My intro primer on the basics of photographic exposure.

1. Introduction to
Photographic Exposure
Michael E. Stern
Photographer Educator Trainer
818-422-0696
CyberStern.com
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2. Introduction to
Photographic Exposure
Exposure: The combination of time, intensity, light sensitivity (ISO) of the medium and
local light energy that creates an image onto light sensitive material.
Time X Intensity = Exposure
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3. Introduction to
Photographic Exposure
Proper Exposure: The specific combination of shutter speed (time), aperture (intensity),
ISO (light sensitivity) and light energy that provides the photographer with the sought
after look, effect, mood or feel.
The specific exposure combination is driven by the technical and reproduction
requirements of an assignment.
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4. Introduction to
Photographic Exposure
Exposure is the sum of these four factors:
1) Illuminance/local light energy: The factor that you typically have no control over. For example,
direct sunlight, standing in open shade, the light of a candle or the brightness during overcast conditions
You can control the amount of light by adding flash, moving you and/or your subject or waiting for different
weather, time of day, etc.
2) Shutter speed: Should be thought of as the time or duration and is usually measured in 1/3 stop
increments. Denoted as fractions of a second: 1/2, 1/50, 1/4000 and so on. Can also be denoted in full
seconds, minutes and hours.
3) Aperture/intensity: Also referred to as f/stop and is the size of the lens opening through which light
passes. Usually measured in 1/3 stop increments.
4) ISO/light sensitivity: This setting controls how well the chip reacts to the combination of the other
three factors. A numerical rating that describes the sensor’s sensitivity to light. Is usually measured in 1/3
stop increments.
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5. Introduction to
Photographic Exposure
Exposure Basics
A stop is a change in exposure or illumination by
a factor of two. One stop more exposure doubles
the amount of light and one stop less exposure
halves the amount of light.
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6. Introduction to
Photographic Exposure
Shutter Speed
One of the basic tenants of photographic theory is the half as much and twice
as much concept. This concept states that a one stop increase in time (slowing
the shutter speed from 1/50 to 1/25) allows twice as much time for light
energy to expose the image sensor. A one stop decrease (increasing the
shutter speed to 1/25 to 1/50) cuts the time in half.
In photography we work with stops as the foundation for changing exposure.
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7. Introduction to
Photographic Exposure
Shutter Speed
1 sec 1/2
A one stop difference
1/2 A one stop difference
1/4
1/4 A one stop difference 1/8
1/8 A one stop difference 1/15
1/15 A one stop difference 1/30
1/30 A one stop difference 1/60
1/60 A one stop difference 1/125
1/125 A one stop difference 1/250
1/250 A one stop difference
1/500
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8. Introduction to
Photographic Exposure
Shutter Speed
Every red number is one stop away from the next red number. In between there are two intermediate stops.
These are referred to as 1/3 stops. There are always two 1/3 stop choices in between each whole stop.
Cameras can also be set to 1/2 stop increments. Precise control over exposure is compromised when
working in 1/2 stops.
When the shutter speed is 1/250 of a second and it’s then changed to 1/125 of a second, the time has
been doubled (the shutter is open twice as long), increasing the exposure value by one whole stop. The
photo will become lighter.
If you move in the opposite direction, then you have cut the time in half (the shutter is open half as long)
and have decreased the exposure by one whole stop. The photo becomes darker.
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9. Introduction to
Photographic Exposure
Shutter Speed & Exposure
Some examples......
From 1/8000 .....................To 1/4000
is one full stop more exposure.
(1/2 the number = twice the time)
1/8000..... is twice as fast as .....1/4000
and lets in half as much light.
1/4000..... is half as fast as .....1/8000
and lets in twice as much light.
Because Shutter Speeds are listed as fractions of a second they relate to one
another in this “twice as much” or “half as much” way at full stop intervals.
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10. Introduction to
Photographic Exposure
Shutter Speed
1/4000............... is twice as fast as ................1/2000
1/2000............... is twice as fast as ................1/1000
1/1000............... is twice as fast as ................. 1/500
1/500................. is twice as fast as ..................1/250
1/250................. is twice as fast as ..................1/125
1/125..................is twice as fast as ...................1/60
When a shutter speed is twice as fast as the previous shutter speed, half as much time is
allowed for exposure. The opposite is true when the shutter speed is half the time (the
fractional number) as the previous shutter speed. At this point twice as much time is
allowed for light energy to travel through the lens barrel back to the image sensor.
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11. Introduction to
Photographic Exposure
Aperture Scale*
Controls the amount of light passing through the lens barrel. Every red number is one stop away from
the next red number. In between there are two intermediate stops. These are referred to as 1/3 stops.
There are always two 1/3 stop choices in between each whole stop. Cameras can also be set to 1/2
stop increments. Precise control over exposure is compromised when working in 1/2 stops.
When an aperture is set to 8.0 and it’s then changed to 5.6, the intensity has been doubled (the iris is larger),
increasing the exposure value by one whole stop. The photo will become lighter.
If you move in the opposite direction, then you have cut the intensity in half (the iris is smaller) and have
decreased the exposure by one whole stop. The photo becomes darker.
