This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
A lens is said to have a fixed focus when near and distant objects are in focus on the ground-glass at the same time, thus doing away with the necessity of altering the distance between the lens and the sensitive plate. Only lenses of short focal length, therefore (lenses made to cover plates not larger than 4x5 inches), can be termed fixed-focus lenses. Objects nearer than 6 feet will, however, be more or less blurred (out of focus). Only the cheap forms of cameras are fitted with fixed-focus lenses, and it is seldom that these are made for plates larger than 3 1/4 x 4 1/4 inches.
A wide-angle lens, embracing the enormous angle of 1350. Manufactured by Goerz.
Achromatic, Single, or View Lens. A lens consisting of a biconvex crown glass cemented to a biconcave flint glass. Corrected for chromatic aberration, but not for curvilinear aberration or distortion.
A lens which is convex on one side and concave on the other, and thicker in the center. A sectional view of the lens has the appearance of the moon in its first quarter.
A lens having greater concavity than convexity.
A lens whose single combinations are free from astigmatic, chromatic and spherical aberration.
An anastigmat type of lens manufactured by Bausch & Lomb.
The name for a particular form of anastigmat lens originated by Zeiss. Manufactured in the United States by Bausch & Lomb.
A lens whose working aperture is large as compared with its focal length is termed a rapid lens. In fact, any lens allowing a large number of rays of light to pass through to the sensitive plate is a rapid one. The greater the rapidity of the lens the less is the depth of focus and definition, and for this reason any adjustment which increases the rapidity of the lens necessarily tends to decrease the depth of focus.