inside, so that they could sketch over the resulting image and use it as the basis for a painting. A study by the artist David Hockney (Secret Knowledge: Rediscovering the lost techniques of the Old Masters, ISBN 0-500-23785-9) argues that the camera obscura, in combination with various mirrors, lenses, and tracing screens, was used by artists from about 1430 onwards - and that the works of many of the Old Masters are effectively photomontages (the extraordinary skill of the artist being that of rendering the optical image in paint). A subsequent study of the use of perspective in Hockney's principle example (Arnolfini & his Wife by Van Eyck) has argued that Van Eyck did not use optical methods in setting out this work (see Scientific American, Dec 2005, p52-59), although an alternative interpretation is that the artist varied his optical setup depending on which part of the picture he was rendering. The word 'photography' means 'drawing with light', ie., dispensing with the artist, and using light-sensitive chemicals instead. Because 'camera' means 'bedroom', Italians use the terms 'fotocamera' and 'maccina fotografica'.

CCTV: Closed-circuit Television, i.e., TV not involving radio transmission or long distances. A somewhat old-fashioned term that confuses the difference between video and TV. Television means vision at long-distance, usually involving transmission by radio, radio carrier in long-range cables, and fibre-optics. The term 'CCTV' is therefore tautologous (short-range long-range vision), and a CCTV camera is nowadays more sensibly referred to as a 'video camera' or a 'surveillance camera'.

Capacitor: An electrical storage device. In a flash unit, energy is stored in a high-voltage capacitor for rapid discharge into the flash tube (See also inverter).

Caveat: Beware. of which to beware.
Caveat Emptor: Buyer beware!

CCD: Charge Coupled Device. An integrated circuit (micro-chip) consisting of a group of charge storage cells (tiny capacitors) with the ability to pass charge from one to the next, in a line, like a bucket-brigade (ie, like a group of fire-fighters passing buckets from one to the next). Originally conceived in the mid 1960s as an analogue delay line for processing Radar images, someone came up with the idea of exploiting the fact that the cells are sensitive to light, and setting them out in an array, like the lines of a TV picture. Thus was born the replacement for the fussy and troublesome video camera tube. Most modern video cameras use a single CCD array with a colour mosaic filter to separate red, green and blue. Early professional cameras split the light into red, green, and blue optically, and used 3 CCD arrays, but as the number of pixels increased (for HD and stills capture), physical alignment of the three pictures became impossible, and single-CCD (mosaic filter) solutions had to be found.

Cell (electrical): An electrochemical energy source. A device with a particular voltage difference across its terminals. Electrochemical cells are often connected in series to form a 'battery of cells', commonly known as a 'battery'. Hence, for example, a 12V battery may be constructed by connecting ten 1.2V cells in series. A cell is composed of two half-cells, each comprisong an electrode (often an electrically conducting rod, plate, film or cannnister) and an electrolyte (a chemical solution). The two half-cells may use the same electrolyte, in which case the two electrodes are immersed in the same solution; or they may used different electrolytes, in which case the two electrolytes are separated by a semi-permeable membrane. The common Leclanché cell (also known as the zinc-manganese cell), has a positive electrode consisting of a porous container filled with manganese dioxide powder with a carbon rod inserted into it for connection, a zinc can for the negative electrode, and a solution of ammonium chloride for the electrolyte.

Cell, Corrosion: When two dissimilar metals in contact are immersed in water containing dissolved salts (an electrolyte), a short-circuited electrochemical cell is created. The action of the cell is to etch or consume one of the metals thereby producing electrical energy which is immediately wasted as heat. One way to stop the corrosion is to reverse-bias the corrosion cell, i.e., apply a voltage in opposition to the voltage generated so that no current flows. This is done by creating an additional corrosion cell using a metal which is more readily consumed than the metal to be protected. Aluminium, for example, can be protected by attaching a block of magnesium to it, this attachment being called a 'sacrificial anode'.

C-Mount: Screw-in lens mount used on some ciné and video cameras. 1" diameter thread with 1mm pitch.

CMY: Cyan, Magenta, Yellow. The 'subtractive' primary colours. Colour-space representation used in conventional print photography.

CMYK: Cyan, Magenta, Yellow, Black. The colour-space representation used by ink-jet and other printers. The letter 'K' is used for black because 'B' already stands for blue. The extra black ink is used to overcome the density limitations of the other inks, i.e., C+M+Y usually comes out dark brown rather than pure black. Also, it is a waste of money to print black and white documents and text using exotic coloured inks.

