mass, and the lens is moving in a straight line: the point source-point lens approximation. In these events, the only physically significant parameter that can be measured is the Einstein timescale . However, in some cases, events can be analyzed to yield the additional parameters of the Einstein angle and parallax: and . These include very high magnification events, binary lenses, parallax, and xallarap events, and events where the lens is visible. Events yielding the Einstein angle Although the Einstein angle is too small to be directly visible from a ground-based telescope, several techniques have been proposed to observe it. If the lens passes directly in front of the source star, then the finite size of the source star becomes an important parameter. The source star must be treated as a disk on the sky, not a point, breaking the point-source approximation, and causing a deviation from the traditional microlensing curve that lasts as long as the time for the lens to cross the source, known as a finite source light curve. The length of this deviation can be used to determine the time needed for the lens to cross the disk of the source star . If the angular size of the source is known, the Einstein angle can be determined as These measurements are rare, since they require an extreme alignment between source and lens. They are more likely when is (relatively) large, i.e., for nearby giant sources with slow-moving low-mass lenses close to the source. In finite source events, different parts of the source star are magnified at different rates at different times during the event. These events can thus be used to study the limb darkening of the source star. Binary lenses If the lens is a binary star with separation of roughly the Einstein radius, the magnification pattern is more complex than in the single star lenses. In this case, there are typically three images when the lens is distant from the source, but there is a range of alignments where two additional images are created. These alignments are known as caustics. At these alignments, the magnification of the source is formally infinite under the point-source approximation. Caustic crossings in binary lenses can happen with a wider range of lens geometries than in a single lens. Like a single lens source caustic, it takes a finite time for the source to cross the caustic. If