Complex sentence: He reported that he stopped taking photographs because keeping his studio working was difficult (he used the wet collodion process) and commercial photographers (who started using the dry-plate process in the 1870s) took pictures more quickly.
Split into simpler sentences: He reported that he stopped taking photographs because keeping his studio working was difficult. He used the wet collodion process and commercial photographers, who started using the dry-plate process in the 1870s, took pictures more quickly.