Almost all modern SLRs, even digital ones like the Nikon D90, have a focal plane shutter. A focal plane shutter goes by that name because of its placement. It is placed right in front of the film, or sensor, in the focal plane. This prevents the film from getting exposed, even if the lens is removed. The light is able to reach the film only when the shutter is released.

A typical focal plane shutter consists of two curtains. In some shutters the curtains are horizontal or vertical. As you can see in the picture on left, one curtain is known as the opening curtain and another curtain behind it is called the closing curtain. At a shutter speed that is slower than the critical shutter speed, or the X sync of the camera, the front curtain opens, exposes the film and then the closing shutter follows after the set time interval. On the other hand, if the shutter speed is at a speed faster than the X sync, then the opening curtain opens and the closing shutter closely follows, thus exposing each segment of the film frame for an equal interval. The diagram on the right shows the shutter mechanism at a speed faster than the X sync. The green curtain is the opening shutter and red one is the closing shutter.

Horizontal focal plane shutters are convenient for motion photography, but problematic for electronic flash guns. This is because as the curtain travels the entire length of the film, the film is only exposed for a brief instant and this is when the flash gun is flashed. Therefore, the time set for the shutter should be higher than the amount of time the 1st curtain takes to open completely. This is the X sync of the camera, the speed at which the film frame is completely exposed for an instant before the closing shutter starts to close.