EFCS
Electronic First-Curtain Shutter (EFCS) is an exposure method used in some digital cameras. In EFCS, the exposure is begun by electronically activating the sensor (clearing it and beginning to record data) - no physical shutter is involved. The exposure is then finished by covering the sensor with a real shutter blind. EFCS is used especially in mirrorless cameras, where the sensor must be uncovered before the exposure for viewfinding. It may also be used in DSLR cameras to permit live-view. At very high shutter speeds EFCS may give rise to non-uniform exposure; the wave-front of activation takes a finite time to pass across the sensor, and in some cameras this is slower than the closing shutter. At very short exposure times therefore, the difference in exposure between the top and bottom of the frame may become significant. This problem affects mirrorless cameras made in the 2010s (for example the Canon EOS M series, where it is noticeable at shutter speeds of 1/2000 second)); improvements in more recent camera make it less of an issue.
