Dichroic fog is a type of image defect seen on photographic negatives, characterised by a purple-green bloom or iridescent sheen. It occurs when silver forms unevenly in the emulsion, often in the presence of acidic conditions during processing or storage.
This phenomenon can reduce image clarity and colour fidelity, making prints appear less sharp or discoloured. Photographers and darkroom technicians aim to prevent dichroic fog through careful chemical handling, proper fixing, thorough washing, and maintaining neutral processing conditions. Understanding this issue is important for preserving negatives and achieving high-quality photographic results.