19 Aquarii 19 Aquarii 19 Aquarii is a star in the constellation of Aquarius. With an apparent magnitude of about 5.7, the star is barely visible to the naked eye (see Bortle scale). Parallax estimates made by the Hipparcos spacecraft put it at a distance of about 300 light years (92 parsecs) away from the Earth. 19 Aquarii has a spectral type of A8V, meaning it is an A-type main-sequence star. These types of stars are bluish-white colored, and have effective temperatures between 7100 and 11500 K. It rotates fairly fast, as its projected rotational velocity is about 155 km/s, so it
Hyperfocal distance to infinity, if the lens is focused to "H"/2, the depth of field will extend from "H"/3 to "H"; if the lens is then focused to "H"/3, the depth of field will extend from "H"/4 to "H"/2, etc. Thomas Sutton and George Dawson first wrote about hyperfocal distance (or "focal range") in 1867. Louis Derr in 1906 may have been the first to derive a formula for hyperfocal distance. Rudolf Kingslake wrote in 1951 about the two methods of measuring hyperfocal distance. There are two common methods of defining and measuring "hyperfocal distance", leading to values that differ only slightly.