Enlarger

Josef Maria Eder, in his History of Photography[2] 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.

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.

[6]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.

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 1870s 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.

R. L. Elliot & Company, 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.

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.

[13] 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.

[13] Their light is blue-rich, in an area of the spectrum to which silver gelatin paper is sensitive, and therefore exposure is shorter compared to 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,[15] and also are Newton's rings' where a glass negative carrier is used.

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.

The image from the negative or transparency is projected through a lens, typically fitted with an adjustable aperture marked with f/ stops, onto a flat surface bearing the sensitized photographic paper.

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.

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.

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.

The claim for the biggest analogue enlargement ever made from a 35mm photograph is that for and Ernst Haas wildlife picture taken in Kenya in 1970.