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Accelerator:   chemical compound that speeds the action of developing agents.

Acutance:   a mathematical measure of sharpness. See our book Quality in Photography.

Alternative processes:   Obsolete or semi-obsolete processes that are still used by some photographers because they like the effects they give.

Anti-curl layer:   the gelatine emulsion on a 120 roll film expands and contracts with humidity, causing the film to curl. Another gelatine layer, the anti-curl layer, is normally coated on the other side of the film to counteract this.

Antifoggant:   see restrainer.

Anti-halation layer:   see Halation

Aperture:   the size of the hole in the lens that lets the light through. Normally expressed as a relative aperture, obtained by dividing the focal length of the lens by the effective size of the hole. This has the advantage that all lenses of the same relative aperture, regardless of focal length, pass the same amount of light. Thus a meter can recommend f/2 whether you are using a 24mm lens, a 50mm lens or a 100mm lens. The progression of f/stops, which seems odd at first sight, is because relative apertures progress in steps equal to the square root of 2, namely 1.414. This in turn is because aperture is a linear measure, whereas the amount of light passed by an aperture of given diameter is proportionate to the square of the radius of the aperture. See also iris diaphragm, f/stop, stop, T/stop.

Argyrotype:   A proprietary variation on a process using iron salts to deposit an image of metallic silver in the image. It is a printing-out process: no development is needed. The sensitizing solution is used to coat heavy watercolour paper, and the results are a rich brown with a limited tonal range.


Bag bellows:   Very much what its name suggests, a non-pleated bellows to allow the use of extreme wide-angle lenses on large format cameras. Where a conventional bellows would bind and jam, a bag bellows allows focusing and even (if the lens has a large enough circle of coverage) the use of movements.

Banquet camera:  A large format camera originally designed for photographing large groups of people, such as banquet parties, so that everyone was recognizable: aren't some explanations simple?. Because there is a limit to how deep you can stack people, the formats are normally wide horizontally but short vertically: things like 12x20 inches. Some people still like these formats for their creative possibilities.

Barn doors:   adjustable flaps on a mounting ring that fits in front of a light, used to make the light more directional by shading off the spill. There are usually four of them.

Baryt, Baryta:   Barium sulphate, the opaque white layer between the paper and the emulsion on most fibre-base papers.

Bellows extension:   The maximum extent and minimum compression of bellows on large format cameras are both important. The maximum extension determines how close the camera can focus, and indeed whether it can focus at all: a 300mm lens requires a 300mm bellows just to focus to infinity, and at least 350mm for any useful focusing range. The minimum extension determines the widest angle lens that can be used: a 75mm lens cannot be focused to infinity on a bellows with a minimum extension of more than 75mm.

Bleach-fix:   chemical bath used in colour film processing (both E6 and C41). Dissolves out both remaining silver halides and the silver image itself, leaving only the dye image,

Blix:   abbreviation for Bleach-Fix, q.v.

Block form film holders:   The standard modern design of cut-film holder, taking two sheets of cut film which are loaded in from the bottom of the holder. Most are of the Graphic pattern.

Blocked (of shadows):   registering as a featureless black in the final image.

Blown (of highlights):   registering as a featureless white in the final image.

Book-form holders:   The traditional form of holder for photographic plates or (latterly) films, so called because they open up like a book. They antedate standardization and will fit only the specific cameras for which they were designed.

Bracket:   to make one or more additional exposures in addition to your best guess at the optimum exposure. The subject of a module.

Broad-area reflected light meter:   exactly what its name suggests: a meter that measures the light reflected back from the greater part or all of the subject in front of the camera. Includes all through-lens meters except spot meters, though in-camera spot meters normally measure a far wider angle than hand-held spot meters.

Brute:   A big movie-type hot light.

Burning:   localized darkening of an image by (i) increased exposure in negative-positive printing, (ii) reduced exposure in positive-positive printing, or (iii) digital image manipulation. The opposite of dodging. The subject of a module, Dodging and Burning.


C41:   Kodak proprietary process for colour negative film processing which has become a de facto standard.

Callier effect:   When a beam of light is shone through a negative in an enlarger, some of the light is absorbed (which is obvious enough) but some is also scattered. The degree of scattering depends on the image structure and also on the degree of collimation of the light. Dye images, as found in Ilford XP2 and other chromogenic films, do not scatter the light at all and therefore have a Callier coefficient (Q) of 1 under any light. Highly collimated light (parallel beams of light, as in a point source condenser enlarger) are scattered most of all (Q is highest); less collimated light (as in a condenser-diffuser enlarger) is scattered less, so Q is lower; and with a completely diffuse light, Q drops to 1 again. The relevance of this is that different enlargers give different degrees of contrast, which must be taken into account when developing the negatives (higher contrast for diffuser enlargers, lower contrast for condenser enlargers) or selecting paper grades.

