You're shooting an After the End movie where almost everyone is gone, and the world has fallen into disrepair. You're telling the story of The Aloner or the last few people, so when you have to shoot in the city you can't have streets full of cars and people in the background. The thing is, clearing the streets of people, making sure lawns look unkempt, and other set dressing for the apocalypse can be expensive and takes a lot of time. How do you get around this but still film the characters outside?
You use a lot of shots from below the actors!note This way you don't have to worry about the busy street behind the actors or the neatly-mown lawns (that should have been untouched for years) on the side of the road. All the audience will see is the tops of buildings (which would probably still be in okay shape), and treetops. You remove the sounds of traffic in editing and replace it with silence. As camera tricks go, you can't get much more simple.
Of course, it can be jarring when used too much or from too low an angle, making it feel like everything is being recorded from the point of view of a child - but when used with proper planning, you barely even notice. Competent directors simply make sure the characters in the foreground or the tall objects in the background are interesting enough and the audience will be distracted.
Also a convenient way to avoid catching undesirable advertisements (such as signs promoting a rival studio's or network's fare) in the background.
Less common these days in big-budget films as a background can be replaced with CGI. For instance, when Tom Hanks climbs to the top of the island in Cast Away, you can see in the DVD extras that he was in a parking lot, but this was replaced by CGI - this is not the trope because the angle was just for effect, not to obscure the parking lot, which was removed in post-production.
Compare with Dutch Angle (the camera is sometimes aiming upwards, and must be canted/tilted to create an uneasy atmosphere), Hitler Cam (the camera is aimed upwards to make one or more figures taller/physically imposing), and Knee-High Perspective (where the camera isn't angled, but is still close to the ground, creating the impression of a small perspective).
- Godspell uses this often with its empty Manhattan, only it's non-apocalyptic.
- The only actual outside shot of 2001: A Space Odyssey was the scene where the proto-human smashes the skull and bones, shot in a field on a raised platform from down low to get the sky in the shot and to avoid the cars and trucks in the background.
- An In-Universe version in the propaganda movie being shown in 1984 in a futile attempt to hide the bombed-out rubble of London the troops are marching through.
- Television, with its long shooting schedule and many shots, uses this a lot. The following shows have many scenes of the characters walking and talking, from a low angle to hide roadsides and the fronts of houses.
- Falling Skies: Mostly season one, in the Boston area. Later seasons spent more time on sets or in the woods.
- Revolution: Used fairly often to save on having to close streets and dress them with trash and rubble.
- The Walking Dead: Often goes on roads out in the country, but still shoots from low so they don't have to dress the ground.
- Battlestar Galactica (2003): When the folks back on Caprica are going through the empty city.
- The Last Man on Earth: The survivors take up homes on a hilltop - this ensures the camera has less of a chance of picking up action in the background of shots.
- In the Blake's 7 episode "Aftermath", location filming was on a beach in Northumberland instead of the usual BBC Quarry. Unfortunately a large crowd of gawkers turned up as the day wore on, so there was an excess of shots like this.