Techtalk7


	Technical Talk Part 7
		Colour positive film, including cross-processing.
	As for colour negative films, Kodak came to dominate the market for colour transparency, also known as slide, chrome, positive or reversal. Gradually, the processes by other manufacturers fell by the wayside, until today only the E6 and K14 processes survive. K14 is used for Kodachrome, the only film I know that's had a song written about it. Run exclusively by Kodak, K14 was only saved by protests from photographers and publishers, who had come to rely on its rich reds and a sharpness that sets the standard. Before looking at what makes Kodachrome so different from E6, we need to look at what makes reversal different from negative. The simple answer is that after processing we end up with a positive image on the film rather than a negative. All reversal film today works to the same principal (except for Kodak B&W Direct Duplicating film and Polaroid Instant slide films, more on them next time). The negative image is developed first, then the unexposed silver halide left in the emulsion is fogged and developed. Now take away the negative image and we are left with a positive. Ordinary black & white film can be reversed to make B&W transparencies and there are several kits on the market. Do read the instructions before exposing the film though. There's also Agfa Scala, which can only be processed at Joe's Basement. The main difference between the C41 negative and the E6 reversal processes, are three extra steps at the beginning. First is a B&W developer, second a wash and third a chemical fogging bath to expose the undeveloped silver halides of the positive image. After that, the steps of developing the colour image, bleaching the silver, fixing and washing, are the same as for colour negative. Although the chemistry is formulated differently in order to get the best quality out of each process, it's similar enough to allow E6 film to be cross-processed through C41 to give a negative or C41 through E6 to give a positive. The differences do mean that the colours and contrast get distorted. The effect obtained varies between film make and type (even batch) and process manufacturer (just like C41, several companies make E6 compatible chemical processes). The effective speed of the film alters and the effects can change if the film is pushed or pulled. C41 process B&W film, such as XP2, can be cross-processed, but not always with a neutral black image colour. Not all labs will cross-process, since it can upset the chemistry and cause problems. There are E6 and C41 compatible processing kits available which can be tried. Much testing is needed and it can be a case of you can't be sure of what you will get until you get it, but it's very popular with fashion, editorial and music photographers. Printing cross-processed film is a little different to normal. C41 film contains an orange coloured filter to correct for certain problems in the printing paper and E6 film doesn't. You can use a blank piece of processed negative film above the cross-processed negative to add the filter, if you want to. For reversal printing, the enlarger filters can be used to affect the colour if you want, although this will also affect the other colours. A lab can do the same when hand-printing, but they have no guide to what sort of distorted colours you want and need help. Who knows what an automatic printer would make of a cross-processed neg. Whichever, be prepared for much more effort and expense to get it the way you want. No doubt a filter for Photoshop will appear sooner or later. Next month I'll continue going in reverse. © Barry Leighton FRPS