It was due to the different intensity of exposure, the less affected areas needed more time for "developing" image, natural developer had to work longer.
Ilikuwa kutokana na kiwango tofauti cha mfiduo, maeneo chini ya walioathirika zinahitajika zaidi wakati kwa ajili ya "kuendeleza" mfano, developer asili alikuwa na kazi tena.