only exception is when a manufacturer is describing the largest opening a
lens has, in which case you might see expressions such as "f/1.7" or
"f/4.5," to name two common maximum lens openings.
These numbers are
accurate because the lens parts are machined to that degree of precision
when it is made.
By the way, when f/numbers are used to describe the
largest opening a lens can provide, the term is called the "lens speed," so
if you see an expression such as "an f/1.4 lens," you know that the largest
aperture the lens can be opened to is f/1.4.
5.
So there you have it. Shutter speeds and f/stops are coordinated to
give you a whole variety of choices when you actually make a photograph.
There will be more about how to decide on a specific choice in Lesson 3.
Learning Event 5:
DESCRIBE THE USE OF A DAYLIGHT EXPOSURE TABLE
1.
We've talked a lot about how to control exposure by changing f/stops
and shutter speeds, and we've talked a lot about equivalent exposures, but
how do you find a correct exposure in the first place? Now you are going to
learn just that. First you have to understand something about film. Film
isn't just film.
It comes in various speeds, and before you try to
determine an exposure, you first need to know the film speed.
"Speed" is
just photographer's jargon for a film emulsion's sensitivity to light. It
is determined by a complex and careful procedure established by the
International Standards Organization (ISO). Every film manufacturer in the
free world rates film using the numbering system established by the ISO.
a. The system is set up so that as the speed numbers double, the film
sensitivity also is doubled.
For example, ISO 400 film is four times as
sensitive to light as ISO 100 film.
(How many stops is this?)
Here's
another example:
If you are shooting ISO 125 film and switch to ISO 64
film, how would you adjust your exposure? (Right, you go on as before, but
you open the lens exactly one stop.) That's why ISO 400 film is "one stop
faster" than ISO 200 film.
b. The ISO system also gives you a really handy way of figuring
out
outdoor exposures without using a light meter. Look at Table 1-7. You
use
this table by first setting your shutter to 1 over the ISO number. That
is,
if your film is ISO 125, start with 1/125 second. If the ISO is 64, use
the
closest speed you have, i.e., 1/60 second.
2.
Now that you have your shutter speed, you need to determine how bright
your subject is. That's the vertical column at the left side of the table.
Here is a breakdown of what those subject brightness labels mean.
a. Average subject. An average subject is one that reflects about 18
percent of the light striking it and absorbs the rest.
This category
includes people in medium-colored clothing, most buildings, landscapes with
trees, and street scenes.
The majority of photographic subjects are of
average brightness.
12
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