Newb- image passing thru frame, not captured

[Nancy Moran G]

I have seen beautiful photos here and have seen the comment, usually regarding a church photo, ' left shutter open so people passing by the altar were not captured in the photo', or something to that effect.
Did I misunderstand the comment? Then, HOW does that happen ?
Explanations in simplistic terms for this newbie is appreciated !
Nancy

[Inkanyezi]

The simple answer is that the image is exposed during a substantially long time, so any person moving within the area would be unnoticed. Depending on how fast people move, times may be from about a minute up to several minutes. Sometimes a ND filter is needed to get the exposure time long enough, and the ISO would be set to lowest possible. It may help to have a black beret to hold in front of the lens if someone in white clothes should get into the area, as white clothing may cause some fogging of the image, but you can eliminate it by covering the lens (without touching the camera) while the person in white is there.

Most cameras cannot expose for more than about a minute, so you might need a remote release with long time ability of some kind, either automatic long time or that you use the B setting. For Canon DSLR Cameras, Magic Lantern is recommended, and for Canon compact cameras CHDK.

Inside a church, sometimes you won't need a very dense grey filter, but outdoors in open spaces, you'd need a very dense filter to reach long exposures. I would get ND8 and ND64 and combine when there is a need.

[jcuknz]

Since there is a build-up of noise when digital cameras make long exposures an alternative way would be to take a series of shots as people 'vacate' different areas of the shot and then with the different frames as layers in your editing programme remove the people in each layer to reveal the emptry area shown in another layer. With care one wouldn't need a tripod to do this as it is fairly easy to line up different layers in editing .... hopefully the light levels will not change unduely as you did this but minor changes can be accomodated for in editing.