Lesson 3/Learning Event 8
Learning Event 8
DEFINE FOCUS
1. When a lens is focused on a point, only that point in the geometric
plane, without depth, is in true sharp focus.
The law of physics allows
depth to be added to this plane in various degrees as determined by the lens
focal length, f/stop, distance focused on, and the permissible size of the
circle of confusion.
2. Circle of Confusion.
The focus of an image on the focal plane film
plane is determined by the size of the circle of confusion. Basically, an
image is an accumulation of many points.
For example, light is reflected
from a subject, refracted by the lens and reproduced on the focal plane as
circles.
a. Light rays seem to produce a "cone" of light " (fig 3-7). The apex
of the cone originates at a point on the subject; the base of the cone is at
the lens.
When light rays pass through the lens, the cone of light is
reversed. The base is still at the lens, but the apex now lies in the focal
plane. An infinite number of these cones combine to produce an image.
Figure 3-7.
Circle of confusion
b. The smallest circle of confusion should intersect at the focal plane
for the best acceptable sharpness. If the cone of light intersects either
in front or behind the focal plane, the light rays form circles rather than
points. These are called circles of confusion. When the circles are small
enough, they are said to be acceptably sharp to the eye and are in focus.
c. If you find circles of confusion perplexing, take a camera and focus
on a point. Now turn the focusing ring and you will notice that the point
enlarges and becomes out of focus.
The more the point enlarges, the more
out of focus it becomes. Now turn the focusing ring to bring the point back
into focus and notice the point decreases in size back to normal as it
becomes more focused.
d. You are not expected nor required to carry a ruler around and do
measurements on circles of confusion to determine if a photograph is in
focus or not.
Everyone will have a different acceptance of what is sharp
focus. However, if the circle is smaller than 1/100 inch on the print, it
appears as a sharp point to the eye.
If the circle is larger than 1/100
inch, their eyes see it as a circle and the image is blurred or out of
focus.
e. The size of the permissible circle of confusion depends on the film
format size and the manner in which the film will be used. The generally
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