Toe curve in photography field

Refers to the curve of the low light part in the exposure characteristic curve. The specific content should be combined with what you saw somewhere in order to have a more detailed answer.

Probably in photography, in order to simulate human visual experience, both camera and film material (if on film, this is an inherent physical feature) have a dark photosensitive characteristic curve with varying slope. But this curve is not objective, or this curve has many standards.

Dark part, middle part and highlight are also called toe, straight part and shoulder respectively.