A magnifying device is a device in photography. It allows by optical projection to produce enlarged images from paper negatives or slides.
In the 19th and early 20th century were due to lack of lighting facilities is extremely difficult or impossible to be made enlargements. It also lacked bright, high-contrast lenses with a sufficiently large field. Therefore, large format cameras with negative formats up to 24 × 30 cm were used and larger, where a large deduction was produce by contact. Only powerful simple lighting enabled enlargers, the 36 mm helped the miniature film with the recording format of 24 × breakthrough. Much was also the appearance of powerful bright lenses with an image field, which corresponded approximately to the screen diagonal in the focal length. An important step was the 1904, developed by Rudolph of Carl Zeiss, Jena Tessar; a triplet variant with 4 lenses.
In principle, a magnifying device is a projector that an illuminated negative (or transparency ) onto a surface ( image plane ), is placed on the photographic paper. The exposure time is controlled by an Exposure timer (time switch) or a light flow meter.
A positive is from the latent image in the photographic paper then caused by a chemical development in general.
Magnifying devices consist of a base plate and a projector ( Vergrößererkopf consisting of lamp house, possibly mixing head, negative stage and lens) which is movably attached to a pillar. Alternative designs have a parallelogram. To prevent camera shake during recording on film of the enlarged negative on the photo paper, the structure must have a high stiffness.
This construction has prevailed, since in this case are both the negative and the photo paper horizontally and can not slip.
The size of the projected image depends on the focal length of the lens and the distance of the image from the stage Photo Paper. Therefore often an indication of the magnification scale is attached to the column or the head of the enlarger. For focusing the lens is set at a distance relative to the negative stage.
Some enlargers allow it to swing the Vergrößererkopf to the horizontal, so that, for example, can be projected against a wall. The distance between negative and wall determines here the zoom magnification, there are distances up to several meters are possible. These devices need to be extra -stiff, because the effects of camera shake increases with the magnification.
For large-screen magnification and horizontal magnification devices were used. The illumination head, the negative carrier and lens here run on rails in front of a fixed projection screen that receives the paper.
Illumination and exposure
The negative must be illuminated as uniformly as possible and light during projection.
Initially, it was a candle that was placed behind the negative or natural light coming through a window into the darkroom, used. The concentrated candlelight was negatively distributed with the help of a diffuser ( opal or " milk glass " ) evenly to the.
In the later transition to incandescent lamps as the light source, the lens was used until Opallampen came into use.
The lighting means is accommodated in the lamp house, is now at least also a plano-convex condenser lens, so that as much of the available light passes through the negative. The negative is on the planning side of this lens. Often a so-called double - condenser is used. With him two mostly identical plano-convex lenses are combined in a common format, with the convex lens surfaces facing each other.
In color enlargers a diffuser instead of a condenser system or a lens is often used to homogenize the color filtering.
To control the exposure time is an exposure timer, which can be combined with a light flow meter. It turns the lamp off after an adjustable time. The exposure time required depends on the zoom magnification, the properties of the photo paper and the desired gradation. The exposure time must be determined experimentally using a new exposure timer for each negative and each scale. To this end, several photo paper test strips are exposed and developed different exposure times.
Amount of light gauge
Amount of light meters measure the amount of light per unit area in the image plane. You receive to the backscattered light from the paper or be placed at magnifications next to the paper image. Light quantity measuring devices integrate the received light and enable scale-independent exact exposure without test strips.
Luminance Meters avoid the error, which may result from the dependent strongly on mains voltage fluctuations brightness of the light bulb.
Black and white enlargements
For increase of black and white negatives is the color temperature of the lighting of little importance; the light must contain the appropriate paper for spectral sensitization. Enlarging lenses are particularly aligned with a plane figure ( flat field lens ). In addition, a good contrast, and high resolution is required, to maintain the reduction in quality of the image contained in the negative by increasing low.
