sensitivity. Highly sensitive film is called fast film; film with low sensitivity is
called slow. For instance, ISO 400 film is relatively fast, and thus requires less
light for good exposure than slower ISO 100 film.
It’s important to understand that all of these factors are interrelated and of
equal importance when establishing exposure. But shutter speed and lens aperture
are the factors that are most often considered since they can be adjusted
from shot to shot.
The relationship between the lens aperture and shutter speed is key to understanding
good film exposure. The combination of these controls determines
just how much light actually reaches the film.
Remember that each full f-stop or full shutter speed setting lets in half as
much light as the full setting before it, and doubles the light of the full setting
after it. Thus a lens aperture set at f/8 lets half as much light through the lens
as one set at f/5.6 and twice as much as one set at f/11; a shutter speed of 1/125
lets in light for half as much time as 1/60 and twice as much time as 1/250.
Each halving or doubling of light is called a stop. So changing the lens aperture
from f/8 to f/11 is making a one-stop difference, as is changing the shutter
speed from 1/125 to 1/60.
In short, the f-stop and shutter speed have a reciprocal relationship. By adjusting
one setting in a particular direction, while adjusting the other by the
same amount in the opposite direction, you keep the total quantity of light striking
the film the same. Thus, you can make the lens aperture one stop smaller Stop
The term stop is broadly used in photography to represent a doubling or halving
of light. For example, you might hear someone say “give it one more stop”
or “cut exposure by a couple of stops.” Probably the most common use of this
term is to indicate a change in the lens aperture, where each full f-stop adjustment
is called one stop. But the term also is commonly used to refer to adjusting
shutter speed or anything that will affect exposure by the equivalent of
doubling or halving the amount of light striking the film.
Changing your shutter speed from 1/250 to 1/125 is referred to as increasing
exposure by one stop, assuming you make no change in lens aperture. Setting
it at 1/60 is a two-stop increase. ISO 400 film is one stop faster than ISO 200
film (because it provides twice the sensitivity to light) and two stops faster than
ISO 100. (letting in half as much light to strike the film), but lengthen the shutter speed
by one stop (letting light strike the film for twice as long). If the correct exposure
for a scene is f/8 at 1/125, all of the following combinations of f-stop and
shutter speed settings will produce equivalent exposures:
f/22 at 1/15 (small lens aperture and slow shutter speed)
f/16 at 1/30
f/11 at 1/60
f/8 at 1/125
f/5.6 at 1/250
f/4 at 1/500
f/2.8 at 1/1000 (large lens aperture and fast shutter speed)
For instance, by adjusting the settings from f/11 at 1/60 to f/8 at 1/125,
you’ve opened up the lens aperture to let in twice the light (changing f/11 to
f/8), but increased the shutter speed to let the light through only for half the
time (changing 1/60 to 1/125).Note that each setting may affect the look of the final image, so you must
make your choice according to the needs of each picture, remembering that lens
aperture controls depth of field, while shutter speed affects the appearance of
movement or motion. For instance, if you want everything in a landscape to be
in focus from near to far, maximize depth of field by choosing a small lens aperture,
such as f/16 or f/22. Conversely, to stop the movement of a running horse,
choose a fast shutter speed, such as 1/1000 or 1/2000.
Remember that each control also affects the total amount of light: The
smaller you set the lens aperture, the slower you must set the shutter speed to
maintain correct exposure; the larger the lens aperture, the faster the required
shutter speed.
It’s possible that your choice of one f-stop or shutter speed over another may
affect the final results in a way you did not intend. Choosing a small lens aperture
may require a slow shutter speed, which can lead to image blur if the
camera or subject moves during exposure. For example, if the correct exposure
is f/4 at 1/60 and you adjust the lens aperture to f/5.6 for more depth of field,
you will need to slow the shutter speed to 1/30 to maintain correct exposure—
and 1/30 may be slow enough to blur the image if your camera or subject
moves during exposure.
Choosing a fast shutter speed may require a large lens aperture, which could
result in shallow depth of field. In the same example, if the correct exposure is
f/4 at 1/60 and you adjust the shutter speed to 1/125 to better freeze the action,
you will need to open up the lens aperture to f/2.8 to maintain correct exposure—
and f/2.8 may result in depth of field that’s too shallow to keep the whole
subject sharp. Film speed has an important role in exposure, because it determines the f-stop
and shutter speed settings. Fast films are more sensitive to light than slow films,
so they require less light for proper exposure, meaning you can use a smaller
lens aperture or a faster shutter speed.
Simple math will tell you just how much faster one film is than another. Film
rated ISO 400 is four times faster (more sensitive to light) than ISO 100 film
(400 ÷ 100 = 4). Four times more light is a difference of two stops: two f-stop
increments, two shutter speed increments, or one of each. Thus, if the correct
exposure is f/5.6 at 1/500 with ISO 400 film, it will be f/2.8 at 1/500, f/4 at
1/250, or f/5.6 at 1/125 with ISO 100 film.
Since fast films need less light, you’re more able to use settings for greater
depth of field and/or less camera or subject movement than with slow films.
And the relatively large lens apertures and slow shutter speeds needed by slow
films are likely to result in more shallow depth of field and/or more camera or
subject movement.
While these are good reasons to choose one film speed over another, keep in
mind another key consideration: Slow films produce images with finer grain
than fast films.
A light meter measures the subject lighting and suggests an f-stop and shutter
speed that should produce the correct exposure for the film speed you are
using. Most 35mm cameras have a built-in meter, called a through-the-lens
(TTL) meter, because it measures the light that passes through the camera lens.
TTL meters usually provide exposure recommendations on either an external
LCD display and/or in the camera’s viewfinder. Separate handheld light meters
also are available.
To use a meter, you must first set the speed (ISO) of your film. The meter
won’t know what settings to recommend unless it knows how much light the
film needs. Most modern 35mm cameras set the ISO automatically. When you
load your film in the camera, an internal sensor reads a bar code printed on the
film cassette that signals the ISO of that film. The bar code is known as a DX
code; virtually all 35mm films have one.
If your camera does not have a DX-code reader, or if you’re using a handheld
meter, you will have to set the film speed manually. Check your camera or
meter’s instruction book for specifics on setting the ISO. On many older
cameras, you set the film speed using a window located on the same dial as the
shutter speed settings. On handheld meters you set the ISO on a dial, but on
some newer model meters and cameras you use a button and a display panel.
Once the film speed is set, you’re ready to take a light reading. Many TTL
meters are activated once you turn on the camera, so you will get a reading just
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