Camera movements:   see Movements.

Chromogenic (black and white) film:   film that can be processed in standard Kodak C41 chemistry.

C.I.:   an abbreviation of Contrast index.

Circle of coverage:.   Any lens throws a circular image which is to some extent dependent on the focal length, but a wide-angle lens (as its name suggests) covers a bigger circle than might be expected from its focal length. Thus a 90mm lens might cover a circle of anything from about 100mm to 200mm or more. Thiis is important when using movements on a large-format camera.

Clip test:   literally clipping a few frames from the beginning of a film to test for exposure accuracy. See the module on Bracketing.

Contrast, contrasty:   This has so many meanings that it has (or will have) a page to itself. Contrast can refer to the subject brightess range; the lighting; the lens or lens/camera combination used to take the picture; the film; the developer; the negative; or the final image.

Contrast index:   A measure of the contrast of a negative; a refinement of gamma.

Crop:   To print or otherwise reproduce only part of the captured image. Cropping should be avoided wherever possible in the interests of technical quality but it should equally be used wherever necessary to improve the aesthetic side of a picture.

Cut-film holder:   see Film holder.


DDS:   abbreviation for double dark-slide, another name for a (usually standardized) film holder for cut film.

Depth of field:   The zone either side of the focused point that is acceptably sharp. Depends on the aperture (the smaller the aperture, the greater the depth of field); image size (the larger the format, the smaller the depth of field); degree of enlargement (the greater the degree of enlargement, the greater the depth of field); the viewing distance (the greater the viewing distance, the greater the depth of field); and the viewer's personal definition of 'acceptable'. Not to be confused with depth of focus.

Depth of focus:   The zone either side of the film or other sensitized surface that allows acceptable sharpness in the negative. The flatness of the film is extremely important: with a flat-field lens, the flatter the film, the greater the depth of focus. Like depth of field, with which it should not be confused, it is affected by aperture, image size, degree of enlargement and personal preference. Depth of field and depth of focus are affected by focal length: other things being equal, at a given distance shorter lenses have more depth of field and less depth of focus while longer lenses have less depth of field but more depth of focus.

Density:   a measure of the transmitted by a negative or reflected by a print. It is expressed as a logarithm. See the module on negative density.

Developer:   A silver halide photographic emulsion will slowly darken ('print out') under the influence of light alone, but long before this happens it has received a latent image which can be revealed by the use of a developer which differentially reduces exposed and unexposed silver halides to metallic silver. The 'amplification' effect of using a developer, as compared with printing out, is often estimated a 1,000,000x. In other words, where a film might require 1/1000 second to capture a latent image, it would require 1000 seconds -- over a quarter of an hour -- to register an image by printing out.

Diaphragm:  An artificial restriction of the light-path inside a lens to reduce the amount of light going through. Usually an iris diaphragm nowadays, but used to be a simple piece of metal with a hole in it: the original 'stop'. Simple, interchangeable stops are often known as Waterhouse stops after John Waterhouse of Halifax, who introduced this system of aperture control in 1858.

Diapositives:   Old name for slides. Often abbreviated to dia. Images viewed by reflected light are called katapositives which is an even more unusual word.

DIN speeds:   a precursor of the ISO speed system (module).

D/log E curve:   A curve that plots density (D) against the logarithm of exposure (log E). See the module on Density, the D/log E curve and gamma.

D max:   the maximum density of a negative or print. See the module on Density, the D/log E curve and gamma.

D min:   the minimum density of a negative or print.

Dodging:   localized reduction of density via (i) decreased exposure in negative-positive printing, (ii) increased exposure in positive-positive printing, (iii) digital manipulation. The opposite of burning.

Double-dark [slide]:   Another name for a (usually standardized) film holder for cut film.

Double extension:   With large format cameras, it is quite common to express the bellows extension as a multiple of the focal length of the standard lens, which in turn is taken as equal to the negative diagonal. Thus the 'standard' lens for 4x5 inch is 150mm (the negative diagonal) and 'double extension' is 300mm, allowing half life size on the film. See also triple extension.

Double lighting:   the lighting which arises when part of a subject is brightly lit and another part is in shadow. See the module on subject brightness.

Dye sensitization:   An 'ordinary' photographic emulsion is sensitive only to blue, violet and ultra-violet light. By adding sensitizing dyes it can be sensitized to green light (orthochromatic, red light (panchromatic) and even infra-red. This is discussed at slightly greater length in the module on choosing film.


E6:   a Kodak proprietary process for slide processing which has become a de facto standard.

