Figure 1-26.Focusing for one object.
I M A G E P O I N T S
( C I R C L E
C O N F U S I O N )
Figure 1-27.Image on film in front of and in back of the point of sharp focus.
FOCUSING FOR ONE OBJECT.Focusing is
done essentially to obtain the proper distance between
the lens and the film. When light rays come from a far
object and pass through a lens, they form a sharp image
close to the lens. When light rays come from a near
object, they form an image farther away from the lens.
This means that the lens must be focused on either the
far or the near object, depending on which one the
photographer wants to have in sharp focus. When a
sharp image of the near object is desired, the lens should
be focused by moving it farther away from the film.
When you want a sharp image of the far object, move
the lens closer to the film (fig. 1-26).
CIRCLE OF CONFUSION.A picture is
basically an accumulation of many points that are exact
images of points composing the subject. After light
strikes a subject, it is reflected from many points on the
subject. A camera lens redirects these reflected rays into
corresponding points on the film. Each of these points
is reproduced by the lens as a circle. When the circle is
smaller than l/100 inch, it appears as a sharp point to
the eye. When the circle is larger than 1/100 inch, the
eye sees it as a circle, and the image is blurred or out of
focus. Each out-of-focus circle on the film is called a
circle of confusion and can be visualized as the cross
section of a cone of a light ray (fig. 1-27).