The human eye and brain relates to an image by imagining oneself in that environment. When a situation contains a strong visual element that is based on a line, the viewer's brain expects the line to be in the same orientation that it knows based on a prior experience. Even if the situation is unfamiliar to the viewer, if the nature of the line is generic enough, the brain draws upon similar visual memories to demand the line's orientation. If the line in the image is of a different orientation, the viewer's response is mixed. The image is either visually disturbing, or the viewer assumes a deliberate attempt, by the photographer, to place the line in this orientation.

Consider this image of Mt.Rainier across Puget Sound. Even if you are unfamiliar with this subject, you will immediately sense that the horizon is not level. The horizon line is one of the most commonly recognized lines in an image. It appears as though the water body is emptying to the right, a visually disturbing perception.

Using a two-way spirit level that slips into the hotshoe on a camera will help level the camera, particularly if it is placed on a tripod. If your camera supports interchangable viewfinder screens, using a screen with grids will help precisely level horizontal and vertical lines in an image.

Compare the two images of Seattle's Space Needle and Experience Music Project building. It will be immediately apparent that in the image on the left, the space needle is not exactly vertical. Despite using a spirit level to exactly level the camera body, the space needle ended up slanting to the left ever so slightly. The reason is keystoning effect. Since I had angled the camera up to capture this image, straight vertical lines converge to the center from either sides. Since the space needle was to the right of the frame, it appears to be leaning towards the left. To capture the image on the right, I didn't level the camera using a spirit level. Rather, I aligned the camera such that the space needle was exactly vertical. This has caused the EMP building to lean more to the right but that doesn't disturb the eye because the building lacks a strong vertical line. While I used the right edge of my viewfinder to align the space needle parallel, a grid screen for my viewfinder would have been perfect for this situation.