As it turns out, most mechanical shutters actually sweep down the sensor, as opposed to sweeping left or right.\u00a0 This is done partly to reduce the effect of rolling shutter (since the distance down the rectangular sensor is shorter than going side-to-side).<\/p>\n
The shutter is composed of a \u201cfirst curtain\u201d that drops down (exposing the sensor) as the exposure starts, and a \u201csecond curtain\u201d that drops down (obscuring the sensor) as the exposure ends.\u00a0 For a fast enough shutter speed (faster than about 1\/200 second), the second curtain starts to close the shutter before the first curtain has reached the bottom.\u00a0 For a very fast shutter speed, like 1\/2000 second), the action of the two curtains makes a slit that travels across the sensor.\u00a0 Here is a rough depiction of a fast shutter in the middle of traversing the sensor.<\/p>\n
If there is motion in the scene you are photographing during the time it takes for the shutter to traverse the sensor, especially sideways motion, the top of the frame will see a slightly different scene that the bottom of the frame.\u00a0 A good example of this is what happens when photographing a fast-moving car.\u00a0 Here is what could happen.<\/p>\n![\"Shutter\"](\"http:\/\/light.elliottmccrory.com\/wp-content\/uploads\/2015\/02\/Shutter.png\")
<\/a><\/p>\n