The graphic below illustrates the operation of a DSLR vertical-travel, focal-plane shutter during the exposure of a scene. The shutter has two curtains, each typically of 5 metal blades. The first curtain is initially closed, blocking light from reaching the sensor. When the shutter release is pressed, the first curtain starts to open, allowing light onto the sensor. After a delay equal to the "shutter speed", the second curtain starts to close to block the light again.
At low shutter speeds, below about 1/200 sec, there is a period during which the shutter is fully open, i.e. the first curtain has opened but the second curtain has not yet started to close.
When using a flash, the flash is synchronized to fire while the shutter is fully open. This is shown below by the illumination of the label "Flash sync". If synchronization is set to 'first-curtain', the flash fires as soon as the first curtain is fully open. On the other hand, if 'second curtain' synchronization is set, the flash fires just before the second curtain starts to close.
At faster shutter speeds, above the flash sync speed, the second curtain follows close behind the first, so the shutter is never fully open. Each pixel of the sensor is exposed to light for a very brief time.
Click the buttons below to repeat the animation at either a slow or fast shutter speed.
With mirrorless cameras, the resting position of the shutter is open, so light can fall onto the sensor during composition and the scene can be viewed through the viewfinder or on the screen. When the shutter release is pressed, the first curtain closes and then the sequence described above starts.