Colour, Color: Colour is what passes for spectroscopic analysis in the rather limited human (i.e., primate) visual system. A pure monochromatic light source will produce an impression of colour that depends on the wavelength of the light falling on the retina. The eye and the brain work together to analyse the input according to the relative levels of stimulation received by three types of cone cell, which have sensitivity peaks at wavelengths of 560 nm (red), 530 nm (green), and 424 nm (blue) (nm = nano metres). The crudeness of this analysis makes colour photography (as we know it) possible, because we can fool the eye into seeing almost any colour simply by superimposing three light sources (R, G and B) and adjusting their relative intensities. Photographic (synthesised) colour is an illusion specifically tailored to (and exploiting the limitations of) the primate visual system; so don't expect non-primate animal species (even other mammals) to make sense of photographs and TV pictures. For more information on the evolution of colour vision in vertebrates and the differences between species, see "What Birds See", by Timothy H Goldsmith, Scientific American July 2006. p50 - 57.

Colour space: The set of colours that can be represented by a particular imaging system. There is considerable latitude of choice in selecting the exact wavelengths used to stimulate the red, green, and blue receptors (cone cells) in the eye. Different choices, and imperfections in the spectral purity of the light sources or dyes used, affect the colour representation capability (colour space) of the system. Synthetic (RGB or CMY) colour systems moreover, cannot properly recreate colours that correspond to the violet (450 - 390 nm) part of the spectrum (magenta = red + blue is the best we can do), which is why photographs of bluebells always come out wrong. When editing images, it is best to use a working colour space that corresponds reasonably closely to that of the intended display device.

Colour Temperature: The temperature of an equivalent hot black body. A theoretical black body is a perfect absorber of light when cold, and hence a perfect emitter of light when hot (i.e., when glowing). The black body is equivalent to the light source in question in the sense that it produces the same relative amounts of red, green, and blue light. Colour temperature is measured in Kelvin, K (degrees above absolute zero).
    If an object is heated in a furnace, or by passing an electric current through it, it glows with a particular colour (red, orange, yellow, white, or bluish white) depending on the temperature. If the emitted light is free from spikes or gaps in its spectrum at particular wavelengths (ie., colours), then it is known as 'black body radiation'. Tungsten light-bulbs produce good black-body radiation. Electronic and gas-discharge light sources (e.g., flash tubes) do not produce perfect black-body radiation, but if there is enough red, green and blue light in the mixture, the source can often be correlated to a black body at a particular temperature. The correct term to express the colour rendering quality of a particular light source is therefore "correlated colour temperature". Standard photographic daylight is assumed to have a colour temperature of about 5500 K. Xenon flash has a correlated colour temperature of about 6000 K, which is fine for most purposes, but sometimes a little too blue. Underwater flash makers sometimes use a yellow reflector or filter, to reduce the flash colour temperature to around 4500 K, to compensate for the blueness of the water. Domestic tungsten light-bulbs have a colour temperature of about 2950 K, which is very orange. Quartz-halogen lamps have a colour temperature of 3200 - 3400 K, which is yellowish, but electronic cameras can compensate for this by adjusting the relative sensitivities of the red, green, and blue channels (white balance). Humans do not usually notice the effective temperature of a light source except by comparison, because our eyes adjust automatically (look at the colour of a tungsten light-bulb while there is sunlight streaming through the windows - the electric light seems orange or yellow in the daytime, but perfectly white at night).

Complementary Colour: The colour of light that, when added to a given colour in the correct proportion, produces white (or grey, i.e., an illumination without a colour cast).
Red + Cyan gives white. Green + Magenta gives white. Blue + Yellow gives white. Hence Red and Cyan are a pair of complementary colours, so are green and magenta, so are blue and yellow.

Compression (of data): The removal of redundant or repetitive information from a data set (e.g., a file) in such a way that the original data can be reconstructed.
Lossless compression: compression that can be reversed without loss of information (e.g., LZW).
Lossy compression: compression that does not permit exact reconstruction of the original data (e.g., JPEG).

Concave lens: A lens in which the principal optical surface curves into the body of the glass, ie., the glass has a a hollow or dent.

Convex lens: A lens in which the principal optical surface bulges outwards.

