Day to Day Photo

To follow up to the previous article on exposure, Photography for Beginners – Part 1: Exposure, today we are going to cover one of the elements, the Aperture. With the understanding of aperture, we would be able to move from the semi-auto Program (P) mode found in all DSLRs to the favourite of many photographers, the Aperture-priority mode (A or Av mode). By turning the dial mode to the A or Av, we are jumping into a new realm of photography where we could exercise our creativity and control our camera to take pictures as we envision them to be.

Photo by Adam Foster

What is Aperture?

Simply speaking, aperture is the opening found in a lens that allows light to pass through and reach the camera sensor or film. The picture above shows the various aperture sizes that can be set on a lens. By changing the aperture, we would be able to alter the depth-of-field of an image and hence we could use it creatively to capture an appealing image such as the one here.

The different sizes of the aperture is the main reason why aperture has the ability to change how a picture would look after it is taken. The size is represented by the ‘f-stop’, such as f/22, f/16, f/8, f/3.5, f/2.8 etc. Here, f refers to the focal length of the lens. Hence the size of the aperture for a given f-stop number would be focal length divided by the f-stop number. In the case of a 50mm lens at f/2.8, the aperture size would be 50 mm/2.8 = 17.86 mm.

Depth-of-Field and Circle-of-Confusion

As mentioned earlier, the size of the aperture determines the depth-of-field of the picture. As we move away in both directions from the focusing distance, everything will begin to lose its sharpness, even though this is not perceived by our eyes. Beyond a certain point, our eyes would begin to notice the loss of sharpness in the image. The depth-of-field then refers to the range of distance in the scene in which the subject would be perceived as sharp. By knowing the depth-of-field, we would then be able to determine how much area is in focus in front and behind the focus distance.

To understand how the aperture affects the depth-of-field, we need to look at the next concept, which is the circle of confusion. Circle of confusion refers to the area on the sensor or film beyond which images would be perceived as unsharp by the eyes.

As seen on the illustration, if the circles on the top and bottom represents the circle of confusion, and the distances to the left of the lens would represent the closest and furthest distance that would be in focus. The reason being any light rays coming further or nearer from these two points would fall outside the circle of confusion and the object would be rendered unsharp.

From this illustration, we can see that increasing the f-stop number and reducing the aperture, it would prevent the light rays that are being refracted at the extreme ends of the lens from reaching the sensor. Since only light rays that would fall within the circle of confusion would pass through the aperture, light rays coming from objects further away and closer than the focusing distance would still fall on the circle of confusion and the depth-of-field would be greater. The reverse is also true since opening the aperture to its maximum size would allow plenty of light rays to land beyond the circle of confusion and therefore the depth-of-field is reduced.

Using the 50 mm at f/2.8 example again, if we focus at a distance of 2 m, the closest and furthest distance of acceptable sharpness would be 1.92 m and 2.09 m respectively, giving us a 0.18 m depth-of-field. That means, anything placed between 1.92 m and 2.09 m away from the focusing plane, which is the camera sensor, would be rendered sharp by the lens.

Controlling depth-of-field using aperture

In summary, a large aperture and a small f-stop number would render a shallow depth-of-field, allowing us to take pictures where only a small portion of the image would be sharp and the foreground as well as the background being out of focus. On the other hand, to obtain a large depth-of-field, we could set the f-stop number to the maximum, thereby reducing the size of the aperture.

Knowing these techniques, we could now apply them to limit the depth-of-field in certain images or capture a scenery where we would want everything in the scene to be tack sharp.

So now go out, shoot some pictures and discover for yourself the infinite possibility of what you can do with aperture-priority mode.

For more information on aperture, depth-of-field and circle-of-confusion, refer to:

• Wikipedia – Aperture
• Digital Photography School – Aperture
• Digital Photography Review – Aperture
• Cambridge in Colour – Understanding Depth of Field in Photography
• The Luminous Landscape – Depth of Field
• Northnet – The Circle of Confusion
• Wikipedia – Circle of Confusion