1 JULY 1938, Page 38

PHOTOGRAPHY IN NATURAL COLOURS

Plotograpby

By W. R. AYLING

UNTIL quite recently, making a photograph in natural colours was a complicated process calling for considerable skill and experience on the part of the amateur. Few general subjects were pos,trkyed in colour and most were of flowers, objets d'art, textiles arid similar specimens, the colour pictures being on glass. These were viewed through a _mosaic screen bound up with the colour photograph like a lantern slide. - Later came the celluloid film which preceded the .present type of colour film. The older type of film needed a special filter through which the picture was taken and viewed. The cost was high, and due to this and the difficulty of securing passable results, colour photography did not make very rapid progres.s. With the introduction of " Dufaycolor," " Agfacolour," and " K:oclachrome " films, colour photography for the amateur commenced to boom. It is now easy for the camera user to make natural colour photographs at reasonable cost and with every chance of success.

The " Dufaycolor " process is an additive process, using the three primary colours, red, green and blue in the form of a reseau or ruled pattern printed on the celluloid, base before the photographic emulsion is applied. The pattern is of microscopic size and ordinarily not divisible by the unaided eye. The idea of utilising such a pattern dates back to 1895, and has been used I believe in many colour processes. Dufay altered the order of colours in the pattern and per- fected this method of filtering the light falling on to the film emillsion. Briefly, the ruling is secured by coloration of the celluloid, ruling with moisture-resistant ink, washing out the dye where it is unprotected by the ink, and then removing the ink. This leaves a very fine line of colour. The process is repeated until the reseau is completed. When finished' the pattern has fifteen lines to the millimeter. Con- siderable magnification is required before the lines are visible.

The photographic emulsion, sensitive to all colours, fine grained and fast in speed is applied to the prepared basep•and .an-anti-scratch layer is afterwards added to protect-the emulsion.

The " Dufay " colour film is -exposed in the camera with the ruled base towards the lens, the microscopic windows of colours formed by Idle reseau acting as filters. It is actually possible to make separation negatives, one for each of the primary colours, from a properly exposed " Dufaycolor "

This is available in roll-film and in 35 mm. size for users of miniature cameras. The film is developed as a negative and reversed. If the emulsion were stripped from the patterned base, the former would be found to consist of blaCk, various tones of grey, and white or transparent. The colour is only in the base.

The new " Agfacolour " film was introduced in 1937, and consists of sensitive layers of emulsion with colour filters between, protected on the surface by an anti-scratch layer. In this case the celluloid base is transparent, and the film is exposed emulsion towards the lens. When it is developed the entire image composed of exposed and darkened silver grains is bleached out, and its place is taken by dyes. The actual formulae for the baths used are secret and the pro- cessing is carried out by the manufacturer. The results are very good and true to colour. This film is at present available_ only in the 35 mm. size. " Kodachrome " is a process of the Kodak Co., and is in many respects similar to the "Agfacolour " process, being available only in 35 mm. size. The film is exposed in a similar manner and is processed by the manu- facturer. Colours are as true to nature as it is possible to get. Both " Agfacolour "' and " Kodachrome " are subtractive processes. " Dufaycolor " and " Kodachrome " can be used in artificial light, using a special film in the case of the latter and a compensating filter in the former. With all three types of colour film there is little if any latitude in exposure. The makers' instructions for exposing should be closely followed. Given bright sunlight and a well-illuminated subject it is easy for any amateur to secure good results. It mutt be remembered that colour photography provides transparencies only, and that colour prints on paper are an entirely different process, difficult of achievement and costly; Definitely not for the beginner in photography. -