CRT: Cathode Ray Tube. Term used for the large glass vacuum tubes that were used in television or video displays and oscilloscopes (but have now been superseded by flat panel displays). The term was coined in the 19^th century, before anyone knew that the mysterious 'cathode rays' that would cause a zinc sulphide screen to glow were actually the sub-atomic particles we now call electrons.  It became impossible to change the archaic name, because the Americans adopted the term 'electron tube' to refer to the device that the British scientist Ambrose Fleming preferred to call a 'valve' (because it only conducts electricity in one direction). Fleming's valve was one of Mr Edison's light bulbs with an extra plate inside it to capture the electrons emitted by the hot filament. A CRT is a type of electronic valve (or electron tube) in which the electrons are focused into a beam and fired at a phosphorescent screen. In a video display, magnetic fields produced by coils of wire fitted to the neck of the tube steer the electron beam rapidly so that it can be used to trace a series of horizontal lines on the screen (a raster).

CU: Close-up.

CVBS: 'Chrominance, Video, Blanking, and Synchronisation', ie., a composite video signal.

---

D8: Digital 8: Digital video recording system using 8mm tape cassette.

DC: Direct current (as opposed to alternating current); an electrical current that does not alternate or change significantly with time (at least over a moderate interval), such as might flow when a light bulb is connected to a battery. Never say "DC current" (direct current current), just say "DC".

DC-DC Converter: An electronic device that converts from one constant voltage to another (usually higher) constant voltage, e.g., 6 V in, 330 V out, as used in typical small flash units. A DC-DC voltage up-converter may also be called an inverter.

Delrin: Acetal Homopolymer. Engineering plastic with sufficient hardness and dimensional stability for the manufacture of precision components (e.g., screw-threads).

Depth of field: No lens can produce a perfectly sharp image. This means that there will be a range of focus settings over which the lens will give no significant improvement over an exact setting. Conversely, for a given distance setting, there will be a range of lens to subject distances over which focusing will be adequate or not-improvable. This range is called the depth of field. The depth of field of a lens improves as the aperture of the lens is made smaller, although maximum depth of field is not the same as maximum optical resolution. The trick, in photography, is to choose the aperture so that the depth of field embraces the whole depth of the subject from the furthest to the nearest point. Good lenses are provided with depth of field markings to assist in this matter, and good SLR cameras have a 'depth-of-field preview' button, which closes the lens down to the aperture that will be used when the picture is taken.

Diffuser: A device that increases the effective area of a light-source, i.e., makes it less point-like. A piece of opal or translucent plastic that can be fitted to the front of a light source. A diffuser helps to even-out the distribution of light intensity in the field of illumination, and reduces the problem of specular reflection from the subject.

Digital zoom: In-camera image cropping (throwing away the outer pixels).  A feature beloved of marketing departments for the purpose of making the camera zoom-range specification seem better than it actiually is.

Diopter:: A unit used to express the power of magnifying glasses. Screw-in magnifying lenses, which can be fitted to the front of a camera lens, are sometimes referred to loosely as 'diopters'.
     The power of a lens in diopters is the reciprocal of its focal length in metres.

Macro Lens	Diopters	Magnification (Diopter / 4) +1	Maximum Focus Distance 1 m / Diopter
	+0.5	1.125×	200 cm
	+1	1.25×	100 cm
	+2	1.5×	50 cm
INON UCL-330	+3	1.75×	33.33 cm
Epoque DML-2	+4	2×	25 cm
Subsea +5	+5	2.25×	20 cm
	+6	2.5×	16.66 cm
	+7	2.75×	14.29 cm
INON UCL-165	+7.37	2.84×	13.57 cm
	+8	3×	12.5 cm
	+9	3.25×	11.11 cm
INON UCL-100 Subsea +10	+10	3.5×	10 cm
	+15	4.75×	6.66 cm
	+20	6×	5 cm
	+25	7.25×	4 cm

