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 defect in a lens when the aperture of a lens shows on the focusing screen as a circular patch of light. When a plate is developed, this spot reproduces practically opaque. This defect will sometimes make its appearance when the camera is pointed in the direction of the sun. It most generally occurs with cheap or inferior lenses.
A glass vessel for holding liquids and usually so constructed as to withstand heat.
(See Support, Flexible or Temporary.)
(See Glass, Flint.)
(See Floral Photography.)
(See Lens, Fluid.)
An optical term signifying that objects from one point of view appear of a different color or tint from that of another.
(See Focus, Equivalent.)
(See Shutter, Focal Plane.) Foci.
A lens has two focal points which have a definite relation, i. e., the points on both sides of the lens at which are situated the object being photographed, on the one side, and the ground-glass or sensitive plate on the other. When reproducing an object actual size, the conjugate foci is double the focal length of the lens and both the ground-glass and object will be located this distance from the center of the lens combination.
An optical term meaning a point in which any number of rays of light meet after being reflected or refracted. An image is said to be in focus when all details are sharp and well defined on the ground-glass.
The distance between the back glass of a lens and the focusing screen, when a distant object is sharply focused upon the screen.
The power of a lens to sharply reproduce near and distant objects at the same time. A lens working at full aperture seldom renders near and distant objects perfectly clear at the same time. The use of a stop or diaphragm which will cut off certain rays of light will give greater depth of focus, but requires increased exposure.