Annan worked under difficult conditions in the dark, cramped closes of Glasgow. Because the narrow streets received little sunlight, he required long exposure times. This technical constraint often resulted in empty streets or blurred, ghostly figures of residents who moved during the shot. He chose the carbon printing process for his later editions. This method used stable pigments instead of silver salts, ensuring that the dark tones and details of the stone architecture remained permanent and did not fade over time.

Modern collectors value Annan for his honest depiction of the industrial city. His photographs of Glasgow provide a window into a vanished world of Victorian architecture and urban life. The images possess a quiet, atmospheric quality that transcends their original purpose as municipal records. They suit contemporary interiors that favour historical depth and monochrome aesthetics. These prints offer a connection to the history of the built environment and the start of the documentary tradition.