Exposure Trio Part Two: APERTURE

The Exposure Trio

A Three Part Series

ISO—APERTURE—SHUTTER SPEED

Part Two: Aperture

As we learned in part one, a film or digital SLR camera lens has an iris style diaphragm inside. This diaphragm opens and closes in steps from largest opening (maximum aperture) to smallest opening (minimum aperture). The size of the aperture opening is measured in f-stops.

Aperture plays an important role in the exposure trio. By controlling the size of the lens opening that we allow light to pass through (f-stop), in combination with how long we allow the light to pass through (shutter speed), and how sensitive the sensor is to that light (ISO), correct exposure is determined.

But Aperture Does So Much More!

Aperture is also a key ingredient in determining depth of field, and depth of field plays a major role in how we compose an image.

My purpose here is not to go into a long textbook dissertation defining depth of field. Instead, what I want to cover is how aperture allows us to take control of the creative process by choosing how much of an image appears in focus.

My short definition of depth of field is this:

A camera lens focuses at an exact distance. Only objects at that—exact distance—are truly in focus. There will be an area in front of the focus point, and beyond the focus point, that appears sharp. This area of apparent sharp focus is called depth of field.

Your aperture selection will increase or decrease depth of field. Let’s take a look at two examples:

Image One

In Image One an aperture of f/2.8 creates a very shallow depth of field. The viewer’s attention is focused on the in-focus section of the railing. The soft focus makes it difficult to concentrate on the path beyond the railing.

By closing down the aperture to f/16 the depth of field is substantially increased. Now the viewer’s eye is led down the railing and into the now sharp path. Which is the correct version? That’s up to the photographer. Aperture choice puts the photographer in control.

(Note: A slight breeze softened the bushes in the f/16 version. This could have been eliminated with a faster shutter speed. To stay at f/16 and increase the shutter speed would have required a higher ISO. This is a good example of how the three parts of our Exposure Trio work together.)

Image Two

Image Two shows two versions of the exact same flower taken seconds apart. The f/16 image shows the flower as it appeared through the viewfinder. By changing the aperture to f/2.8 the background is rendered out of focus. The f/2.8 image isolates the flower bud; the f/16 image shows the plant as it appeared before the photographer. When you understand how aperture controls depth of field you have the power to choose how you want to present the image.

As a photographer you must ask yourself before releasing the shutter… Is depth of field important to this composition? If the answer is yes, then here are a few basic guidelines to follow:

To minimize depth of field…

  • Open your lens to its widest aperture.
  • Maximize subject to background distance.
  • Move closer to your subject and/or use a longer lens to zoom in on your subject.

To maximize depth of field…

  • Stop down your lens to a small aperture.
  • Minimize subject to background distance.
  • Increase camera to subject distance.
  • Use a wider angle lens.

Stay tuned for part three of our Exposure Trio: Shutter Speed

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2 Responses to “Exposure Trio Part Two: APERTURE”

  1. rommelrutor Says:

    Reblogged this on Words and Images.

  2. all answer about photography Says:

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