The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

	Anastigmatic	Angle
	Anastigmatic (dud, in the sense of back, and vriypri, a dot or a mathematical point). A lens should be said to be anastig-matic - or simply stigmatic - when it refers every point on the scene accurately to a corresponding point on the plate ; but in practice the terms are used as signifying freedom from that aberration called astigmatism. (See Lens.) Angle, Critical. See Critical Angle. Angle, Mid. This is a term applied to a lens which, for a plate of the same size, has a focal length intermediate between the ordinary rapid rectilinear and the so-called wide-angle. Angle of View, or Width of Angle. It is sometimes essential to know the angle inclosed by a lens, and for this purpose it is only necessary to divide the diameter of its field by the focus, when reference to such a table as the following will at once give the result: -
	Soret's Tables of Angles included by a Lens.
	As the full field of the lens would be a circle corresponding to the diagonal of the plate, this diagonal would be taken by some 33                                     D