Chemical focus refers to the point at which a lens brings the actinic (photographically active) rays to focus on the film or sensor. In modern, fully corrected lenses, the chemical focus aligns with the visual focus, ensuring that what the photographer sees through the viewfinder matches the area of sharpest detail captured in the image. Understanding chemical focus is important in film photography and lens design, particularly when working with black-and-white emulsions or early photographic processes.