Element   (lens): normally, a single glass in a compound lens. Sometimes used mistakenly as a synonym for a lens group, which consists of two or more elements cemented together: thus a Tessar is a four-glass, three-group lens (two of the glasses are cemented together to form a doublet) while a Dagor is a six-glass, two-group lens (two cemented triplets).

Emulsion:   Strictly a misnomer: an emulsion is a suspension of a liquid in a liquid, like mayonnaise. It is however firmly established as the standard name for the suspension of silver halide in gelatine that forms the basis of the vast majority of non-digital photography. Some emulsions add synthetic latexes to the gelatine but to the best of our knowledge there are no gelatine-free conventional emulsions for conventional photography, though there may be in graphic arts.

Extension:   see Bellows extension.


FB:   Abbreviation for Fibre base (paper).

fb+f:   abbreviation for film base plus fog.

Factorial development:   A technique for determining development times. The film (or paper) is developed for a multiple of the time it takes for the first traces of an image to appear, otherwise known as the induction time. See also Watkins factor.

Fibre base    (FB): paper base that is not coated with a waterproof plastic layer, and therefore absorbs processing chemicals more freely, necessitating longer wash times. Probably longer lived than resin coated papers and many photographers prefer the look of FB too.

Field camera:    Originally any camera (though usually large format) that was designed to be taken out of the studio and used 'in the field'. Now generally used of folding large-format cameras, as distinct from monorails.

Film base plus fog:   even a clear film base, the substrate on which film emulsions are coated, absorbs a small amount of light and therefore has a density of its own. Some bases are tinted grey to reduce halation and light piping: this is normal with 35mm. The combination of the film base sensity and the fog density after exposure and processing is understandably known as film base plus fog density or simply fb+f.

Film holders (for cut film):   Since the 1930s these have largely been standardized on the Graphic or Graflok pattern, and there are several standard sizes. The first size, the smallest, takes 6.5 x 9cm, 2.25 x 3.25 or 2.5 x 3.5 inch film: external dimensions are the same but obviously internal dimensions vary and the ground-glass must be marked accordingly. The next size up is quarter-plate, also standardized but the film is vanishingly rare today. The third standard size is holders for 9x12cm and 4x5 inch. Again these share the same external dimensions but again they vary internally. The fourth size, again standard on the ouside but varying internally, is 13x18cm, 5x7 inch and half-plate. A fifth size, whole-plate, is standardized but film is very hard to find now. The sixth size is again made to accept two film sizes, 18x24cm and 8x10 inch, and the biggest standardized size is 11x14 inch. But see also Graflex and Book-form holders.

Fish fryer (FF):   A medium-to-small soft box. Originally a trade name from Strobex/Strobe Equipment. A bigger version was the SFF or Super Fish Fryer.

Fixer:   a chemical bath used after the developer to dissolve out the silver halide that has not been developed. An un-fixed or inadequately fixed film looks milky when it comes out of the developer (or fixer) and will gradually darken and stain. The classic fixer was hypo (sodium thiosulphate) but most modern fixers are based on the much faster-acting ammonium thiosulphate. There is more about fixing in the module on black and white film development.

Flag or French flag:   Opaque sheet of material to block or 'flag off' the light falling on a particular part of the scene.

Flood:   A broad area light, which literally 'floods' the scene with illumination. In many ways the opposite of a spotlight or spot.

Focal length:  The distance at which a lens focuses on infinity. Thus a 150mm lens (unless it is a telephoto or retrofocus design needs to be 150mm from the film plane. Longer focal lengths give a bigger image of a smaller subject area on the film; shorter focal lengths ('wide angles') give a smaller image of a bigger area.

Fog:   Even unexposed grains of silver halide will develop to some extent, giving rise to a uniform density throughout the film even where it has not been exposed. Developers that give rise to a minimum of fog are said to be 'clean working'. Fog can be reduced or eliminated by adding antifoggants or restrainers to film and paper developers. The classic inorganic restrainer is potassium bromide; the classic organic restrainer (used in much smaller concentrations) is benzotriazole.

f/stop:   The standardized way of measuring the light transmission of a lens. Crudely, it is obtained by dividing the diameter of the diaphragm or stop by the focal length of the lens. Thus a 50mm lens with a maximum aperture of 25mm would be f/2; reducing the stop to 12.5mm would bring it down to f/4. See also Stop


Gamma:   a measure of negative contrast. The higher the contrast, the higher the gamma. Typical negative contrasts are between gamma = 0.55 and gamma = 0.70, though gammas of 1.0 and above are sometimes encountered for printing out papers (POP) and other 'alternative' processes. See the module on Density, the D/log E curve and gamma.

