41
Chapter 3 Lenses
See Figure 3-24. A 100 mm lens will produce
an image twice the size of a 50 mm lens.
Aperture Control
The lens aperture is a circular open-
ing in a diaphragm that is usually located
between the lens elements. See Figure 3-25.
On a camera, the diaphragm is a cover made
up of metal. Its openings control the amount
of light that can pass through the lens to the
fi lm or sensor. A large aperture (opening)
will admit more light than a small aperture.
Aperture size is specifi ed by f-stops. The
f-stops are shown as f-numbers on the lens
barrel. See Figure 3-26. Some lenses made just
for digital cameras may not have any external
adjustable f-stop ring. See Figure 3-27. Inter-
nal motors control the adjustment of the aper-
ture on these lenses.
Figure 3-24. The focal length of the lens affects the image size.
Figure 3-23. A standard or normal lens gives about
the same perspective as the human eye. A 43 mm
lens, such as this one, is the perfect focal length for
a normal lens, but most manufacturers round this up
to 50 mm. (Pentax)
Image as it
appears at
focal plane
Lens with a short focal length reduces the image size.
Lens with a long focal length increases the image size.
50 mm
focal length
100 mm
focal length
Lens
Lens
Focal plane
Focal plane
Image as it
appears at
focal plane