b. Think about the formula shown in Figure 1-2. If you have less light
(intensity), then you must allow more time to get the same exposure. Or, if
you have less time, then you must increase the intensity.
It is obvious
there are many combinations of time and intensity which will give the same
exposure.
Any two combinations which do this are called equivalent
exposure.
From the standpoint of merely darkening a piece of film, one
equivalent exposure is just as good as any other.
But there are artistic
considerations involved in making a good photograph which can make one
equivalent exposure distinctly better than another.
You will learn more
about this in Lesson 3.
Learning Event 2:
DESCRIBE HOW TO CONTROL LIGHT INTENSITY
1.
You know you can control exposure by controlling intensity and time.
But how is this done? Let's first talk about controlling light intensity.
a. A lens is a lot like a window.
The bigger the window, the more
light will come in. It's the same with a lens. The larger the opening in
the lens, the greater the light's intensity on the film.
And, like a
window, you can control the intensity by opening and closing the opening in
the lens (called the aperture).
A diaphragm is the mechanism which opens
and closes the aperture precisely, and provides for a numerical measure of
the aperture opening.
(1) Early diaphragms were simply thin pieces of metal with various-
sized holes in them.
These holes allowed the photographer to adjust the
amount of light passing through by selecting the hole size desired and
inserting it in the lens' light path.
(2) Modern lenses use an iris diaphragm.
This works much like the
iris of your eye. A series of thin metal plates are assembled to allow the
lens to open and close continuously (fig 1-3).
Figure 1-3.
Diaphragm of a camera
3
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