In this article, we look at the focal plane shutter, providing a guide to what it is for, and how it works


The AF sensor from a Nikon DSLR

Like shutters on a window, the focal plane sensor either lets the light through or keeps it out. The simplest kind of camera shutter is a lens cap. In the old days you took a photo by taking the lens cap off for the duration of the required exposure, sometimes for several minutes. A focal plane shutter is situated right inside the camera just in front of the focal plane, the surface of the image sensor or the film emulsion. The focal plane shutter is a modern electronically governed and very precise device comprising a series of very thin overlapping metal blades. Older styles were made from rubberised cloth and is why the opening and closing of a focal plane shutter involves the terminology of ‘curtains’, of which there are two and exposure happens by separating each curtain. It needs to be ‘cocked’ into a spring-tensioned, ready-to-fire state. This used to be done manually but now is operated by a motor. For longer exposures it simply opens up completely, exposing the whole frame for as long as is required. But there is a limit to how fast the focal plane shutter can open and close the curtains, so for the fastest shutter speeds, it changes mode and passes an open gap between the curtains across the frame. The smaller the gap or slit passing across the frame, the lower the amount of light that gets through and so there is a desired reduction in exposure. The smaller the slit, the better the shutter is able to freeze the action.

Thanks to the focal plane shutter’s precision electro-mechanical construction, it can reduce the exposure to as little as 1/8000sec. The fastest it can open and close fully before changing to slit-mode is also known as the X-sync speed and is the fastest shutter speed available for electronic flash synchronisation. Digital developments mean the focal plane shutter is now working in tandem with the camera’s sensor and some cameras offer what is called a first curtain electronic shutter. That means it uses only one curtain, with the sensor acting electronically as the other curtain. This can reduce shutter shock and audible noise but can result in digital artefacts in the image under certain conditions.