Camera Exposure Basics: Shutter Speeds, Aperture, F Stops, and ISO
In photography, the term exposure is a reference to how much light, and how long that light is exposed to the
cameras' image sensor when the camera shutter release is pressed.
In simple terms, the camera's exposure value settings determine if an image will be too light, too dark, or correctly
exposed according to normal standards.
The correct or desired exposure is obtained by using a combination of the camera's Lens Aperture setting, the
Shutter Speed, and the ISO setting. Those three functions are commonly known as the "Exposure Triangle".
All digital cameras have automatic shooting modes which enable you to just pick up your camera and start taking
pictures without having to worry about the settings for a correct exposure. That would include basic compact cameras
like the Canon Powershot Elph 135.
Still, it is good know what the camera is doing to obtain the exposure even with a basic compact camera. That is just
in case you need to change from the full automatic mode to a scene mode like night landscape or back lit portrait to
adjust for various light conditions.
However, Digital SLR cameras like the Canon T5i, or Mirrorless Compact System cameras like the Sony NEX5T allow
you to manually adjust all of the camera's exposure settings. Some "Bridge" type cameras also allow you to manually
choose specific settings to adjust the exposure.
With most basic compact cameras, the exposure settings are automatically set and cannot be manually adjusted.
What are the functions of the Aperture, Shutter, and ISO rating?
As previously mentioned, the camera lens aperture, the camera shutter, and the ISO function work together to
produce a correctly exposed image. Lets take a look at each of these functions individually, and then how they work
in relation to each other.
The Lens Aperture: The camera lens aperture, or more specifically, the
size of the lens aperture opening determines the amount of light that reaches
the camera's image sensor. The camera lens aperture diagram on the right
shows the relative size of the aperture openings at different settings.
The various lens aperture settings are called "F Stops". The aperture settings
with the lower F Stop numbers allow more light to reach the image sensor
than the settings with the higher F Stop numbers.
Listed below is an aperture chart. The F Stop numbers in red represent a full
stop and the numbers in black are 1/3 increments between the full stops.
A full stop in either direction will allow twice as much light, or half as much light
to reach the image sensor. For example, F4 allows twice as much light to reach
the cameras' image sensor than F5.6. On the other hand, F8 allows half as
much light to reach the sensor than F5.6.
Take a look at picture # 1. It was taken with the lens aperture set to F10 and is much darker than picture # 2, which
was taken with an F5 aperture setting. That is because the wider aperture opening of F5 allows more light to reach
the image sensor than the smaller aperture opening of F10. In this example, the shutter speed and ISO settings are
the same in both pictures.
You can see that even though the only setting that was changed was the lens aperture, the images look totally
Another important feature of the camera lens aperture is its effect on the Depth of Field in an image. Depth of field is
a reference to how much of the image, (subject, foreground, and background) will be in focus. Using a wide aperture
such as F2.8 or F3.5 can produce an image similar to
picture # 3. Notice how the flower is in focus but the
background area is out of focus. That effect is referred
to as narrow, or shallow depth of field.
Using a smaller aperture opening like F11 or F16 might
produce an image like picture # 4 where just about
everything is pretty much in focus. That effect is called
wide depth of field.
However, take note that there are other factors such as
the lens focal length,the lens to subject distance, and
the type of camera lens being used that will also affect
the depth of field in an image.
The Camera Shutter: The shutter speed determines the length of time that light will be exposed to the camera's
image sensor. Shutter speeds are listed in seconds or fractions of a second. Those times reflect how long the camera
shutter will be open during an exposure. Listed below is a chart showing shutter speeds in full steps and in 1/3 step
increments. The numbers in red represent a full step or stop.
A shutter speed of 1/60th of a second keeps the shutter open longer than a shutter speed of 1/125th of a second.
Changing the shutter speed by a full or partial step in either direction is the equivalent of changing the lens aperture
setting by a full or partial F Stop in the same direction.