*Also known as the f/stop scale
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12. Introduction to
Photographic Exposure
Inside every lens is
a diaphragm which
controls the opening
of the iris. This in
turn controls the
flow or intensity of
light traveling
through the lens
barrel to the light
sensitive recording
media (the sensor).
Question: Why does a smaller number represent a larger opening?
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13. Introduction to
Photographic Exposure
F/Stop
The ‘f’ number
represents the
amount of light
and is figured by
the size of the
opening
(aperture)
divided by the
focal length of
the lens.
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14. Introduction to
Photographic Exposure
F/Stops
Due to the specifics of lens design, apertures let in the
same amount of light regardless of f/stop.
F/5.6 lets in the same intensity of light on a 200mm lens
as a 50mm lens.
This is important to remember.
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15. Introduction to
Photographic Exposure
Exposure
Before we discuss how ISO’s, factor into exposure control,
you must appreciate that there is a reciprocal relationship
between aperture and shutter speed.
1/50 @ f/8.0 = 1/100 @ f/5.6
This reciprocal relationship is the way to balance exposure.
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16. Introduction to
Photographic Exposure
Exposure
So How Do Shutter Speed and Aperture Work Together?
The rule of thumb for proper exposure:
If we choose a slower shutter speed 1/50 IRIS
(lengthening the amount of time light
travels through the lens barrel), then we
have to choose a smaller opening to let in
1/25 IRIS
less light. A larger opening in the lens is a
smaller f/stop (f/4.0) number. And vice-
versa.
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17. Introduction to
Photographic Exposure
ISO
Determines the sensitivity of film and digital chip. It is represented by a numerical
value in 1/3 stop increments. Each red number is one whole stop away from the next
red number. In between there are two intermediate stops. These are referred to as
1/3 stops. There are always two 1/3 stop choices in between each whole stop.
Cameras can also be set to 1/2 stop increments. Precise control over exposure is
compromised when working in 1/2 stops.
If ISO 400 is “twice as much” as ISO 200, at ISO 400 the chip in the camera is
“twice as sensitive” as the same chip at ISO 200.
This same principal is applied to shutter speed and f/stops.
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18. Introduction to
Photographic Exposure
ISO
If ISO 400 is “twice as much” as ISO 200, at ISO 400 the chip in the camera is “twice
as sensitive” as the same chip at ISO 200.
It stands to reason then, that a shutter speed of 400 is twice as fast as a shutter speed of
200.
With twice the chip sensitivity (and assuming the f/stop remains constant), it only
requires the shutter to be open half as long to produce the exact same exposure.
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19. Introduction to
Photographic Exposure
At ISO 200
Exposure = 1/200 @ f/16
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20. Introduction to
Photographic Exposure
At ISO 400
Exposure = 1/400 @ f/16
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21. Introduction to
Photographic Exposure
ISO 400 ISO 200
So, side by side, it’s clear to see that the exposure is the same if illumination is a
constant no matter what the ISO. If we learn to understand how ISOs, shutter speeds,
and f-stops relate, we will then be fluent in understanding the basics of photography.
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22. Introduction to
Photographic Exposure
Exposure
In the world of camera controls
Half as Much
and
Twice as Much
inherently applies in one stop increments.
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23. Introduction to
Photographic Exposure
Exposure
Let’s try a problem...
1/200 @ f/16
is the same as
1/? @ f/5.6
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24. Introduction to
Photographic Exposure
Exposure
Returning to the f/Stop scale
f/2.8 f/4.0 f/5.6 f/8.0 f/11 f16
f/5.6 is 3 stops more light than f/16. Therefore,
our shutter speed has to be 3 stops faster.
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25. Introduction to
Photographic Exposure
Exposure
On The Shutter Speed Scale
100 200 400 800 1600
Which number is 3 stops faster than 200?
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26. Introduction to
Photographic Exposure
Exposure
The answer:
1/200 @ f/16
is the same as
1/1600 @ f/5.6
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27. Introduction to
Photographic Exposure
Equivalent Exposures
1/1600 @ f/5.6
Equivalent Exposures 1/200 @ f/16
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28. Introduction to
Photographic Exposure
Equivalent Exposures
As one camera control changes, either shutter speed
or f/stop, the other camera control must move in an
equal but opposite direction to maintain a balance
to the exposure. This concept is called equivalent
exposures.
Example to follow.
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29. Introduction to
Photographic Exposure
Equivalent Exposures
In addition to earlier illustrating the concept
of Depth of Field, these two images also
demonstrate the concept of Equivalent
Exposure.
Equivalent exposure refers to maintaining
the same exposure value despite changing
camera controls.
As one camera control moves (shutter
speed), the other camera control (f/stop)
moves in an equal but opposite direction.
-or-
To maintain equivalent exposures, when we
increase our shutter speed we must also
decrease our f- stop (aperture) by the same
amount. Conversely, if we decrease our shutter
speed we must then increase our f-stop
1/60 @ f/5.6 ISO 200 (aperture) by the same amount.
.6 sec @ f/36 ISO 200
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30. Introduction to
Photographic Exposure
Michael E. Stern
Photographer Educator Trainer
818-422-0696
CyberStern.com
30