What developing agent controls the shoulder of the characteristic curve?

The developing agent that controls the shoulder of the characteristic curve is hydroquinone. In photographic chemistry, hydroquinone is known for its ability to influence the density and contrast of the film image. The shoulder of the characteristic curve represents the area where the film begins to saturate, indicating that further exposure will result in minimal increase in density. Hydroquinone is particularly effective in creating higher density and enhancing contrast in this region.

This characteristic is significant for film and photographic processes as it allows for better detail recovery in highlights, which is crucial for overall image quality. While other developing agents like metol and phenidone play roles in contrast and developing speed, hydroquinone's specific properties make it the primary choice for managing the shoulder area of the curve, thereby improving the quality of the photographic output.