The neat thing about diopters is that if you stack several lenses, the new magnifying power can be obtained simply by adding together the diopters of the individual lenses; i.e.,
D = D1 + D2 + . . . . . . . etc.
This formula holds good only if the lenses are close together, i.e., well inside each other's focal distance. Thus, if you stack a 1 diopter lens with a 2 diopter lens, you get a 3 diopter lens; but that doesn't mean it's a good idea. Stacking lenses like this is a useful practice while you're trying to determine the magnification power needed for a particular job; but if you want best corner to corner image sharpness, you should then obtain a single lens of the right power.
     Lenses that are overall convex (bulging) have positive diopters. Lenses that are overall concave (indented) have negative diopters. The low-cost close-up lenses designed to screw into the camera filter ring are usually convex on one side and concave on the other, but the convex part is more curved than the concave part, so overall, it's a magnifying glass. This concave-covex structure is called a `meniscus' lens, and is used because it gives the best minimisation of chromatic aberration for a lens constructed from a single piece of glass. Needless to say, adding such a lens to your camera causes a reduction in optical resolution. Far superior results can be had, at a price, by using two-element achromatic close-up lenses, such as those manufactured by Nikon and Canon.

Meniscus lenses for screwing into the camera-lens filter thread. The lens holder has a male thread at the back and a female thread at the front, to allow stacking and the use of additional filters.

Dispersion: The separation of white light into its component colours. Dispersion in optical media (such as glass) occurs when the refractive index varies with wavelength. It is generally undesirable in lenses, since it leads to chromatic aberration.

DNR: Digital Noise Reduction. Clean-up process used when transferring analog video signals into the digital domain.

Dome port: Lens port constructed as part of a sphere of uniform thickness. The dome port corrects for the magnifying effect that occurs at an air-water boundary, and thus preserves the image geometry and angle of coverage. Contrary to popular myth, the dome port does not correct for all optical aberrations at the boundary, and actually introduces some. Introduced aberrations are reduced as the radius of the dome increases, but the problems of excessive buoyancy and delicacy of the outer optical surface impose practical limits on dome size.

Duo connector: Sea & Sea proprietary underwater connector used on sync. cables and flash units. It is called 'Duo' because the electrical specification supports both the Motormarine II and Nikonos V flash interface protocols.

DV: Digital Video

DX-coding: Pattern printed on the side of a 35mm film cassette that is used by the camera to sense the required ISO/ASA film-speed setting. The film speed is sensed by means of electrical contacts pressing against the canister, so dirt can cause mis-sensing. DX-coding was meant to simplify photography; but if no over-ride was provided, it could be a pain for advanced photographers who wanted to deviate from the film manufacturer's recommendations.

---

Electromagnetic Radiation:
Radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays are all manifestations of the same thing: electromagnetic radiation. The difference between the various types is a consequence of the difference in frequency (or wavelength). The theory was worked out in the 19^th century by James Clark-Maxwell; who showed that electricity and magnetism are related and that light consists of electric and magnetic fields linked together as they travel through space. Maxwell's work set the stage for the development of radio and countless other technologies that we nowadays take for granted. The relationship between frequency and wavelength is:
c = f λ
where c is the speed of light (299 792 458 metres per second), f is the frequency, and λ (the lower case Greek letter lambda) is the wavelength, ie., the distance the wave travels as it undergoes one complete cycle. The middle of the medium-wave broadcast band is about 1MHz (one million cycles per second) and the corresponding wavelength is 300 metres. The wavelength at the middle of the visible spectrum is about 500 nm (nanometres = thousand millionths of a metre), and the corresponding frequency is
6 × 10¹⁴ Hz, ie., 600 000 000 000 000 Hz (600 THz or 600 Tera Hertz).
Radio signals are normally characterised by their frequency, but light is normally characterised by its wavelength because the numbers for frequency are messy to write down without using scientific notation.
     Electromagnetic radiation is also sometimes split into two classes: ionising radiation, ie., radiation that has sufficient energy to knock electrons out of atoms and molecules, and non-ionising radiation, which hasn't. Radio and infrared are classed as non-ionising radiations (although some things can be ionised by infrared), whereas visible light, UV, X-rays and gamma rays are definitely ionising radiations. Our body senses are divided according to this distinction; in that we sense infrared through the skin by its heating effect, whereas we sense visible light through our eyes by its ability to split molecules of light-sensitive pigment. Insects can also see in the UV part of the spectrum, whereas to the human body, anything of shorter wavelength than visible light is simply harmful.

E/O connector: Electro-Oceanic: A type of wet-connector with two contacts, in appearance like a rubber jack-plug.

E/O Connector

E&OE: Errors and omissions excepted. Legally, this means that the information given cannot form part of a contract, i.e., it is not binding on the seller because it may require corrections.

