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All enlargers consist of a light source, normally an incandescent light bulb, a condenser or translucent screen to provide even illumination, a holder for the negative or transparency, and a specialized lens for projection. 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.
Types of Enlarger
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.
A diffuser enlarger's light source is diffused by translucent glass or plastic, providing even illumination for the film.
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.
Diffuser enlargers produce an image of the same contrast as a contact print from the negative.
Dedicated color enlargers contain an adjustable filter mechanism - the color head - between the light source and the negative, enabling the user to control the amount of cyan, magenta and yellow light reaching the negative. Other models have a drawer where cut filters can be inserted into the light path. These enlargers can also be used with variable-contrast monochrome papers.
Enlarger physical arrangements
Most modern enlargers are vertically mounted with the head pointing downward and adjusted up or down to change 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.
Principles of operation
The image from the negative or transparency is projected through a lens fitted with an adjustable iris aperture, onto to 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 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 enlargng 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.
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.
After exposure, photographic paper is developed, fixed, washed and dried using the gelatin silver process.
Automated print machines
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 image may be printed to a size different from the negative or transparency. Without an enlarger, only a contact print would be possible, and large images would require large size negatives and hence very large cameras.
- Local contrast and density of various parts of the print can be easily controlled. Changing the amount of light exposing the paper in various areas will change the image density in those areas. A mask with a hole can be used to add extra light to an area "burning", which will have the effect of darkening the regions with additional exposure, while the use of a small wand to reduce the total exposure to a region is called "dodging" and has the effect of lightening the regions with reduced exposure. The tool is kept moving to avoid producing a sharp edge at the region boundary. Using these techniques it is possible to make significant changes to the mood or emphasis of a photographic print. Similar methods are available with contact printing, but it is more difficult to see the image as it is being manipulated.
- It is also possible to make composite photographs by overlaying the print with a hand-cut mask, performing an exposure, and then using the inverse of that mask to perform another exposure with a different negative. This is much more difficult to do well using photographic methods than it is now by using the methods of modern digital image manipulation.
Image enlargement limits
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, and the intended viewing distance of the final product.
For example, a 5 by 7 inch print intended for viewing in a scrapbook at 18 inches may be unsuitable for enlargement as an 8 by 10 inch print to be hung on a hallway wall to be viewed at the same distance, but usable at a larger 5 by 7 feet (twelve times larger) on a billboard to be viewed no closer than eighteen feet (twelve times more distant).
As the photographic market shifts away from film-based towards electronic imaging technology, many manufacturers no longer make enlargers for the professional photographer. Durst, who made high quality enlargers, stopped producing them in 2005, but still supports already sold models. Manufacturers old and new include:
- De Vere
- Gnome Photographic Products [a]
- Kaiser Fototechnik
- Kienzle Phototechnik
- Lucky (now owned by Kenko)
- Ōmiya Shashin-yōhin K.K.
- Paul Teufel & Cie Photogerätebau
|This section requires expansion. (June 2008)|
- Gelatin silver process for an overview of the dominant photographic printmaking process;
- Photographic printing for an overview of analogue photographic printmaking methods;
- Contact printer for a non-enlarging method of producing photographic prints;
- Projector for a directory of projector types.
- Established in pre-WW2 Stuttgart, moved to UK in 1938, closed in 1994 
- "Diffuser vs Condenser Enlargers". Applications Printing In Black & White Darkroom Equipment. Ilford Photo. Retrieved October 4, 2015.
- "De Vere 504 DS Digital Enlarger". Odyssey Sales. Archived from the original on 7 August 2008. Retrieved 21 September 2015.
- "Black and White World's Enlarger Guide". Retrieved 2008-07-29.
- Jacobson, Ralph E. (2000). "6 - Optical aberrations and lens performance". The manual of photography : photographic and digital imaging (9th ed.). Boston, Mass.: Focal Press. p. 80. ISBN 978-0-240-51574-8.
- "Gnome Pixie". The Camera Shelf. Retrieved 4 October 2015.