G-bar:   a refinement of gamma.

Gelatine:   an organic extract of bones, hoofs, horns and hides used as a medium for the suspension of silver halides in conventional photography. It has the extremely useful property of greatly increasing the sensitivity of the silver halides, probably as a result of sulphur impurities. Militant vegetarians sometimes have a problem with this but only the stupid ones believe that anyone makes non-gelatine film emulsions.

Glass:   apart from the obvious meanings, another name for a lens element.

Gobo:   a piece of card, wood or metal with holes in that is used to interrupt light and create dappled or shaded effects. Also used of perforated metal screens used in projectors and focusing spotlights to create definite shaped lighting effects such as windows. Sometimes used as a synonym for flag. See our book Lighting for Photographers (same book as Learning to Light).

Grades:   see Paper Grades

Graflex:   Reflex cameras by Graflex, which took film holders that were substantially identical to Graphic/Graflok holders except in one critical respect: instead of a locating ridge on the film holder fitting into a corresponding groove in the camera, there was a locating ridge on the camera that fitted into a corresponding groove in the film holder. An unnecessary, infuriating and unexpected problem!

Graflok:   An improved version of the Graphic back, introduced in 1949. The ground-glass can be removed and thick roll-film holders, Polaroid backs, etc. can be secured via two sliding bars.

Graphic:   The standardized fitting for film holders (including roll-film holders) established by Graflex for their Speed Graphic press-type cameras. It seems to have originated as early as 1912, and eventually became an ASA (later ANSI) standard, apparently in 1951. It sets external dimensions (width and height, but not thickness), the position of the locating ridge (see Graflex) and the all important register or distance from the front face of the film holder to the position of the film, 3/16 inches = 0.1875 inches = 4.7625mm. In practice register seems to vary between 4.6 and 5 mm, depending on the manufacturers' allowances for film thickness and film curl, and general attitude towards tolerances.

Grain:   the gravelly effect you can see when you enlarge a photographic negative. Some people distinguish between true grain, made up of metallic silver, and apparent grain, made of dye clouds, but there seems little advantage in doing so. Strictly, what you are seeing is clumps of silver grains or dye clouds, not the grains or dye molecules themselves, but again this is a pointlessly pedantic description. The terms 'fine grain' and 'coarse grained' are self-descriptive but they can be heavily influenced by the marketing department.

Group (lens design):   two or more glasses cemented together. See also element.


Halation:   When light strikes a photographic film, some passes straight through the emulsion and penetrates to the substrate. If this is reflected back through the emulsion, it will create a halo effect around light sources. An anti-halation layer absorbs this light and prevents it wreaking quite so much havoc. Anti-halation layers may be between the emulsion and the substrate or behind the substrate. During processing the dyes in the anti-halation layer are partly washed out, and the remainder is rendered colourless by the processing chemicals. Otherwise they would block the light needed for printing. See also irradiation.

Half frame:   18x24mm format on 35mm, so called because it is half the size of a standard or 'full-frame' 24x36mm frame. Equally accurately called single frame because it is the size of a single movie frame: die-hard half-framers call 24x36mm 'double-frame'. Killed by the difficulty of finding commercial printing and the rise of ultra-small full-frame cameras such as the Rollei 35 and Minox 35.

Hand-and-stand camera:   Obsolete term used for cameras, usually quarter-plate or larger, that could be used hand-held or on a tripod (stand). Another nice easy one!

Halide:   see Silver Halide

Hard:   Of paper, contrasty; of a high paper grade; with a low ISO(R). Of water, rich in dissolved solids.

Highlights:   by convention, the lightest areas in a print and therefore the darkest areas in a negative.

Honeycomb:   A sort of deep grid with a pattern like a small egg-box without a bottom; screen with honeycomb-shaped holes, for making light more directional.

Hot light:   Tungsten incandescent lamp.

Hydrolysis:   The tendency of chemicals to break down in solution. Some are completely stable in solution; others break down more or less quickly. It is mostly of concern with developing agents. Unlike the products of oxidation, which are often brown, the products of hydrolysis may well be colourless. This explains how a clear developer may be completely and inexplicably inactive.

Hypo:   abbreviation of 'Hyposulphite of Soda', an old (and incorrect) name for Sodium Thiosulpha.


Incident light meter:   a meter that measures the light falling on the subject instead of the light reflected from it.

Induction period:   When you start to develop a photographic paper or film, it takes a while before any image starts to appear. The interval between the immersion and the first appearance of an image is the induction period. See also factorial development and Watkins factor.