EV: Exposure value. A change in exposure of 1 EV corresponds to a halving or doubling of the intensity of light falling on a film or sensor, i.e., a 1 EV change is equivalent to a change of 1 f-stop, or a halving or doubling of shutter speed or ISO film speed.

ewa-marine: Pronounced "ay-va marine".  German manufacturer of flexible underwater camera housings.  Such housings are known in the film and broadcast industries as "splashbags".

EVIL: Electronic Viewfinder, Interchangeable Lens.

EXIF: Exchangeable Image File. An extension to the JPEG file format to include information provided by a digital camera: time, date, firmware version, focal length, aperture, shutter speed, exposure value, etc.

Exposure: The product: light intensity × time.

Exposure bracketing: the practice of taking a series of photographs, usually at 1 EV or ½ EV increments on either side of the expected correct exposure, in order to obtain a range of images with slightly different exposures from which the best may be chosen. Exposure bracketing is of particular value when using a recording medium with limited exposure latitude, such as slide film, or digital cameras that only give 24 bit output.

Extension tube: A lens focuses at infinity when the distance from the exit pupil of the lens to the film or sensor (the focal plane) is equal to the focal length. As the lens is moved away from the focal plane, closer objects come into focus, but the amount of extension permitted by the focus barrel is limited at some point by a decision made by the lens manufacturer. To make a lens focus closer than the designer originally intended, an extension tube can be fitted between the lens and the camera. Extension tubes for system (SLR and electronic viewfinder) cameras can have have control linkages or electrical connections passing through them to retain any automatic functionality of the lens. Manual extension tubes are simply tubes, with a male lens mount on one end and a female mount on the other. Different length tubes give different ultimate magnifications with a particular lens. Note that unless the lens is designed for the purpose, it will be taken out of its optimum focusing range when a tube or bellows extension is used and will therefore not give its maximum possible resolution.

---

Fasteners: Generic term or collective noun for nuts, bolts, screws, washers, and other parts or devices used in attaching one object to another.

FE: Fisheye

Fe: Iron (ferrum)

Field of View, FOV - of a lens: same as angle of coverage.

Filter: Any device that modifies a signal is a filter. Therefore, it is legitimate (but not good usage) to describe a close-up lens as a filter, the real reason for doing so being that close-up lenses screw into the camera filter-ring. In photography, the term 'filter' is normally reserved for a device (usually a sheet of glass or plastic) that attenuates (reduces) all or part of the light passing through it. A Neutral-Density (ND) filter causes an equal reduction across the whole visible spectrum and is used when the light is too bright for the camera equipment, or to modify the required iris setting as a way of controlling depth-of-field. Most other filters modify the colour or the light in some way, or remove troublesome invisible components. A UV filter removes ultraviolet light, to prevent haze, but leaves the visible spectrum virtually unchanged. An underwater colour-correction (UWCC) filter has an orange or magenta colour to compensate for the cyan-blue or green colour cast introduced by photographing through water; an there are a host of other colour-modifying filters for different applications.

Filter thread: Except for the products of awkward manufacturers, camera filter screw threads are international standard (ISO) metric (M) threads with a pitch distance of 0.75 mm. If you measure the inside diameter of a female filter thread (using engineer's calipers), adding the pitch distance to the result gives the nominal thread diameter plus about a 10^th of a mm. E.g., a camera filter thread measures 57.35 mm. Adding the pitch distance gives 58.1 mm. The filter thread size is fully specified as M58×0.75, but usually abbreviated to M58.

FireWire: See i.Link

Fisheye: Wide angle lenses can be designed in two ways: They can be corrected to make all straight lines appear straight (rectilinear correction); but this type of correction introduces perspective distortion, ie., assuming a flat object, it exacerbates the fact that points at the edge of the field are further away than points in the middle. The alternative is to avoid rectilinear correction, in which case perspective distortion is minimised; but rectangles appear to bulge in the shape of a barrel, and the only lines that remain straight are those that pass through the exact centre of the field of view. The latter type of lens is called a 'fisheye' lens.

