What, in fairly objective terms, is the use of cross-processing transparency films? Is it purely to get quirky colors? I don't want to use 'lomo' in a pejorative sense, but I'm wondering if these films accomplish something for relatively conventional photography. The Rollei films seem to be the predominant examples of this. It doesn't help that the manufacturer claims 'fine grain' and the few remarks I can find on the interblag say 'grainy'.
There is nothing "conventional" about cross processing. Cross processing is the deliberate processing of a film in a chemical process for which it was not designed.
The most common types of cross processing are :
- Processing color negative films with the E6 process
- Processing color reversal films with the C41 process
Processing color negative film in E6 will result in a positive image on the film but with a distinctly orange cast due to the orange mask layer incorporated into negative films.
Processing color reversal film in C41 will result in a negative image on the film but the film will not have the typical orange cast found on most film negatives.
It is possible to cross process other types of films in chemistry for which they haven't been designed (e.g. color film processed with black and white developers), but in all cases it's done to achieve a creative effect.
Note, there are some films, particularly black and white films, for which there are multiple developers recommended depending on the desired use case. For instance, Kodak Technical Pan Black and White film can be shot at a variety of exposure indexes ranging from EI 16-200 then processed with various developers to achieve a range of contrasts from a typical photographic contrast index of about 0.5 to an extreme contrast typical of that of a lithographic film (CI ~2.5). This, however, is not usually considered "Cross Processing"