Enlarger
An enlarger is a device used in photography . It enables enlarged paper images to be produced from negatives or slides through optical projection .
In the 19th and early 20th centuries, enlargements were very difficult or impossible to produce due to the lack of lighting equipment. There was also a lack of bright, high- contrast lenses with a sufficiently large field of view . For this reason, large-format cameras with negative formats of up to 24 × 30 cm and larger were used, where a large print had to be made by contact. Only powerful, simple lighting made enlargers possible, which helped 35mm film with a recording format of 24 × 36 mm to achieve a breakthrough. Another important factor was the appearance of powerful, bright lenses with an image field that roughly corresponded to the image diagonal in focal length. An important step was the Tessar , a triplet variant with 4 lenses , developed in 1904 by Paul Rudolph at the Carl Zeiss company in Jena .
In principle, an enlarger is a projector that projects an illuminated negative (or slide) onto a surface ( image plane ) on which photo paper is placed. The exposure time is controlled with an exposure switch (timer) or a light quantity meter .
A chemical development then usually produces a positive from the latent image in the photo paper .
construction
Magnifying devices consist of a base plate and a projector (magnifying head, consisting of lamp housing, if necessary color mixing head , negative stage and lens ), which is movably attached to a column. Alternative constructions have a parallelogram guide. In order to avoid blurring during the enlarged exposure of the negative onto the photo paper, the structure must have a high degree of rigidity.
This construction has prevailed because both the negative and the photo paper lie horizontally and cannot slip.
The size of the projected image depends on the focal length of the lens and the distance between the image stage and the photo paper. A display of the magnification scale is therefore often attached to the column or the head of the enlarger . The lens can be adjusted at a distance from the negative stage for focusing.
Some enlargers allow the enlarger head to be pivoted horizontally so that, for example, you can project against a wall. The distance between the negative and the wall determines the magnification scale, distances of up to a few meters are possible. These devices have to be designed to be particularly rigid, because the risk of blurring increases with the magnification.
Horizontal enlargers have also been used for large-screen enlargements. The lighting head, the picture stage and the lens run on rails in front of a fixed projection screen that accepts the paper.
Illumination and exposure
The negative must be illuminated as evenly and brightly as possible during projection.
Initially, a candle placed behind the negative or daylight falling through a window into the darkroom was used. The concentrated candlelight was evenly distributed over the negative with the help of a diffuser ( opal glass or "milk glass").
In the later transition to incandescent lamps as light sources , the diffuser was continued to be used before opal lamps came into use.
At least one plano-convex condenser lens is part of the lighting equipment housed in the lamp house , so that as much of the available light as possible passes through the negative. The negative is on the flat side of this lens. A so-called double condenser is often used . In it, two mostly identical plano-convex lenses are combined in a common mount, the convex lens surfaces facing one another.
In color enlargers, a diffuser is often used instead of a condenser system or a diffuser to homogenize the color filtering .
Exposure clock
An exposure clock, which can be combined with a light meter, is used to control the exposure time. It switches the lamp off after an adjustable time. The required exposure time depends on the magnification, the properties of the photo paper and the desired gradation. When using an exposure clock, the exposure time must be determined experimentally for each negative and each scale. For this purpose, several photo paper test strips are exposed to different exposure times and developed.
Light meter
Light quantity measuring devices measure the amount of light per area in the image plane. They receive the light backscattered from the paper or are placed next to the paper image when the section is enlarged. Light quantity measuring devices integrate the received light and enable an exact exposure independent of scale without test strips.
Light quantity measuring devices avoid the error that can arise from the brightness of the light source, which is strongly dependent on fluctuations in the mains voltage.
Black and white enlargements
The color temperature of the lighting is of little importance for the enlargement of black and white negatives ; the light must contain the appropriate spectral components for paper sensitization. Magnifying lenses are especially designed for a flat image (flat field optics ). In addition, a good contrast and high resolution are desired in order to keep the reduction in quality of the image contained in the negative due to the enlargement to a minimum.
The lens has an adjustable aperture . You can fade in for easier focusing and stop down for exposure for optimal imaging, which reduces lens errors. In addition, it achieves a greater depth of field , which largely compensates for defects in the flatness of the negative or in focus.