Fish-eye distortion of rectangular test card

Fish-eye photograph with vanishing point placed at the centre of the image

Flare: A major problem in lens design is that of minimising reflections from the surfaces of the lens elements. Such reflections result in multiple images, and in light from bright objects outside the field of view arriving at the film or sensor. The various optical effects that can occur due to internal lens reflections are known collectively as 'flare'. Reflections occur at boundaries where there is a sharp change of refractive index (e.g., on going from air into glass) - the sharper the boundary the stronger the reflection. Modern lenses have anti-reflection coatings, which make the change of refractive index more gradual and so reduce (but do not eliminate) flare. All compound lenses will exhibit some flare when shooting into the sun. Strong reflections from scene highlights will also cause visible flare. Some lenses can be fitted with an external 'anti-flare hood', which is designed to cut off all light-rays that do not originate from objects within the field of view.

Focal length, f: The distance at which a lens brings light from infinity to a point. The focal length of a simple symmetrical lens, is the distance between the image plane (the surface on which the image is formed) and the middle of the lens, when light from infinity is brought to a focus. You can estimate the focal length of a simple lens by projecting an image of the sun onto a heat-resistant surface, and measuring the distance from the middle of the lens to the image (do not look at the sun through a lens, do not focus an image of the sun onto your skin). Strictly, the place to measure from is the exit-pupil of the lens, which is where the iris (aperture) appears to be when you look into the back of the lens. Most camera lenses are not symmetrical however, i.e., the actual focal length is different if you turn the lens around, in which case, the figure quoted is the focal length of an equivalent symmetrical lens. Many camera lenses also are of the retrofocus type, i.e., they have additional optics at the rear to allow them to sit close to the film plane, and the simple method for estimating focal length given above will produce misleading results.
     Photographers often talk of focal length in the context of a given camera format (usually 36 × 24mm), in which case the term serves a shorthand for the associated angle of coverage. Note however that the commonly understood relationship between focal length and angle of coverage breaks down when you change the optical medium; i.e., in the absence of a correcting element (such as a dome port) lenses of a particular specified focal length are not as wide-angle underwater as they are in air.

Format: The 'format' of a film or camera is the dimensions of the image area (or frame). The 35 mm (full-frame) film format used by stills cameras is 36 × 24 mm. The 35 mm (half-frame) format, used by motion-picture cameras is 24 × 18 mm. The 2¼" square (medium) format is 57.15 × 57.15 mm (not 60 × 60 mm as is sometimes stated).
     Digital cameras use a wide variety of formats, making it extremely difficult to relate the focal lengths of the lenses used to the actual angles of coverage achieved. This problem is usually solved by giving a focal length multiplier, i.e., a number that when multiplied by the actual focal length, gives the equivalent focal length for the 35 mm film format.  
     For a list of format sizes and focal length multipliers, and the implications of format choice with regard to image quality, see the Image formats article.

Four Thirds: An "open-standard" image format and common lens-mount specification for high-quality digital cameras, intended to make lenses from different camera manufacturers interchangeable (see http://www.four-thirds.org/en/). It is supported by Olympus, Kodak, Fujifilm, Panasonic, Sanyo, Sigma, and others. The specification dictates a senor diagonal of 21.63mm (half that of the 35mm format, giving a focal length multiplier of 2). The original Four Thirds system was intended for SLR cameras. A variant "Micro Four Thirds" has a reduced lens to focal-plane distance specification and is designed for for mirrorless live-preview (electronic viewfinder, interchangeanle lens) cameras. Four Thirds lenses can be used on Micro Four Thirds cameras by means of an adapter (an extension tube with electrical contacts).

FOV: Field of view. Usually synonymous with Angle of Coverage.

f-stop: The illuminating power of a lens is a function of the focal length of the lens and the diameter of the hole (aperture, iris) through which the light is allowed to pass. The amount of light falling on a film or electronic sensor can be adjusted by changing the diameter of the aperture; but instead of recording the actual diameter, we can treat all lenses equivalently if we record the aperture diameter as a function of the focal length. Thus we record apertures as f/n (focal length divided by a number n). eg., a 50mm lens at f/2 has an aperture of 25mm, an 80mm lens at f/2 has an aperture of 40mm, but both have the same illuminating power. A standard series of apertures has evolved such that the illumination doubles or halves with each click-stop (f-stop). If you double the diameter (or radius) of a circle, you quadruple the area (area = πr²), and it is the area that determines the level of illumination. Thus, to get from one f-stop to the next, the number n is not doubled, but multiplied by the square root of 2 ( √2 = 1.414). The standard f-stop series thus comes out as: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, etc., ie, the number doubles for every 2 steps in the series. The smaller the number n, the larger the aperture. The aperture designation "f/22" incidentally is an ancient mistake. A more exactly calculated aperture value is f/22.6 which, when rounded to the nearest whole number, should really be called "f/23".

