An enlarger is a specialized transparency projector used to produce photographic prints from film or glass negatives, or from transparencies.
All enlargers consist of a light source, normally an incandescent light bulb shining though a condenser or translucent screen to provide even illumination, a holder for the negative or transparency, and a specialized lens for projection, though some, such as the Rapid Rectilinear or Aplanatcould be used in both camera and enlarger. Enlarger lenses, like the dialyte construction, are generally symmetrical in design or nearly so, optimised for sharp focus at 2x to 10x magnification. The light passes through a film holder, which holds the exposed and developed photographic negative or transparency.
Prints made with an enlarger are called enlargements. Typically, enlargers are used in a darkroom, an enclosed space from which extraneous light may be excluded; some commercial enlargers have an integral dark box so that they can be used in a light-filled room.
Josef Maria Eder, in his History of Photography attributes the invention of photographic enlargement to Humphry Davy who realised the idea of using a solar microscope to project images onto sensitised paper. In June 1802 Davy published in the first issue of the Journals of the Royal Institution of Great Britain his An Account of a Method of Copying Paintings upon Glass, and of Making Profiles, by the Agency of Light upon Nitrate of Silver. Invented by T. Wedgwood, Esq. With Observations by H. Davy in which he described their experiments with the photosensitivity of silver nitrate. Eder credits the first mention of enlargements after the announcement of the daguerreotype (unique images on metal plates) to John William Draper who in 1840, during, wrote prophetically in the American Repository of Arts; "Exposures are made with a very small camera on very small plates. These are subsequently enlarged to the required size in a larger camera on a rigid stand. This method will probably contribute very much to the practice of the art"  In March 1843 Americans Wolcott and Johnson patented a means of copying and enlarging daguerreotypes.
In June 1843 Henry Fox Talbot in his patent for an enlarger for his calotype process which produced a paper negative, mentions that using lenses it is possible to produce a large negative from a smaller, so having made such enlargements has a priority claim to be the inventor of a system for making an enlarged print from a negative, though it did not go into production and was not practical given the lengthy exposures required. In 1848 Talbot recommended to fellow photographer Thomas Malon the enlarging camera made by Thomas Ross of lens manufacturers Ross, Andrew & Thomas.
The advent of collodion negatives on glass in the 1850s made enlargement more practical. Achille Quinet's invention of 1852 used artificial light, but was inefficient, requiring very extended exposures. David Acheson Woodward's 1857 'solar enlarging camera' addressed that problem by tapping the brightest light-source then available – the Sun – with mirrors and a condenser.
Main article: Solar camera
Solar cameras, introduced in the late 1850s, and ancestors of the darkroom enlarger, were necessary because of the low light sensitivity of albumen and calotype materials used. A larger version of the 18th century solar microscope, they were first freestanding, a design analogous to picture-taking cameras but with the relative position of negative and lens reversed so that sunlight shone through the glass plate to be projected onto photo-sensitive paper inside the instrument. Mounted on a stand, they could be rotated to continuously face the sun.
Woodward's 1857 solar enlarging camera was a large instrument operated out-of-doors that could produce life size prints from quarter plate and half plate negatives with an exposure of about forty-five minutes, improved in the 1860s and 70s with a clockwork heliostat to rotate the mirror in synchronisation with the sun's passage to concentrate its light on the condenser lens, while Désiré van Monckhoven's 1863 patent was for a modification of Woodward's design that had an appearance more like a modern horizontal enlarger.
The instrument was used by significant photographers Disderi and Nadar. By 1890, artificial light sources – gas, petroleum, limelight, magnesium, and electric light bulb – were commonly used in enlargers, but even at the turn of the century simple folding daylight enlargers still found a use among amateurs to easily produce prints of a fixed size. Some cameras were made convertible to use in a similar manner.
In the 1870s hand-coloured enlargements from carte-de-visite prints and daguerreotypes as well as existing negatives were offered for sale in London for two shillings for an A4 print, and three pounds for a life size bust, and R. L. Elliot & Co., of King’s Road could print up to 25" x 20" from quarter plate negatives in 1878 using limelight, as suggested by John Benjamin Dancer.