Lets refer back to picture # 1 in the lens aperture section. It is clearly too dark. The underexposure was corrected
by changing the aperture setting from F10 to F5 which allowed more light to reach the image sensor. However, If
you wanted to lighten that same picture without changing the aperture setting, you could simply change your shutter
Changing the shutter speed from 1/400 to 1/100 will allow the light to be absorbed by the image sensor for a longer
period of time. That change will yield the same basic result as changing the aperture from F10 to F5. The point here
is that changing the shutter speed by any given amount of "steps" or increments will have the same effect as
changing the aperture by an equal number of steps. (as long as the change is in the same direction)
The shutter speed also has an effect on the look of an image when there is motion in a scene. If there is fast
movement in a scene, you might need to use a fast shutter speed to avoid motion blur. Take a look at the moving car
in picture # 5. The car in the picture is blurred because the shutter speed of 1/60 was not fast enough to "freeze" the
subject. The car in picture # 6 was shot using the faster shutter speed of 1/250 which was fast enough to "freeze" the
movement of the car.
On the other hand, there may be times when using a slower shutter speed might be your best or only option. For
instance, if you try to take pictures at night using a shutter speed like 1/250,they will almost certainly come out black or
severely underexposed. Sometimes that will happen even if you use your widest lens aperture opening.
Picture #7 was taken in a fairly well lit area by night photography standards.
The shutter speed was 1 full second with a somewhat wide aperture setting
of F 3.4. Can you imagine what the image would look like if it had been taken
using a shutter speed of 1/250? (assuming the same lens aperture and ISO
settings were used)
The ISO Function: The ISO setting determines the image sensors' sensitivity
to the available light. Lower ISO number settings will require more light for
the image sensor to produce a good image than when the camera is set to
a higher ISO number. A camera set to ISO 400 will need less light to produce
a correctly exposed image than when it is set to ISO 100.
The following are a few of the most common ISO settings. (there may be more
that show in your camera but we will just use these to keep things simple)
It is usually best to use lower number ISO settings when taking pictures. That is because the overall quality of the
image will be better. Higher ISO number settings tend to produce grainy images that have tiny specks called digital
camera "noise". (Check the tutorial "What is ISO" for an example)
However, there may be times that using a higher ISO setting might be the best way to go. A simple example might be
when you are in a poorly lit place and your pictures are coming out too dark. If your camera is set on ISO 100, change
it to ISO 200, ISO 400 or whatever setting gives you a lighter picture.( that is, in situations where the shutter speed
and aperture settings are not the primary concern) Remember, the higher number ISO settings require less light to
produce a well exposed picture.
Here is another example of when you might need to change the ISO setting. Lets say you are at an indoor sporting
event with lots of fast movement. Your camera is set to ISO 100. The cameras' exposure metering system suggests
a lens aperture setting of F4 and a shutter speed of 1/30 for a correctly exposed picture. (assume that F4 is your
maximum aperture width)
You know that a shutter speed of 1/30 won't be fast enough to "freeze" the action. However, a shutter speed of
1/250 should work well. Change your ISO setting from ISO 100 to ISO 800. Next, change your shutter speed from
1/30 to 1/250. You will probably be able to take a properly exposed picture with the action "frozen" instead of blurred.
As you have probably noticed, changing the ISO setting by a certain number of steps is the same as changing the
shutter speed the same amount of steps. That principle also applies to the ISO in relation to the aperture setting.
Aperture Priority, Shutter Priority, and Program modes: The majority of material in this tutorial refers to using
your camera in the Manual mode where you can set the lens aperture, shutter speed, and ISO separately.
Just remember that you don't always have to set each function individually each time you take a picture. Try using
one of your cameras' semi automatic modes. If the shutter speed is most important to you for a particular shot, try
using the Shutter Speed Priority mode. When using the Shutter Speed Priority mode, you set the desired shutter
and the camera will automatically set the aperture that should produce a correctly exposed image.
On the other hand, if the lens aperture setting is most important to you, try using the Aperture Priority mode. You
choose the lens aperture setting you want to use and the camera will set a shutter speed that should produce a
correctly exposed picture.
Lastly, if you aren't concerned about shutter speeds or aperture settings but want to change your ISO at times. Just
use your camera's program mode. In the program mode the camera will automatically choose the shutter speed and
aperture setting for the best exposure. In the Program Mode you will still be able to change the settings for things like
the ISO and White Balance.
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