Choosing ISO/ASA, Shutter Speed, and Aperture

Choosing ISO/ASA, Shutter
Speed, and Aperture
Rebecca Scoggin McEntee
The Camera Operates Like Eyes
Camera Obscura: dark room
Your Cornea is like the front
element of a lens.
Together with the lens they are the eye's focusing
elements. The cornea takes widely diverging rays
of light and bends them through the pupil, the
round opening in the central portion of the
colored iris.
The iris and pupil act like the
aperture of a camera.
The iris is a muscle which allows adjustable control of the
quantity of light entering the eye so that the eye can work
in a range of viewing conditions, from dim to bright light.
The retina is the sensory layer
that lines the very back of our
It acts like the imaging sensor chip in a digital camera
Light adjustment:
Both the eye and the camera can adjust quantity of
light entering the camera or eye.
On a camera it is done with the aperture control built
into the lens, while on the eye, it is done by making the
iris of the eye smaller or larger.
The eye is a subjective device.
Eyes work in harmony with the brain to create the images
The eye is the best automatic
The brain even chooses it’s film speed. If you are outside
in bright light, you squint to protect your eye from getting
too much light into the eye.
If it is night time, your eyes are wider. Your brain is
helping to choose which amount of light your eye needs
to be ready to receive. In that way, you choose your own
• Old: American Standards Association (ASA)
• New: International Organization for
Standardization (ISO)
• Based on earlier research work by Loyd Ancile Jones
(1884–1954) the American Standards Association (now
named ANSI) defined a new method to determine and
specify film speeds of black-and-white negative films in
The ISO difference in eyes:
Eyes do not have definite ISO levels, they are flexible
Eyes do have a great ability to naturally adjust to ambient light
levels even under the most severe lighting conditions
The human eye has special abilities: it can modify its own light
sensitivity. After about 15 seconds in lower light, our bodies
increase the level of rhodopsin in our retinas
So eyes are around 600 times more sensitive at night than
during the day
Inside buildings, typically your
ASA/ISO needs to be at a higher
level, such as 800 or 1600.
Even 3200 if you have a good
camera that will keep good
resolution in the dark.
Outside in brighter light, you can
choose a lower number such as
100 or 400, or something in
Eye shutter speed
• In a still camera a single “snapshot” of the
scene relies on an exposure determined by
aperture and shutter speed
• Our own eye/brain combination refreshes
every 1/50th of a second.
Choose manual – “M”
After ISO, choose Shutter, then
Curtain shutters
Leaf Shutters
Some didital controls for ISO, shutter
speed and aperture
Electronic shutters just turn the
sensor on and off to capture the
exposure much like the brain
The image builds up as light is
captured by the sensor.
Normally, after you have chosen your
ISO, you will choose your shutter
speed, depending on the action you
might be photographing.
With more experience you can choose
aperture first, to establish depth of
Which do you want?
You choose a fast or slow
shutter speed depending on
whether you want to stop the
action or not:
(you can practice on
To stop action of talking subject,
you may have to choose a shutter
speed of 125.
A sport will require and increase in
shutter speed to 500 or more.
Apertures are next: this is f.32
Once you have chosen your shutter
speed, you look at your meter and
watch the pluses and minuses move
as you move the dial on the camera
that controls them, to see which
f.stop gets you closer to the mid
range of the meter.
The Light Meter in Camera
• A light meter is an instrument inside your
camera that tells you if the amount of light
reaching the film will be enough or too much
to properly expose your image. The light
meter takes into account your shutter speed,
film speed, and aperture settings.
Light Meter Inside the Camera
Depth of Field
• Depth of field (DOF) is the distance between
the nearest and farthest objects in a scene
that appear acceptably sharp in an image.
• The higher the number on the f.stop, the more
depth of field in general.
Apertures and Depth of Field
Various Depth of Field
on same subject
Now you know why and how to
choose ISO, shutter speeds,
and f. stops.

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