Diffusion of Focus

When an image is not sharp, or in other words, when the image is dull or spread out, it is said to be diffused. Certain portrait lenses are so constructed as to render a slightly hazy, soft appearance to the portrait, thus the focus is diffused.

Dividing the Focus

There are times when it is desired to have near and distant objects in sharp focus, but the lens will not sharply cut the objects located at these extremes. The usual course of procedure is to focus on an object approximately midway between the two extremes and then use a stop small enough to have all objects sharply defined. This is termed dividing the focus.

Equivalent Focus

Focal Length, or Focal Distance

An optical term applied to a doublet lens. The distance from the optical center of a lens and the ground-glass when focusing on an object over one hundred feet away.

Principal Focus

The point at which parallel rays of light meet after having passed through a convergent lens. The distance from the lens to this point is termed the focal length of the lens.

Visual Focus

The point at which (with any given lens) the most strongly visible light rays come to a focus.

Focusing Cloth

An opaque cloth to throw over the head and camera to exclude stray rays of light while focusing.

Focusing Glass

A magnifying lens used while focusing to aid in securing sharpness.

Focusing Scale

A graduated scale fastened to the baseboard of the camera which allows of focusing for any given distance without looking on the focusing screen.

Focusing Screen

A ground-glass placed in the back of a camera on which the image is cast. The image is focused on this before exposing the plate.


A darkening of the silver salts either all over the plate or in certain portions which should not have been affected by the light.

Chemical Fog

Fog caused by too energetic a developer too much alkali in the developer or other spontaneous reduction of the sensitive silver salts independent of the action of light.

Dichroic Fog

A difficulty met with in negative making a deposit on the film which appears red by transmitted light and green by reflected light. Generally due to the plates being old, or through imperfection of manufacture; also to the forcing of development.

Light Fog

Caused by the use of an unsafe light in the dark-room; also by defects in the camera or plate-holder, over-exposure or accidental exposure to light.


Bringing out detail in an under-exposed plate by using an additional amount of accelerator during the process of development. Care must be exercised, however, in the addition of an accelerator, for chemical fog is very liable to be caused.


CH2O - II, 805; IV, 20.

Formic Aldehyde or Methytic Aldehyde, or Formaldehyde. Clear, colorless liquid with pungent odor. Use. Renders gelatin insoluble to even boiling water and for this reason is used for hardening films, preventing frilling, blistering, etc.