Learning Event 4
CALCULATE APERTURE OPENINGS
1. The aperture diaphragm is an adjustable device made of thin, curved,
metal blades which overlap each other which function much like the iris of
your eyes (fig 3-3).
The aperture controls the intensity of the light,
through the lens.
Figure 3-3.
Iris and iris diaphragm (aperture)
a. The diameter of the diaphragm opening is controlled by the aperture
ring which is scaled in increments referred to as f/stops.
Rotating the
aperture ring in the direction that reduces the diameter of the diaphragm
opening is referred to as stopping down and moving the aperture ring in the
direction which increases the diameter of the diaphragm opening is referred
to as opening up.
b. When the diaphragm is set at the largest aperture, the lens is said
to be wide open.
This largest opening is the maximum working aperture of
the lens and is called the lens speed or speed of the lens. The lens speed
of any lens is determined by dividing the lens focal length by the diameter
of the lens.
A lens with a focal length of 100mm and a lens diameter of
25mm will result in a lens speed of 4. The largest opening and lens speed
of this lens is expressed as f/4.
2. The f/system is a factorial system devised for the marking of the
f/value of the many different lens aperture sizes possible. These f/value
markings are found on the aperture ring and are referred to as f/stops. The
f/value is determined by dividing the diameter of the diaphragm opening by
the lens focal length. Thus a lens set at f/8 means that the diameter of
the diaphragm opening is 1/8th of the lens focal length. F/16 would be a
lens diaphragm diameter opening of 1/16th of the lines focal length. As the
f/stop increases in size the diameter of the diaphragm decreases in size and
visa-versa (fig 3-4).
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