---

GIF: 'Graphical Interchange Format'. A compressed image file format best suited to diagrams and images with a restricted colour range. In GIF compression, the colour range of the image is first reduced to 256 colours or less, then the remaining information is further reduced in size by a scheme known as LZW (Lempel-Ziv-Welch) compression. LZW compression is lossless, which means that you can edit and re-save the image without degrading it, but the initial colour-space compression makes the format unsuitable for high-quality colour photographs. Note however, that 256 levels in a black and white image can be indexed as colours, which means that the GIF format is lossless for 8 bit continuous-tone monochrome images. The LZW algorithm was patented by Unisys. The US patent expired in 2003, but international patents required the payment of licence fees in other countries for some years after that.  That situation led to the development of the alternative open-source PNG format.

Gland: In underwater engineering parlance: an joint or interface that prevents water ingress; e.g., control gland, cable gland, etc. A sealing structure that permits passage through a bulkhead (vessel wall), usually a combination of machined metal or plastic parts and O-rings.

GTO: Gate Turn-Off Thyristor. A power switching device used to switch-off (quench) the current in a flash tube in order to achieve automatic or TTL flash exposure control. Now largely replaced by the IGBT.

Guide Number: A number used to represent the illuminating power of a photographic flash. When electronic flash is used, the camera shutter opens (i.e., the exposure begins), the flash fires, and the shutter closes; but because the flash duration is very short (and assuming relatively little ambient light), the 'shutter' does not control the amount of light falling on the sensor or film. Consequently, the level of exposure due to the flash can only be controlled by the lens aperture (f-stop) setting. In metric countries, the flash guide number is the aperture setting required for ISO 100 sensitivity when the flash to subject distance is 1m (in North America, the guide number is the 100ASA aperture setting for a distance of 1 foot. Divide the American guide number by 3.28 to get it in metres). Using the Guide number G, the required aperture setting for any ISO speed and distance (in air) is obtained thus:



This equation is used to produce the guide table that is sometimes attached to the flash.

---

HAD: Hole Accumulation Diode. Diode structure used in some types of CCD image sensor.

Hand-wheel (nut , bolt ): A threaded nut or bolt that can be tightened by hand, usually in the form of a knob with a gripable (knurled) surface, or with protrusions (wings).

Handy: Quasi-English term (malapropism) used in Germany to mean 'Mobile Telephone', and used in other countries to mean 'hand-held'. In native English idiom, handy simply means 'useful'.

Helical Scanning: A method for recording analog video signals onto magnetic tape.
     The problem in recording a video signal is that it has enormous bandwidth in comparison to an audio signal. This means that the tape must move at a very high speed relative to the recording head. Tape in an audio cassette moves at 47.625 mm/sec (1+7/8 ips). To record analog video, the tape would need to move at about 5 - 10 m/sec (11 - 22 mph) depending on the required picture quality, which is impractical.  The solution is to mount recording heads on a drum, which is set at a slight angle to the path of the tape and spins rapidly while the tape moves by slowly.. The result is that the recording heads execute a helical path relative to the movement of the tape, and thereby write the video information at high speed as a series of diagonal stripes.  The drum rotation is synchronised with the picture, so that the changover from one stripe to the next occurs in a non-displayed part of the video waveform (the field blanking interval).  All analog video-tape recorders from the late 1950s onwards used helical scanning, and the technique was also used to write CD-quality audio on to tape in DAT (digital audio tape) machines.

Hi-8: High-band Video 8.

HID (Lamp): High Intensity Discharge Lamp.

High-band video recording: Analog video signals are written onto tape as a frequency-amplitude modulated (FAM) radio-frequency signal. The frequency modulated (FM) part represents the luminance (brightness) information, and the amplitude modulated (AM) part represents the chrominance (colour) information. Early systems used a relatively low-frequency range for the FM, but as the sizes of the magnetic particles in the tape became smaller with improvements in the technology, higher frequency recordings became possible, with consequent improvements in the available video bandwidth. This led VTR manufacturers to release high-band versions of their recorders (e.g., S-VHS and Hi-8). The high-band machines can record and play the earlier tapes in the low-band format, but the old low-band machines cannot play the high-band tapes, and cannot record or erase them properly because higher magnetic field-strengths are required. A high-band machine can however, make a low-band recording on a high-band tape for playback in a low-band machine.

