A leaf shutter is a type of camera shutter mechanism consisting of a series of thin, overlapping metal blades - known as leaves - arranged in a circular formation within or immediately adjacent to the aperture diaphragm of a lens, typically positioned near the optical centre of the lens elements. Because of this positioning within the lens itself rather than in the camera body, the leaf shutter is also widely referred to as a between-lens shutter or interlens shutter, distinguishing it from the focal plane shutter found in the camera body of most SLR and mirrorless cameras.
The operation of a leaf shutter is elegantly simple in principle. At rest, the overlapping blades are held in a closed position, forming a light tight barrier that prevents light from passing through the lens to the film or sensor. When the shutter is released, a spring mechanism causes the blades to rapidly pivot outward from the centre, opening the aperture to its full extent and admitting light for the duration of the selected exposure time, before returning to their closed position to end the exposure. The smoothness and speed of this opening and closing cycle determines the accuracy and consistency of the shutter at different speeds.
One of the most significant practical advantages of the leaf shutter is its ability to synchronise with electronic flash at all shutter speeds, including the fastest speeds the shutter is capable of. Because the leaf shutter opens and closes as a complete unit, fully exposing the entire film or sensor area at every speed, there is no point during the exposure cycle at which only part of the frame is uncovered - unlike the focal plane shutter, which at faster speeds uses a travelling slit that covers part of the frame at any given instant and limits flash synchronisation to slower speeds. This unrestricted flash synchronisation capability makes leaf shutter lenses particularly valuable for professional portrait, wedding, and commercial photographers who frequently use flash in combination with ambient daylight and need to use fast shutter speeds to control the balance between the two light sources.
Leaf shutters are most commonly found in medium and large format camera lenses, where they are a standard feature of the lens design, as well as in certain compact and rangefinder cameras where the between-lens position is practical and advantageous. They are generally limited to maximum speeds of around 1/500th of a second, somewhat slower than the highest speeds achievable with modern focal plane shutters, but their flash synchronisation advantage and the smooth, vibration free nature of their operation make them the preferred shutter type for many professional applications.