Infra-red:   Literally 'beneath red', that is, of a wavelength too long for the human eye to see, but which can nevertheless be picked up by a suitably dye sensitized photographic emulsion. In practice it is used somewhat sloppily: the sensitivity of the human eye does not cut off suddenly, but rather, declines fairly quickly. There is however some sensitivity well beyond 700nm, often used as the cut-off point for infra-red in photography.

Iris diaphragm:    The kind of infinitely variable aperture control or stop seen on all modern lenses. It looks a bit like the iris of an eye as it opens and closes but of course it works completely differently, being made up of anything from five (rarely fewer) to a dozen (rarely more) overlapping plates of very thin metal.

Inky:   short for inky-dinky: A small focusing spotlight.

Irradiation:   As light penetrates a silver halide photographic emulsion, it tends to scatter, so a point is surrounded by a faint mist caused by scattered light. This inevitably reduces sharpness and is one reason why thin emulsions are highly desirable: the thinner the emulsion, the less the irradiation. See also Halation.

ISO(R):   a measure of paper contrast. The lower the ISO(R), the harder the paper (the higher the contrast grade). Grade 5 is typically ISO(R) 35 to 50, grade 0, 160 or more. There is a module on paper grades.

ISO speeds:   Internationally agreed system of standardized film speeds, the subject of a module. There are also ISO paper speeds, which are determined and denoted in a rather different way, but no-one pays them much attention.


Large format:   Generally accepted as film sizes of 9x12cm or 4x5 inch and above. Roll-film and small cut film sizes are normally referred to as medium format.

Latent image:   When light strikes a photographic emulsion, it creates 'development centres' which will, under the influence of a suitable developer, yield a visible image. Until the image is developed it is said to be latent. It is impossible to distinguish visually between a film that holds a latent image and one that does not, and by far the easiest test is to develop it.

Latent image regression:   An exposed but undeveloped film slowly loses detail and contrast, especially in the least exposed areas, as the electrons that have been knocked into higher-energy orbits by photons of light drop back into their older, lower-energy orbits. This is a slow process and you need not worry about it for weeks or months, and indeed we have often developed films that were exposed over a year before without seeing any detectable loss of quality.

Latitude:   the range of exposures across which an acceptable image is obtained. Varies greatly with the film in question and the photographer's definition of 'acceptable'.

Light piping:   any transparent medium in rod or sheet form will 'pipe' light down from the open end: this is how fibre optic transmission works, after all. There are however small losses along the length of the pipe. Now imagine a 35mm film with the end sticking out of the cassette and you can see the relevance of light piping.

Line pairs per millimetre (lp/mm): see Resolving power.

lp/mm:   abbreviation for line pairs per millimetre. See Resolving power.


Medium format:   Normally used to describe roll-film, usually 120 but also 220, 70mm and some obsolete formats. Also used (with less conviction) for small cut film formats, traditionally up to and including even the old quarter-plate size of 3.25 x 4.25 inches, 8.3 x 10.8cm.

Miniature formats:   Today, all but synonymous with 35mm (if the expression is used at all). Until the 1950s it was also used to apply to some roll-film formats, especially the nominal 6x6cm or 'two and a quarter square' format, 12-on-120.

Minox format:   8x11mm on imperforate 9.5mm film, so named after Walter Zapp's original Minox camera in the 1930s. The Minox cassette has become something of a standard for sub-miniature cameras.

Monorail:   As its name suggests, a camera (usually large format though some are medium format or even designed for use with digital backs), mounted on a more or less substantial rail. There are normally two standards, the front or lens standard and the rear or film standard, linked by a bellows, though for very long extensions, a double bellows with an additional middle standard (bellows standard) may be employed. Monorails were the staple of commercial studio photography (except portraiture) for most of the second half of the 20th century. Their great advantage is extensive movements front and rear but compared with a field camera most are bulky and unwieldy. Exceptions to this (which we use) include Linhof's Technikardan and the Toho line of cameras.

Movements:   Most large-format cameras, and some medium-format, allow the lens to be moved parallel to the film plane (decentring movements: rise, fall, cross) and also for the lens axis to be moved away from a right angle with the film plane (out-of-parallel movements: swing, tilt). Rise, fall and cross are used for perspective control in much the same way as a PC lens on a 35mm camera, e.g. to avoid having to tilt the camera up, which results in buildings looking as if they are falling over backwards. Swing and tilt are used to vary the plane of focus. As long as the lens axis is at right angles to the film plane, the plane of focus is on the film plane and corresponds to an image plane parallel to the film. Swing and tilt can be used to hold a receding plane in focus (see also Scheimpflug rule), or to restrict focus to (for example) the eyes in a portrait.

Multigrade:   see Variable contrast.


