What is the function of the electronic front curtain shutter?

In general, focal plane shutters use two mechanical shutter curtains: front and rear curtains located in front of the image sensor.

In products with translucent mirrors or in mirrorless cameras, the shutter stays open during live view before shooting. When the shutter button is pressed, the front curtain closes once, and then opens to expose the image sensor to light for a given exposure time (known as shutter speed). Then, the rear curtain closes when the exposure time finishes.
NOTE: You may hear the shutter sound twice when shooting because of this operation, one from the front shutter curtain and the other from the rear shutter curtain, depending on the shutter speed.

In the electronic front shutter curtain function, operation of the front shutter curtain is avoided through electronic control of the image sensor. Instead of mechanical operation of the front shutter curtain, the exposure is started electronically by the image sensor and finished by closing the rear shutter curtain.

NOTE: You hear the shutter sound only once in this operation because only the rear shutter curtain operates mechanically.

IMPORTANT: Set Electronic Front Curtain Shutter to OFF in the following situations.

• When you shoot at high shutter speeds with a large diameter (aperture) lens attached
  If you shoot images with Electronic Front Curtain Shutter set to ON, the ghosting of a blurred area may occur, depending on the subject or shooting conditions. In such
  cases, set this function to Off.
• When shooting at a fast shutter speed
  If you shoot images with Electronic Front Curtain Shutter set to ON, the brightness of the image may be uneven, depending on the shooting conditions.
• When a lens made by another manufacturer
  If you use a lens made by another manufacturer (including a Minolta/Monica-Minolta lens), set this function to Off. If you set this function to On, the correct exposure may not
  be set or the image brightness will be uneven.

By shooting with the electronic front shutter curtain function, because there is no mechanical operation of the front shutter curtain, you can shoot images with a short release time lag, the time after the shutter button is released until exposure is started.