A press focus lever is a feature found on many large-format cameras with between-the-lens shutters. This lever allows the photographer to hold the shutter blades open for focusing, regardless of the shutter speed setting.
By keeping the shutter open during composition and focus, the press focus lever enables precise adjustments on the ground glass or focusing screen. This is particularly useful in studio, architectural, and landscape photography, where accurate focus and careful framing are critical. The mechanism ensures that exposure timing is separate from the focusing process, giving photographers greater control over image sharpness and composition.