Photographic Exposure Triangle - Wild Ones | Illinois Prairie Chapter

Photographic Exposure Triangle - Wild Ones | Illinois Prairie Chapter
EXPOSURE TRIANGLE
Photographic Workshop – Wild Ones – Sugar Grove Nature Center
Introduction
With a modern digital cameras set to
automatic modes, many things happen
when you press the shutter.
Lens is focused
Light sensors measure the light and
then the following are set
according to parameters specified
in different program modes:
• ISO speed (light sensitivity of
the digital “film” sensor
• Shutter speed (length of
duration of the exposure)
• Lens aperture / f-stop (how
much light is allowed through
the lens)
In most circumstances, the resulting
images are quite good, and the electronics
in each new generation of digital cameras
are increasingly sophisticated. However,
photographers need to understand the
Exposure Triangle in order to capture
images that meet their expectations.
Be sure to read in the instruction manuals
for your digital cameras how the different
program modes manipulate ISO, shutter
speed, and f-stop to suit certain
photographic situations. In addition, most
simple digital cameras let you have some
control over the ISO setting
Many also have “exposure compensation”
settings that allow you to make images
that are lighter or darker than the regular
automatic setting. . Since I prefer images
slightly darker and richer, I always leave the
exposure compensation set to negative 0.7 on
all of my digital Nikon cameras.
Single-lens reflex (SLR) and more
expensive compact cameras have settings
for ISO speed, “aperture priority,”
“shutter priority, and full manual control.
Creative photographers need to make full
use of these options.
Shutter Speed
The shutter speed is the period of time
that light is allowed to reach the film or
digital sensor. This is typically a fraction
of a second, such as 1/30, 1/60, 1/125,
1/250, 1/500, 1/1000. Slower shutter
speeds are best for low light conditions,
while high works best in bright light and
to stop action.
Aperture / f-stop
The aperture is the diameter of an iris
diaphragm in the lens that controls the
amount of light passing through the lens.
It is analogous to the iris of an eye. The
numbers used are called f-stops, and each
step to a higher number lets in half as
much light as the preceding. The normal
range is: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22,
32.
The aperture controls depth-of-field, with
low numbers having shallow depth-offield and high numbers having greater
depth-of-field.
ISO
In film, ISO is the speed of a film, that is,
how sensitive the film is to light.
Examples in increasing sensitivity: 25, 64,
100, 200, 400. .
In digital photography, ISO is the
sensitivity of the image sensor. The range
is much greater: 64, 200, 400 ,800, 1600,
3200, 6400.
The lower the ISO number, the higher the
image resolution (allows for larger
enlargements), the better the color, and
the finer the grain / noise. A higher ISO
number allows the camera to stop action
in low light conditions.
EXERCISES
1. Take a series of photos of the same
subject in the same light
conditions by varying the setting
for ISO speed.
2. Set a camera on a tripod and frame
a subject that has depth. Set the
ISO to a low setting, such as 64 or
100. Take a series of photos with
the camera set to “shutter
priority” and taking an image with
the shutter speed at 1 second,
1/15, 1/60, 1/125, 1/250, 1/500,
1,000 (or whatever range your
camera allows).
3. Repeat with camera set to
“aperture priority” and changing
the aperture (f-stop) from 4, 5.6, 8,
11, 16, 22, 32 (or whatever range
your camera allows).
Kenneth R. Robertson
10 June 2013
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