35
Chapter 3 Lenses
converge or focus at the same spot. This com-
mon spot is called the image point. The fi lm
or sensor in the camera is at this point. See
Figure 3-12.
A simple lens is made of a single piece
of glass or plastic. It may not meet all of the
Light ray
Plate glass
Ray bent
inside glass
Ray parallel
to original path
Figure 3-9. Light rays passing through a sheet of
glass bend, but exit traveling parallel to the incident
light.
Ray emerging
from other
surface
Incident ray
striking one
surface
Figure 3-10. Light rays passing through a prism are
bent at each surface.
Figure 3-11. A simple lens acts like two prisms
placed base-to-base.
requirements for good photographs. Several
lenses that refract light differently may be
necessary to correct defects.
The Compound Lens
A compound lens is made up of two or
more simple lenses, as shown in Figure 3-13.
Each simple lens in a compound lens is called
an element.
Individual lenses are classifi ed into two
major groups:
• A positive lens, also known as a
convergent lens is thicker at the center and
gathers light. See Figure 3-14. Convergent
means it bends the light inward.
• A negative lens, also known as a
divergent lens is thicker at the edges and
spreads light. See Figure 3-15. It is called
divergent because it bends light outward.
Lens Aberrations
Defects or fl aws in a lens are called
aberrations. These defects are not, in most
cases, the result of a manufacturer’s careless-
ness. They are caused by the behavior of light.
A lens maker can correct for most aberrations
by using many elements.
Coated Lenses
Modern lenses are also coated to protect
against stray light. A thin layer of a chemical
substance, usually magnesium fl uoride, may
eliminate all but about one percent of these
refl ections. The amber, blue, or magenta tints
seen on a lens indicates the presence of this
coating.
Lens Quality
Lens quality depends upon the char-
acteristics of the optical glass used and the
lens design. Making one batch of glass often
requires weeks. Even with rigid controls, most
of the batch cannot be used to make quality
lenses. The glass that can be used is remelted
and cast in molds. The rough glass casting is
ground and polished into the fi nished product.