Figure 2-19.
Image points nearer to and farther from focal point.
If the lens is then moved nearer to or farther from the ground glass or film,
the light rays intersect in front of or behind the ground glass or film, and
the image is blurred. This blur, caused by overlapping circles of confusion,
can be overcome by changing the distance between the lens and the focal plane
to give a minimum circle of confusion. This is called focusing.
(2) Lens speed.
Another factor controlling circles of confusion is the
speed or aperture of a lens.
The smaller a lens opening, the less light
reaches the film and the narrower the light rays falling on the film.
The
narrower these rays, the smaller the circles of confusion. In practice, this
means that a small lens opening permits the recording of several objects at
varying distances.
Even if the rays from some objects do not intersect
perfectly at the film plane, the circles of confusion intersecting before or
behind the film are negligible and still appear sharp.
(3) Photographic prints.
Another factor to consider when dealing with
circles of confusion is the manner in which the negative is to be printed.
Negatives used primarily for contact printing do not require as small a circle
of confusion as negatives from which enlargements are to be made.
f.
Size of Circle. A circle of confusion that is 1/250 inch in diameter is
generally small enough for negatives made with 4x5-inch cameras. For miniature
cameras, a 1/1000-inch circle of confusion is acceptable. A uniform standard
of definition in enlarging is automatically obtained by establishing the value
of the circle of confusion as 1/1000 of the focal length at the photographic
recording lens of larger cameras, and 1/2000 of the focal length for miniature
cameras. This is better than using arbitrary figures that vary with different
focal length lenses.
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