Non-substantive (colour) film:   film in which the dyes are added during processing, instead of being formed from dye precursors already present in the emulsion: cf substantive. The only modern non-substantive film is Kodachrome.

North light:   another name for a soft box. Named by analogy with a north-facing window that never receives direct sun (in the northern hemisphere).


Ordinary:   an old-fashioned term for films that are sensitive to blue, violet and ultra-violet only, because the emulsion has not been dye sensitized.

Orthochromatic:   a film or emulsion that has been sensitized to green (but not red) by the use of dye sensitization. Literally 'right-coloured'.

Oxidation:   The breakdown of a chemical by oxygen, whether absorbed from the air or dissolved in the water used to make up chemical solutions. Mostly of concern with developers, which usually turn a familiar nasty brown colour.


Panchromatic:   a film or emulsion that has been sensitized to green and red by the use of dye sensitization. Literally 'all-coloured'.

Paper grades:   using different paper grades enables you to get a good or excellent print from a range of negatives of widely different contrast. Contrasty negatives require a soft paper, typically grades 1 and 0; negatives that lack contrast require a hard paper, typically grades 4 and 5. 'Average' negatives should print on grades 2 or 3. There will be a module on paper grades; see also variable contrast paper, below.

Pixel density: Not all pixels are created equal. Bigger pixels gather more light, so the signal has to be amplified less, which means that the image is 'cleaner'. Thus, you will normally get better pictures, especially at high ISO speeds, from (say) 18 megapixels on a 24x36mm sensor (lower pixel density) than from 18 megapixels on a 16x24mm sensor (higher pixel density).

Plane of focus:   The plane in which the image lies. In most cameras, parallel with (and ideally coincident with) the film plane, and forming an image of a subject plane parallel with the image plane. By the use of out-of-parallel movements the parallelism of the image plane and the subject plane can be destroyed, i.e. it is possible to focus at the image plane a subject plane which slopes towards or away from the image plane.

POP:   The usual abbreviation for printing-out paper.

Pre-flash:   A means of getting better highlight detail by giving the paper a weak, uniform exposure before the actual printing exposure. The subject of a module.

Printing-out paper:   Literally, a paper that prints out (darkens) under the influence of light, without development. The negative is sandwiched against the POP, usually with a very thin layer of Mylar or other acetate sheet between the two to prevent possible staining of the negative by the paper, and exposed to sunlight or a UV lamp. Processing consists of washing, toning, fixing and washing again.

Push:   to push a film is to over-develop it in order to get extra usable speed. The penalties are higher contrast, bigger grain and reduced shadow detail. There is quite a bit more about pushing in the module on black and white film development.


RC:   see Resin coated

Register:   In large format photography, the distance from the front of the film holder to the actual film plane: see Graphic for more information. In 35mm and medium formats, the distance from the front of the lens flange to the film, e.g. 28.8mm for Leica screw, 1mm less at 27.8mm for Leica bayonet and 46.5mm for Nikon F. It is more or less easy to make an adapter for a lens intended for a camera with a greater flange-to-film register than the host camera (hence Leica's 1mm drop), but a lens made for a camera with a smaller flange-to-film distance than the host camera will not focus to infinity unless there is an additional (and image-degrading) optical system in the adapter.

Relative aperture: see Aperture

Replenished, replenisher, replenishment: A chemical bath is said to be replenished when it is fortified with concentrates to refresh its action. Normally a fixed volume is drawn off every day (or every few dozen films or prints, or whatever) and replaced with a replenisher.

Resin coated:   Well, it sounds better than plastic coated, doesn't it? But that is what RC paper is. The Germans are more honest: they call it PE, for polyethylene coated. Its enormous advantages are stability in processing and the fact that because the base doesn't absorb water and processing chemicals, it washes many times faster than paper (10x to 20x) and dries faster and flatter too: just a few minutes in a flat-bed dryer like the one in our darkroom (free). But it doesn't feel as nice, and although it can be hard to tell RC from FB (fibre base) under glass, FB generally looks nicer nude. We do the vast majority of our printing on RC, because it is for reproduction where RC has no advantages, but we still use FB for prints for exhibition.

Resolving power:   the ability to resolve fine detail such as twigs against a sky. Often and most comprehensibly described in terms of line pairs per millimetre (lp/mm), the number of line pairs (one black, one white) that a film, lens, or film-lens combination can deliver. Although the best lenses can resolve 400 lp/mm or more, few films except microfilms can top 200 lp/mm and the practical limit in the real world is set by the accuracy with which film is located in the camera. Figures in excess of 100 lp/mm cannot be delivered reliably, though 125 lp/mm may occasionally be seen.

Restrainer:   chemical compound that slows the action of developing agents, thereby inhibiting the formation of fog but also reducing the effective emulsion speed.