 Histogram: In digital photography; a graph of the number of pixels occupying each possible brightness level. Experienced picture editors learn to look at the image histogram and assess the 'health' of the image in terms of what it will look like when printed. It there are serious gaps in the histogram (brightness levels not used or 'not populated') the picture will not be able to give the impression of continuous tonal gradation, and the print will appear 'posterised', ie., made out of discrete blobs of colour with obvious boundaries between them.

Hot Shoe: A camera accessory shoe with electrical connections for a flash unit. Different manufacturers use different connector pad layouts and electrical signals. A standard accessory shoe without any electrical connections is sometimes also erroneously called a hot-shoe, so that, for example, Sea & Sea made a flash arm to fit on the accessory shoe and called it a "hot-shoe mini-arm".

Nikon hot shoe

Olympus hot shoe

House reef: Term used, particularly in the Maldives, to refer to the underwater surroundings of a small coral island diving resort.  Also, through adoption as diver's idiom, used to refer to the reef local to any diving resort.  It is almost certainly a homophone of the German 'Zuhause riff' (home reef).

Housing (underwater): That part of an underwater camera or other equipment in which the waterproof seals are located. A casing or box with waterproof seals, designed to contain a camera or other equipment in such a way that it can be used underwater.

Hydrophone: Underwater microphone. Because air is an easily compressible medium, but water is not; sound waves (pressure waves) travelling in water involve small displacements of the water molecules, whereas sound waves travelling in air involve relatively large displacements of the air molecules. Consequently, if you put an ordinary microphone in a box, the sound waves in the water won't move the walls of the box very much, won't compress the air in the box very much, and so won't move the diaphragm of the microphone very much. The solution is to have a specially designed microphone in which the diaphragm is actually in contact with the water and is sensitive to very small displacements.

Bulkhead mounting hydrophone.

Hyperfocal distance (of a lens): The distance setting that gives the greatest possible range of distances at which objects appear to be in focus. Good lenses have means of indicating the depth of field for a given aperture setting. The hyperfocal setting for a given aperture is obtained by placing the most distant limit of the depth of field range on ∞. The corresponding 'hyperfocal distance' is the actual distance setting when the hyperfocal criterion has been met. Note that wide-angle lenses have an extremely large depth of field, and therefore, unless being used for extreme close-up work, are best set to the hyperfocal distance and left alone (refocusing is only necessary if the aperture is changed).

Hz, Hertz, c/s: Hertz is the unit of frequency, and is synonymous with 'cycles per second' (c/s). The unit was established in commemoration of the work of Heinrich Hertz, whose scientific experiments in the late 19th century were the basis of radio telecommunications).

---

ICS: Ikelite connector system.
ICS2: A 2-pin wet connector used with early Ikelite manual flash units.
ICS4: A 4-pin O-ring sealed connector used with early Ikelite automatic flash units.
ICS5: A 5-pin O-ring sealed connector used with current Ikelite TTL flash units and video lights.

ICS2 Connector (3rd pin is for reversal prevention only)

 

ICS5 connector

Nikonos V (or 5) Connector

UWCC: Underwater Colour Correction (filter). An optical filter designed to introduce a colour cast having the complementary colour of the cast introduced by the water. The colour cast introduced by so-called 'blue' water is somewhere between blue and cyan, hence the complementary colour is somwhere between yellow and red, i.e., orange. The complementary colour for green water is magenta.

UR/Pro UWCC filter disks

YS: Yellow Submarine (we all live in one). Beatles inspired name for Sea & Sea's first ever product (a flash unit). Sea & Sea strobes are still called "YS", even though most of them aren't yellow any more.

YS-mount: The attachment between Sea & Sea, Epoque, Sony and Olympus flash-units and lights and the lighting support bracket (arm). Fixing-bolt thread is M8. Distance between clamping cheeks is 12mm.

YUV: Y=Luminance, U=Normalised B-Y, V=Normalised R-Y. The colour space representation used in PAL video and television transmission.

Z: North Americans pronounce the letter "Zee", and certain abbreviations such as E-Z P-Z (easy-peasy) only make sense when you use the American pronunciation.

Z-mount: Mounting attachment on Inon flash units. M6 socket with facility for an anti-rotation pin.


 DWK


© D W Knight, 2001 - 2011, 2018

---

Optics & Photography