Long-exposure photography or time-exposure photography involves using a long-duration shutter speed to sharply capture the stationary elements of images while blurring, smearing, or obscuring the moving elements. Long-exposure photography captures one element that conventional photography does not: time. The paths of bright moving objects become clearly visible. Clouds form broad bands, head and tail lights of cars become bright streaks, stars form trails in the sky and water smoothes over. Only bright objects will form visible trails, however, dark objects usually disappear. Boats during daytime long exposures will disappear, but will form bright trails from their lights at night.
Whereas there is no fixed definition of what constitutes "long", the intent is to create a photo that somehow shows the effect of passing time, be it smoother waters or light trails. A 30 minute photo of a static object and surrounding cannot be distinguished from a short exposure, hence, the inclusion of motion is the main factor to add intrigue to long exposure photos. Images with exposure times of several minutes also tend to make moving people or dark objects disappear (because they are in any one spot for only a fraction of the exposure time), often adding a serene and otherworldly appearance to long exposure photos.
When a scene includes both stationary and moving subjects (for example, a fixed street and moving cars or a camera within a car showing a fixed dashboard and moving scenery), a slow shutter speed can cause interesting effects, such as light trails.
Long exposures are easiest to accomplish in low-light conditions, but can be done in brighter light using neutral density filters or specially designed cameras.
Long-exposure photography is often used in a night-time setting, where the lack of light forces longer exposures, if maximum quality is to be retained. Increasing ISO sensitivity allows for shorter exposures, but substantially decreases image quality through reduced dynamic range and higher noise. By leaving the camera's shutter open for an extended period of time, more light is absorbed, creating an exposure that captures the entire dynamic range of the digital camera sensor or film. If the camera is stationary for the entire period of time that the shutter is open, a very vibrant and clear photograph can be produced.
In this technique, a scene is kept very dark and the photographer or an assistant takes a light source—it can be small penlight—and moves it about in patterns. The light source can be turned off between strokes. Often, stationary objects in the scene are illuminated by briefly turning on studio lights, by one or more flashes from a strobe light, or by increasing the aperture.
Water and long exposure
Long exposures can blur moving water so it has mist-like qualities while keeping stationary objects like land and structures sharp.
Solargraphy is a technique in which a fixed pinhole camera is used to expose photographic paper for an extremely long amount of time (sometimes half a year). It is most often used to show the path taken by the sun across the sky. One example of this is a single six-month exposure taken by photographer Justin Quinnell, showing sun-trails over Clifton Suspension Bridge between 19 December 2007 and 21 June 2008. Part of the Slow light: 6 months over Bristol exhibition, Quinnell describes the piece as capturing "a period of time beyond what we can perceive with our own vision." This method of solargraphy uses a simple pinhole camera securely fixed in a position which won't be disturbed. Quinnel constructed his camera from an empty drink can with a 0.25mm aperture and a single sheet of photographic paper.
On February 3, 2015 a pinhole camera used in a Georgia State University solargraphy art project was blown up by the Atlanta bomb squad. The device, one of nineteen placed throughout the city, had been duct-taped to the 14th Ave. bridge above I-75/85; traffic was shut down for two hours, and the remaining cameras were later removed by authorities.  
- School of Photography
- Greenspun, Philip (January 2007). "Studio Photography". Photo.net.
- Digital Photography Review
- Lucy Dodwell (4 October 2008). "Watching the sun go by". New Scientist (Reed Business Information) 200 (2676).
- "Stunning photographs of landmark captured over six-month period". London: Telegraph. 21 November 2008. p. 1. Retrieved 9 September 2010.
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Media related to Long-exposure photography at Wikimedia Commons
- Long Exposure Photography Ideas
- Article about series of solargraphs in the ESO Messenger
- Bob Fosbury's Solargraphs
- Tarja Trygg's Solargraphy page
- Long Exposure Photography inspirations
- Solargraph taken from Bayfordbury, UK at the APOD archive
- TIME IN A CAN International project of solargraphy
- Polish solargraphy forum