To determine the image section and focus, a red filter can usually be swiveled into the beam path without exposing the enlarging paper; the emulsion of black and white paper is mostly largely insensitive to red light. This filter is swiveled either in front of the lens or between the lighting and the film plane.
Adjustable mask straps are often used in the negative stage . This allows exact partial sections to be determined and enlarged and the enlarger can thus be used for different film formats.
Color enlarger
In the case of color enlargements, it is also important that the color temperature of the enlarging light is adapted to the sensitivity of the photo paper used for the light of the visible spectrum. In addition, the so-called masking (the mostly orange basic coloring of the film material) has to be compensated for with negatives. For this purpose, there is a so-called color mixing head on color enlargers , with which the operator can control the composition of the light to eliminate color casts or to consciously create them (color balance).
Magnifying lenses
In order to largely maintain the quality of the negative during enlargement, the enlarging lens must be of high quality. However, it does not require its own distance setting, as this is usually built into the device. Lenses with a zoom function are only common in printer laboratories so that changes in magnification are possible in these largely rigid constructions without changing the objective.
Magnifying lenses are usually optimized for a close range of 1:10 to 1: 3. They are specially designed for low distortion and high image field flattening in order to depict the flat surface of the negative on the flat surface of the paper in sharp focus. The open aperture is usually only used for adjustment. The optimal aperture is from 1: 5.6 to 1:11. Enlarger lenses should also be coated (anti-reflective coating) in order to preserve the image contrast, among other things. It is also favorable if the device has an image field diaphragm with which only the desired image section is illuminated. It prevents overexposure. Over-exposure in the image due to insufficient compensation and missing field stop can hardly be influenced by changing the paper gradation, exposure and development. They are particularly noticeable when the lights come in from dark areas.
The light intensity (max. Aperture value) is of little importance; lower light intensities can be compensated for by longer exposure.
The aperture can help prevent blurring by stopping down after focusing, which increases the depth of field.
Automation in large laboratories
The process of enlarging negatives is largely automated in the so-called photo finishing in large laboratories for economic reasons . This also improves reliability and quality assurance.
The automation extends not only to the mechanical-optical and chemical processes, but also color corrections and an exposure adapted to the subject are carried out by means of image analysis. Here z. For example, when it comes to color contrast, subjective factors are also taken into account, which are often tailored to the expectations of the largest proportion of customers and standard motifs.
Special requirements for a print, such as section enlargement, compensation of inhomogeneous exposure through so-called dodging or targeted soft focus effects, lower color saturation , etc., can only be carried out with individual enlargement in a specialist laboratory or privately.
Enlargement in the specialist laboratory
With manual and individual elaboration, the enlarger offers the possibility to intervene in the projection process.
By dodging or re- exposure , the brightness-related emphasis of individual parts of the image can be influenced or inhomogeneous exposures of the negative can be compensated. The same applies to filtering in color moods, provided that it is a question of color enlargements.
The often high color contrast in automatic production can be kept lower in the case of individual production through the choice of material or certain effects can be created through partial use of colors.
To achieve the desired contrast, black and white images can be achieved by using different types of photo paper with different gradations (increasing contrast from “soft” to “medium” to “hard”). Gradation convertible papers, in which the image contrast is increased by varying the illumination color, are widespread today . It is hardly possible for different developers to influence the image contrast with modern papers due to the stabilizers embedded in the photo layer.
Image compositions through extreme contrasts are possible. Softening (targeted blurring) in the darkroom process usually does not produce the desired effect because - conversely to the recording - the lights are closed.
With variable-contrast photo paper, brightness contrasts in selected parts of the image can be partially adjusted through appropriate light filtering. Further enlargements and photo montages are possible.
Softly outwardly tapering photographs, as they were popular in the past, are achieved with jagged frames that are held over the paper image during exposure and are moved slightly. However, in many cases the desired shape can be achieved easily by using the hands, without being tied to a fixed mask size.
Manufacturer (selection)
- Enlargers
- Agfa , Beseler, Durst , DeVere, Hengstler, Jobo , Kaiser, Kienzle, Krokus, Liesegang , Linhof , Meopta , Omega, Teufel, LPL, Rowi
- Lenses
- Schneider-Kreuznach , Rodenstock , Leitz , Nikon , Meopta , Vivitar
