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Black and White Films and Papers - Push-Processing, Film Design, Anti-halation Protection, Chromogenic Films

color speed emulsion image

MIKE GRISTWOOD
Formerly of Ilford Imaging UK

Push-Processing

This is a popular technique with photojournalists and other users of high-speed and ultra high-speed films working under unfavorable conditions. It involves exposing the film at a higher than usual speed rating and then giving extended development, preferably in a speed-increasing developer. The extended development increases the effective speed of the film a little and expands the density range of the negative which is then reduced by deliberate underexposure. In the days of graded papers the increased development required was that which would ensure that the negative would print on the same grade as if it had not been underexposed or, for more extreme underexposure, one or two whole grades higher. The result of this constraint on the choice of development times was often rather grainy negatives. The advent of high-quality variable contrast papers with the flexibility of smaller contrast increments allowed the use of shorter development times giving better image quality. Indeed, for some films the manufacturers’ recommendation is that no increase in development time is required for a one-stop push.

Although extravagant claims have been made for some developers marketed as suitable for push-processing, the absolute limit is a three-stop push, i.e., EI 3200/36 for an ISO 400/27 film. However, the image quality will be very poor and it is generally preferable to use an ultra high-speed film at this rating and above.

Film Design

Most films use a combination of a fast and a slow emulsion. The slow emulsion improves overexposure latitude and allows the designer to achieve the required curve shape in the highlight region of the characteristic curve. These two emulsions can be blended together and coated as a single emulsion layer or, more common, as a bipack with the slow emulsion underneath. Alternatively it can be coated as a partial blend with each emulsion layer containing both emulsions in variable proportions. Although two emulsion components is the most common design, up to four emulsions have been used in some of the modern ultra high-speed films.

Anti-halation Protection

Anti-halation protection is provided by the use of a dyed base (typically gray, blue-pink, or blue) on most 16, 35, and 70mm films on both acetate and polyester base. 120 and 220 roll films and sheet films are coated onto clear base and have a dyed backing layer that bleaches during processing. This layer also balances out the curl imparted by the emulsion layer and can, if required, be modified to accept retouching.

Chromogenic Films

Chromogenic films, which were first introduced by Ilford and Agfa in 1980, use color film technology to produce a black and white image. These films are significantly finer grained than conventional films of the same speed, but this comes at the expense of lower sharpness. These films are increasingly popular with social photographers looking for a cost-effective way to produce black and white pictures for their customers. They vary significantly in their degree of complexity. Some use a simple bipack assembly with a blend of color couplers designed to give an essentially neutral image tone suitable for printing onto black and white paper. This is the approach that gives the best image sharpness. The arrival of chromogenic films coincided with the rise of the one-hour mini-lab, which provided users with a quick and convenient way of getting their film processed and a set of prints made on color paper. However, these films were never designed with printing onto color paper in mind, and it took some skill on the part of the mini-lab operator to get acceptably neutral prints. Also, even when this was achieved with correctly exposed negatives the color varied on prints from under- and overexposed negatives. An alternative approach adopted by other manufacturers was to use a multi-layer assembly that prevented competition between the color couplers causing color variations. This gives a more consistent image tone and improved printing onto color paper. Typically these films are even finer grained than the simple type but at the expense of still lower sharpness. Finally, there are films designed solely for producing black and white prints on color paper. These have a distinctive yellow/ orange appearance similar to a color film and produce acceptably neutral prints on color papers using the typical printer settings used for making color prints.

These films also offer the best performance for scanning, especially using less expensive film scanners, because the non-scattering dye image is more suitable for the specular light sources used.

However, in spite of all of its advantages chromogenic film has failed to supplant conventional film because of a number of significant disadvantages. For example, one of the joys of black and white photography for many users is the scope for experimentation provided by the almost inexhaustible supply of different developer formulas. Similarly, aficionados of the Zone System felt that its benefits outweighed those of the chromogenic films. There are also concerns over the permanence of dye images compared to silver images. Also counting against these films for some users is the fact that they cannot be push-processed.

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