The lens has an adjustable aperture. Here you can fade in for easier focusing and stop down for exposure for optimal imaging, which for lens defects decrease. In addition to leading to an increased depth of field, which compensates for shortcomings in the flatness of the negative or in focusing for the most part.
For the determination of the image section and the focus position, a red filter can usually swivel into the beam path without exposing the magnification paper; the emulsion of black and white paper is usually largely insensitive to red light. This filter is rotated either on the lens or between lighting and film plane.
Frequently find adjustable mask bands in the negative stage application. For an accurate display parts can be determined and the Magnifier enlarges and can therefore be used for different film formats.
For color prints is also important that the color temperature of the light, the sensitivity magnification of the photographic paper used in each case is adapted to the light of visible spectrum. It must also be balanced with negatives, the so-called masking ( the most basic orange coloring of the film material ). For this purpose there is to color enlargers a so-called color mixing head, with which the operator can control the composition of the light for eliminating color casts or their conscious generation ( color balance ).
To maintain the quality of the negative at increase largely, the magnifying lens must have a high quality. However, it does not need a removal setting, as it is generally installed in the device. Lenses with zoom function are common only in Printer laboratories, thus magnifying changes in these largely rigid structures is possible without changing lenses.
Enlarging lenses are usually optimized for a close range from 1:10 to 1:3. They are specially designed for low distortion and high flatness of field to map the flat surface of the negative on the flat surface of the paper sharp. The open aperture is usually used only for setting. The optimal aperture is 1:5.6 to 1:11. Also Vergrößererobjektive should be remunerated (anti -reflection coating ) to obtain the image contrast, among others. It is also favorable if the device has a field stop, with only the desired image section is illuminated. It prevents glare. Glare in the picture due to lack of remuneration and lack of field stop can be influenced by changing the paper - gradation, exposure and development barely. They are recognizable especially when the lights from dark areas to run.
The light intensity ( max. aperture value) has a low importance, low light levels can be compensated by longer exposure.
The aperture can help prevent blur by dims after focusing, which increases the depth of field.
Automation in large laboratories
The process of enlarging from negatives automates the so-called photo-finishing in the large laboratories for economic reasons largely. In this way, the reliability and quality is improved.
The automation extends not only to the mechanical- optical and chemical processes but by image analysis and color correction and a motif adapted Ausbelichtung be performed. Here, for example, the color contrast and subjective factors which are often tailored to the expectations of the largest customers share and on standard designs.
Particular requirements for a deduction, such as zoom, exposure compensation inhomogeneous by the so-called Dodge or selective blur effects, lower color saturation, etc., may only individual magnification in a specialized laboratory or done privately.
Magnification in specialized laboratory
With manual and individual drafting the magnifying device offers the possibility to intervene in the process of projection.
By Dodge or burn the brightness- related emphasis on particular parts of the image can be affected or there may be non-uniform exposures of the negatives are compensated. The same applies to the Einfiltern of color scenes, unless it is color enlargements.
The often high color contrast in the automatic production can be kept low by the choice of material for individual manufacturing or certain effects are produced by partial use of colors.
To achieve a desired contrast ( increasing contrast of " soft " to " medium" to "hard" ) can in black and white images by using photo paper of different gradation can be achieved. Are in widespread use today Gradationswandelpapiere in which the variation of the illumination color of the image contrast is increased. An influence on the image contrast by different developers is hardly possible in modern papers by the embedded in the photo layer stabilizers.
Image designs by extreme contrasts are possible. Blur (selective focus blur) in the darkroom process takes usually not the desired effect, because - reversed to accommodate - to pull the lights.
In kontrastvariablem photo paper contrasts in brightness can be adjusted by partially corresponding light filtering in selected image parts. Furthermore, enlargements and photo montages are possible.
Soft leaking to the outside photographs, as they were formerly popular, are achieved with jagged borders that are held during the exposure over the paper image and it moved easily. However, it can easily be reached by the use of the hands to the desired shape, without being bound to a solid Maskenmaß here in many cases.