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Aperture is the hole inside your camera lens. This hole, known as the diaphram is formed by a number of overlapping blades that adjust to control the size of the hole opening. Depending on the type of lens, the hole opening or what we will refer to as aperture can be adjusted manually on the lens (Manual Lenses) or automatically (Automatic Lenses) by your camera using buttons or dials (on the camera). When you do this adjustment, the aperture either increases or decreases which in turn controls how much light can pass through the lens and onto the digital sensor or film.

Aperture settings are expresssed in f-stops and are represented by numbers (i.e. f/2, f/5.6, f/11, etc). The smaller the f-stop number, the bigger the lens opening. See Figure 1 below.

Lens Aperture Openings
FIGURE 1: Lens Opening - Aperture Setting

How does Aperture affect Exposure?

The example illustration in figure 2 shows you the effect of aperture on the exposure of a photograph. From left to right, the images are shot from large to small aperture, in this case from F/2.8 to F/11. To isolate the effect of aperture, the shutter speed and ISO was left unchanged.

Effect of Aperture on Exposure
FIGURE 2: Effect of Aperture on Exposure

Refering again the figure 2 above, the middle image taken with aperture of F/5.6 has the optimal exposure. As you can see, with the shutter speed and ISO the same, the larger aperture lets in more light thus having the effect of overexposing the image. When the aperture is decreased, less light enters the lens and hits the sensor so it has the effect of underexposing the image.

Aperture and Depth of Field

Aperture has a creative effect to the photograph called Depth of Field. Depth of Field is the distance where objects in your scene are in focus. The size of the lens opening is what determines the depth of field. The larger lens opening translates to less area of focus on your photograph. This is also known as shallow depth of field. This is because only the light that hits the area in focus will be rendered sharply on the digital sensor or film; all the other light in the scene will be spread out across the sensor or film. This unfocused light renders as blurry images. On the other hand, smaller lens opening, will allow for a deeper focus which means that more things in your photo to be in focus. This is because the amount of light entering the lens is considerably minimized resulting in an image that has a greater area of sharpness and detail because there were no extra unfocused light that was spread out across the sensor or film.

Depth of Field
Figure 3: Depth of field

Below are some examples showing photos taken with different apertures resulting in different levels of focus or depth of field.

Exposure 1
F/2.8, 1/25 sec, ISO 1600
Exposure 2
F/4, 1/13 sec, ISO 1600
Exposure 3
F/5.6, 1/6 sec, ISO 1600
Exposure 4
F/8, 1/3 sec, ISO 1600
Exposure 5
F/11, 1.6 sec, ISO 1600

FIGURE 3: Depth of Field Sample Photos

The image taken with the largest aperture of F/2.8 has the least of amount of depth of field where a only a small area is in sharp focus. Only the bottom half of the left lens of the reading glasses is in sharp focus. The right lens is very blurry and the background is indistinguishable. The next couple of shots at F/4 and F/5.6 has more of the reading glasses in focus but the background is still extremely blurry. At F/8, almost all of the reading glasses is in focus and the background is starting to become distinguishable. The last photo taken at F/11 has the reading glasses in sharp focus and the background is still a little hazy but you can pretty much tell what it is.

Aperture Priority Mode

Aperture Priority Mode Dials

Aperture Priority Mode is a semi-automatic setting on your camera that gives you the ability to manually adjust the aperture setting (F/stop number) while the camera automatically selects the appropriate shutter speed to come up with an optimal exposure. Setting your digital camera to Aperture Priority Mode gives you the ability to close the aperture to let in less light, or open it up to let in more light. It allows you to control the depth of field of your photograph.

To set your camera to Aperture Priority Mode, look for an Av or an A symbol on your camera dial. Different camera manufacturers may use another symbol so read your camera's product manual to figure out how to set Aperture Priority Mode on your camera.

Real world uses of Aperture Priority Mode

Suppose you want to take a photo of a landscape scene where you want everything in focus. To accomplish this, with your Camera set to Aperture Priority Mode, you simply need to use a small aperture, say f/8 or f/22 and take the picture.

Landscape Photo taken at F/10
FIGURE 4: Landscape Photo taken at f/10

Now, suppose you want to take a portrait where you want to focus on your subject only and have the background seemingly disappear into fuzziness. To accomplish this, with your Camera set to Aperture Priority Mode, you simply need to use a large aperture, say f/1.8 and take the picture.

Portrait Photo
FIGURE 5: Portrait sample

What to watch out for in Aperture Priority Mode

When you start using smaller and smaller aperture, the camera will select a longer and longer shutter speed to match in order to get a proper exposure. There may come a point where the shutter speed is not fast enough for you to avoid camera shake. Camera shake usually happens when you do not use a tripod and if the shutter speed is too slow. The solution to this problem is either to adjust to a larger aperture so that the shutter speed increases or use a tripod to steady the camera while shooting.

Related Topics
  1. Online Course #1: Understanding Exposure
  2. Shutter Speed & Shutter Priority Mode
  3. What is ISO?