Retrofocus:   A specific design of wide-angle lens in which a negative (diverging) lens or lens group is placed in front of the main image forming group in order to allow a longer back focus (more room behind the lens). This is essential for wide-angles on single-lens reflexes, where there must be room for the flipping mirror. For example, a 15mm lens for a non-reflex might be 20mm thick and the rear element might only be 7mm from the film plane -- but the mirror needs 35mm in which to move. The answer is a Retrofocus design, so named after the first lens of this type (designed by Angenieux) for 35mm SLRs. Also known as reverse-telephoto, because it is the reverse of a telephoto lens where the negative group is behind the imaging group and serves to shorten the back focus,

Reversal processing:   The film is first developed to a negative; then fogged, by exposure to light or chemically; then redeveloped to create the positive image. The basis of slide processing.

Reverse telephoto:   See Retrofocus.


Scheimpflug rule:   When the subject plane, the plane of the lens panel and the image plane all coincide at a single line, everything in the subject plane will be in focus in the image plane. This necessitates the use of camera movements.

Scrim:   a piece of more or less transparent cloth used to soften, reduce or diffuse a light. See our book Lighting for Photographers (same book as Learning to Light).

SDS:   Abbreviation for single dark-slide.

Seasoned:   A developer is said to be seasoned when it has reached an equilibrium state after repeated replenishment. A seasoned developer is normally slower acting than a fresh one, and gives lower emulsion speed.

Sensitometry:   The science of how films, papers, etc., respond to light.

Shadows:   by convention, the darkest areas in the print (and the thinnest in the negative). They may or may not be actual shadows: black velvet in the best-lit part of the print may still be darker than the worst-lit parts.

Sharpness:   Strictly, a measure of the rapidity of transition from a dark area to a light area at a line: another word for acutance. Normally used subjectively to describe a cocktail of fine grain, resolving power, contrast and acutance.

Short stop:   see Stop Bath

Silver halide:   compound of silver (chemical symbol Ag) and one of the four halides: fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). All dissociate under the influence of light to give metallic silver and the elemental halide, which normally reacts with anything handy.

Single dark-slide (or just 'Single Dark'):   An obsolete form of film holder taking just one sheet of film (or plate). These were semi-standardized in the 9x12cm size from the 1920s to the 1950s are are generally freely interchangeable.

Single frame:   see half frame.

Slide:   transparent film, usually colour, with a positive image for projection or scanning. Nowadays usually produced by reversal processing but can be produced by printing -- as it normally is for movies. Also known as a transparency, diapositive or just dia.

Snoot:   a means of shaping or modifying light, usually in the shape of a truncated cone. The wide end of the cone fitrs over the light; the narrow end is open. The effect is like a (not very efficient) spotlight.

Sodium thiosulphate:   the classic fixer, discovered by Sir John Herschel in 1819.

Soft box:   large diffuse light source giving a shadowless light.

Speed:   for sensitive materials, a measure of the sensitivity of a film or paper to light. A fast film -- one with high speed -- requires little light to register a latent image; a slow one requires more light. See also the module on film speeds. For lenses, a measure of the light-gathering power: a fast lens has a wider aperture than a slow one.

Spill:   light that goes where it is not wanted, off to the sides. See also barn doors.

Spill kill:   a shallow reflector that stops light spilling sideways from a lamp.

Spotlight:   A light that projects a relatively narrow, focused beam of light, literally throwing a pot of light onto the subject. Most spotlights can be focused to vary the diameter and hardness of the spot. In many ways the opposite of a flood.

Spot meter:   a reflected light meter that reads a very narrow angle of view, usually 1 degree. Most useful in determining negative exposure (module) but also usable for determining exposure for slides and digital images (module).

Soup:   an archaic term, still sometimes used by older American photographers or by younger photographers who don't realize how old-fashioned it is, denoting developer. Also used as a verb, 'to soup', meaning to develop.

Standard lens:   A lens with a focal length equal or close to the diagonal of the negative size: thus 43mm for 35mm, 80mm for 6x6cm, 101 or 105mm for 6x9cm, 150mm for 4x5 inch, 210mm for 5x7 inch, 300mm for 8x10 inch. In the days of contact prints, a 'standard lens' was reckoned to give the most natural perspective, the assumption being that the print would be examined from roughly the same distance as the focal length of the taking lens. All this is somewhat shaky, now that the vast majority of prints are enlarged from smaller negatives or digital sensors.

