What causes yellow or brown stains on a developed film?

Yellow or brown stains on a developed film are primarily caused by old developer. When the developer solution is past its effective life, it can lead to improper development of the film emulsion, resulting in discoloration. This is due to the breakdown of the chemical components within the developer that are responsible for reducing and developing the latent image. As these chemicals degrade, they may not fully process the silver halide crystals in the film, leading to the formation of these stains.

While other factors can affect film quality, they typically do not produce the same yellow or brown discoloration. For instance, static electricity tends to cause marks or artifacts on the film surface but does not result in staining. Fixer cut off relates to improper fixing processes, where part of the film may remain undeveloped but does not usually lead to a uniform yellow or brown stain across the film. Reticulation of emulsion is caused by sudden temperature changes during processing and results in a cracked or wrinkled appearance rather than staining. Hence, the indication of yellow or brown stains strongly points to the inefficiency of an aging developer solution.