Scrying is the use of magic to view images of situations remote from the viewer in space, and, potentially, in time. This is one of several techniques of clairvoyance known to unicorn magic, and the one affording the longest range. It has the disadvantage of requiring equipment, however, unlike the simple use of the unicorn's magical senses for remote viewing.
Range Limitations
In the late classical period of pony history, noted crystallomancer and optician Neighstradamus (f. 200 AE) set forth the Scryer's Equation, which establishes the theoretical limits on scrying range. The material of the crystal ball, the radius of the ball, and its deviation from perfect sphericalness affect the resolution available at a given range. A near-perfect sphere of a meter in diameter is sufficient to scry anywhere on Equus; a small crystal ball produced by ordinary means of a few centimeters diameter has an effective range of a few kilometers. The Scryer's Equation is a limit; an actual unicorn will fall short of the range theoretically attainable by a greater or lesser degree depending on her magical acumen.
Limitations on the Target
A place or thing can be scryed. The scryer must be familiar with the place or thing; it is acceptable for the familiarity to be separated by time. (E.g. a picture of someone as a child can be used to scry them as an adult.) Direct familiarity with the present situation of the target makes it much easier to get a "lock" on the target.
Effect Produced
The thing scryed upon appears in the crystal ball. Others, not just the mage, can see the image, and it can be recorded impersonally (e.g. with a photographic camera.) The crystal ball gives a 3D view of the subject; by moving around the ball, one can see it from different angles (the angle and field of view can also be adjusted by the mage.) Telescopes can be aimed at a crystal ball in use, as can other sorts of optical instruments, although they must have a working distance adequate for the radius of the ball, which sets a limit on how close the external optics can approach the thing scryed. (E.g. an ordinary microscope would not work because its objective lens would be too far away from the target of the spell, and the surface of the crystal ball would prevent the microscope being brought close enough.)
The scrying effect is one-way; it is not possible to shine a light on the crystal ball and thereby illuminate the target remotely. Further, as the scrying image is not generally as bright as the thing observed and it is best conducted in a darkened room.
Detection and Countermeasures
An area can be shielded against scrying with appropriate enchantments. Magically sensate creatures can detect the onset of scrying and the maintenance of the effect, and potentially also the perturbation in the local magical field associated with the existence of the scrying "wormhole." Although designed to be one-way, it can be traced by a skilled mage, and used to obtain information about the scryer's direction and very approximate distance. If a crystal ball is available to horn, the scryer may even commence counterscrying, using the existing conduit to circumvent the ordinary requirements for information about the location or thing to be scryed. Sophisticated scrying spells in turn include counter-countermeasures to detect tracing in progress, counter-counter-countermeasures to disguise the tracing, and so on.
Scrying Diachronically
The past can be scryed, but it is much more difficult than scrying the present (i.e. synchronically) and fails more often. It is theorized that the mere presence of a skilled counterscryer causes some attempts to scry back in time to fail, because this person would have detected the scrying attempt and reacted; since they did not, the orderly, paradox-free progression of causality demands that no successful scrying took place, and it is impossible for any future-scryer to scry that person or her surroundings except with methods beyond the target's ability to detect.
The future can also be scryed, a form of divination. It is considered dangerous, and, in more civilized times, was so highly illegal as to cast the whole field of scrying into some degree of disrepute despite its utility. It is generally believed that scrying a point in the future fixes that event irrevocably. One of the catalysts for the banning of divination was a noble-minded attempt in early Equestria to use future-scrying to detect and avert crimes and accidents. Not only was it found impossible to prevent any of the incidents foreseen, but many bizarre and improbable secondary accidents plagued attempts to do so.
Although one tends to see the most probable outworkings of the present situation in the future, it is thought that the expectations of the scryer might influence to outcome as well. This theory was put forth after statistical analysis of the future accident prevention experiment showed a precipitous increase in the specific kinds of unfortunate events the scryers were looking for, but further study of the concept was quashed by the government, citing ethical concerns and possible further unknown dangers associated with scrying the future.