Focal plane
The focal plane is the flat plane onto which a lens focuses its image. The focal plane sits at a 90-degree angle to the optical axis.
A focal plane shutter gets its name from the fact that it sits directly in front of the focal plane (ie, the film) of the camera. This is an advantage for removable lens cameras, because lenses do not need to have shutters built into them.
The most common type of focal plane shutter is the two-curtain type first used by Leica. It uses two curtains travelling in the same direction across the focal plane. The first curtain opens across the focal plane, and then the second curtain closes behind it. The time between the release of the first curtain and the second curtain is determined by the shutter speed. Cocking the shutter moves the curtains back to their starting position.