Fast bromide and chloride printing papers largely superseded albumen emulsions in the 1880s.
A condenser enlarger consists of a light source, a condensing lens, a holder for the negative and a projecting lens. The condenser provides even illumination to the negative beneath it. Condenser enlargers produce higher contrast than diffusers because light is scattered from its path by the negative's image silver; this is called the Callier effect. The condenser's increased contrast emphasises any negative defects, such as dirt and scratches, and image grain.
A point source enlarger is a variation of the condenser enlarger designed to cut light diffusion above the negative. Contrast is enhanced and grain in the resultant print is sharper than with a conventional enlarger, and the transition from light to dark at the edge of the shadow areas is dramatic.
An unfrosted clear lamp with a tiny filament is used without diffusers. As the illuminant is narrow the lamp must be precisely positioned both vertically and horizontally, because the condensers project only that single small filament rather than light that fills the whole housing. However the lens must be kept at full aperture to avoid projecting an image of the light source restricted to the centre of the baseboard, which will cause vignetting and falloff in the print. Exposure is controlled through duration or using a variable transformer.
A diffuser enlarger's light source is diffused by translucent glass or plastic, providing even illumination for the film. Diffuser enlargers produce an image of the same contrast as a contact print from the negative.
Cold light or cold cathode enlargers employ diffusion enlarger heads with a coiled Fluorescent lamp tube rather than a conventional light bulb. Their light is blue-rich, in an area of the spectrum to which silver gelatin paper is sensitive, and therefore exposure is shorter comparative to that with other light sources, ideal for making large mural prints which require extended exposure, and heat is reduced which is beneficial in avoiding buckling or 'popping' of negatives, and also are Newton's rings' where a glass negative carrier is used. They produce a softer (less contrasty) print.
Color enlargers typically contain an adjustable filter mechanism - the color head - between the light source and the negative, enabling the user to adjust the amount of cyan, magenta and yellow light reaching the negative to control color balance. Other models have a drawer where cut filters can be inserted into the light path, synthesize colour by additive mixing of light from colored lamps with adjustable intensity or duty cycle, or expose the receiving medium sequentially using red, green and blue light. Such enlargers can also be used with variable-contrast monochrome papers.
Digital enlargers project an image from an LCD screen at the film plane, to produce a photographic enlargement from a digital file.
Most modern enlargers are vertically mounted with the lens pointing downward. Moving the head on the column up or down changes the size of the image projected onto the enlarger's base, or a work table if the unit is mounted to the wall.
A horizontal enlarger consists of a trestle, with the head mounted on crossbars between two or more posts for extra stability. A horizontal enlarger structure is used when high quality, large format enlargements are required such as when photographs are taken from aircraft for mapping and taxation purposes.
The parts of the enlarger include baseboard, enlarger head, elevation knob, filter holder, negative carrier, glass plate, focus knob, girder scale, timer, bellows, and housing lift.
The image from the negative or transparency is projected through a lens, typically fitted with an adjustable aperture, onto a flat surface bearing the sensitized photographic paper. By adjusting the ratio of distance from film to lens to the distance from lens to paper, various degrees of enlargement may be obtained, with the physical enlargement ratio limited only by the structure of the enlarger and the size of the paper. As the image size is changed it is also necessary to change the focus of the lens. Some enlargers, such as Leica's "Autofocus" enlargers, perform this automatically.
An easel is used to hold the paper perfectly flat. Some easels are designed with adjustable overlapping flat steel "blades" to crop the image on the paper to the desired size while keeping an unexposed white border about the image. Paper is sometimes placed directly on the table or enlarger base, and held down flat with metal strips.
The enlargement is made by first focusing the image with the lamp on, the lens at maximum aperture and the easel empty, usually with the aid of a focus finder. The lamp is turned off, or in some cases, shuttered by a light-tight mechanism.
