

Anastigmatic 
Angle 



Anastigmatic (dud, in the sense of back, and vriypri, a dot or a mathematical point). A lens should be said to be anastigmatic  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 socalled wideangle.
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 