Stop:   Originally, a small sheet of metal with a hole in it that was inserted between the elements of a lens to cut down the amount of light transmitted by the lens, a means of controlling the aperture: this was a Waterhouse stop. Waterhouse stops are now almost entirely superseded by the iris diaphragm. The word is also widely used to describe an exposure step that doubles or halves the light reaching the film: thus one stop is 2x, two stops are 4x, three stops are 8x and so forth. This exposure step may be achieved via the diaphragm or the shutter. Finally, it is sometimes used to describe variations in density. One stop corresponds to a log change of 0.3 because the logarithm of 2 is 0.3.

Stop bath:   an optional chemical bath between developer and fixer. The vast majority of developers will only work rapidly, if at all, in an alkaline environment: a stop bath is a dilute solution of a weak acid such as acetic acid or citric acid which abruptly arrests the action of the developer. Stop baths are further discussed in the module on film development.

Strobe:   Inaccurate name, often used in the United States, for electronic flash. Strictly a stroboscope flashes repeatedly like a disco light.

Sub-miniature:   A pleonasm used to describe still-camera film formats smaller than 35mm (usually 16mm or 9.5mm) and sometimes for smaller-than-usual formats on 35mm. Thus 18x24mm half-frame or single-frame is sometimes called sub-miniature, as is the 14x21mm Tessina format.

Substantive   (colour) film: a film in which the dye precursors are incorporated in the emulsion and are released during processing: cf non-substantive. All modern colour films except Kodachrome are substantive.

Substrate:   the fancy name for the support under an emulsion. With film, it is normally sodium tri-acetate, PET (polyetheylene terepthalate) or PEN (polyethylene napthalate). Most 35mm films are tri-acetate because of problems with light piping down PEN and PET, but 120 may well be PEN/PET and cut film almost always is. With paper it is, well, paper.

Swimming pool:   a very large soft box. Originally a trade name from Strobex/Strobe Equipment.


T50:   the wavelength at which a filter cuts off 50 per cent of the light. Thus a filter with a T50 of 715 nm cuts off 50 percent of light at 715 nm (near infra-red). In practice such a filter would pass almost nothing below 715 nm and the great majority of the light beyond that wavelength.

T-stop:   A refinement of relative aperture (see aperture) which takes account of the actual measured transmission of the lens after allowing for light absorbed by the glass and reflected away. Typically an f/2 lens might be T/2.2, one-third of a stop slower.

Telephoto:   A specific type of optical design in which a negative (diverging) lens or lens group is placed behind the main image-forming group in order to make a physically shorter, lighter and handier lens. Thus a 400mm non-telephoto would need a 400mm focusing mount or bellows, while a 400mm telephoto might need only 250mm. There is inevitably a loss of image quality and contrast in a telephoto lens as compared with a non-teleohoto, but with the right design this can be rendered negligible. Many lenses are mistakenly called telephoto or tele lenses (especially 'short tele' lenses for 35mm, in the 75 to 100mm range) when they are actually just long lenses.

Tonality:   the hardest to quantify of all the attributes of an image, but one of the most easily recognizable. An image with good tonality 'sings' and has an almost sensuous quality: poor tonality may look muddy or flat or harsh but is never pleasing.  Discussed at slightly greater length in the (paid) module on film choice.

Triple extension:   The maximum bellows extension for a large format camera is often expressed as a multiple of the focal length of the standard lens, which is in turn taken as equal to the negative diagonal. Thus the standard focal length for a lens for 8x10 inch is 300mm and triple extension would be 900mm, allowing life size on the film.


Ultra-largeformat[s]:    Another name for Very large format[s]

Ultra-violet (UV):   literally, 'beyond violet', of a shorter wavelength than is perceptible by the human eye. Silver halides are sensitive to a wide range of ultra-violet as well as violet and blue, but the shorter UV wavelengths are absorbed by glass.


Variable contrast paper:   Printing paper where the contrast can be controlled via the colour of the light used for printing. Will be discussed at greater length in a planned module on paper grades. See also paper grades, above.

VC paper:   see Variable contrast paper

Very large format[s]:   Formats larger than 8x10 inch, including 10x12 inch, 11x14 inch, 12x16 inch, 16x20 inch and 20x24 inch as well as banquet formats such as 12x20 inch.

View camera:   Originally a camera used for taking 'views' or landscapes. Now generally used of any large format camera, usually one equipped with movements.

VLF:   see Very large format[s]


Waterhouse stop:  see Diaphragm and Stop

Watkins factor:   The factor used in factorial development. Thus a Watkins factor of 5 implies developing the film for 5 times as long as it takes for the image first to appear. It is determined empirically for any given film/developer combination.


Zone System:   a system of development and exposure devised by Ansel Adams; a simplified version of basic sensitometry. The subject of a module which explains why we don't use it.

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© 2012 Roger W. Hicks