The image is focused by changing the distance between the lens and the film, achieved by adjusting the length of a light-tight bellows with a geared rack and pinion mechanism.
The lens is set to its working aperture. Enlarging lenses have an optimum range of apertures which yield a sharp image from corner to corner, which is 3 f/ stops smaller than the maximum aperture of the lens. For an enlarging lens with a maximum aperture of f/2.8, the optimal aperture would be f/8. The lens is normally set to this aperture and any color filtration dialed in, if making a color print or one on variable-contrast black-and-white paper.
A series of test strips, and/or a stepped series of exposures made on the one sheet of paper, are undertaken to determine exposure, and then contrast or colour filtration. Alternatively a custom incident light meter (densitometer or 'colour-' or 'darkroom analyser') may be used in setting exposure once the degree of enlargement has been decided, and in colour printing may also be used to establish a base neutral filtration from the negative rebate.
The enlarger's lamp or shutter mechanism is controlled either by an electronic timer, or by the operator - who marks time with a clock, metronome or simply by counting seconds - shuttering or turning off the lamp when the exposure is complete. The exposed paper can be processed immediately or placed in a light-tight container for later processing.
Digitally controlled commercial enlargers typically adjust exposure in steps known as printer points; twelve printer points makes a factor of two change in exposure.
If a greater or lesser enlargement from the same negative is then required, a calculator – analogue, digital or in app format – may be used to quickly extrapolate from the original settings the exposure without the need for labour-intensive re-testing.
After exposure, photographic paper is developed, fixed, washed and dried using the gelatin silver or C-print process.
Automated photo print machines have the same basic elements and integrate each of the steps outlined above in a single complex machine under operator and computer control.
Rather than project directly from the film negative to the print paper, a digital image may first be captured from the negative. This allows the operator or computer to quickly determine adjustments to brightness, contrast, clipping, and other characteristics. The image is then rendered by passing light through the negative and a built-in computer-controlled enlarger optically projects this image to the paper for final exposure.
As a byproduct of the process a compact disc recording may be made of the digital images, although a subsequent print made from these may be quite inferior to an image made from the negative due to digitization noise and lack of dynamic range which are characteristics of the digitizing process.
For better images, the negatives may be reprinted using the same automated machine under operator selection of the print to be made.
The practical amount of enlargement (irrespective of the enlarger structure) will depend upon the grain size of the negative, the sharpness (accuracy) of both the camera and projector lenses, blur in the image due to subject motion and camera shake during the exposure.
The intended viewing distance for the final product is a consideration. For example, an enlargement from a certain negative as a 12 x 18 cm (approx. 5 by 7 inch) print may be sufficient for a scrapbook viewed at 50 cm (20 inches), but insufficiently detailed for an A4 print hung on a hallway wall to be viewed at the same distance, though usable at a larger 120 x 180 cm (ten times larger) on a billboard to be viewed no closer than 5 metres.
Since the inverse square law applies to illumination intensity at increasing distance, enlargement beyond a certain size becomes impractical, requiring extended exposure times and dependent on the extent to which dampening of enlarger supports may eliminate vibration causing blur in the resulting print.
The claim for the biggest analogue enlargement ever made from a 35mm photograph is that for Ernst Haas' wildlife picture taken in Kenya in 1970. It required a 5-hour exposure using the Kodak Colorama process, for a giant transparency. The 508-times enlargement consisted of 20 vertical panels of 3 feet width and 18 feet height (91.4 x 548.6 cm) for a total size of 18 x 60 feet (5.48 m x 18.28 m). Displayed at Grand Central Station in New York in 1977, it was Illuminated from behind with 61,000 watts of light; it was the first time a 35 mm picture had been used for an ongoing series of Kodak advertising displays there c.1950–1990. The transparency print was destroyed after exhibition.
As the photographic market shifts away from film-based towards electronic imaging technology, many manufacturers no longer make enlargers for the professional photographer. Durst, which made high quality enlargers, stopped producing them in 2005, but still supports already sold models. Manufacturers old and new include:
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