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[Lesson 3] Learning about Aperture

"Aperture" refers to a group of tiny blades that are built into the lens. Though small in size, the aperture plays a vital role in photographic expression. (Reported by: Ryosuke Takahashi)

The Mechanism & Effect of Aperture

Regulates the amount of light that travels into the camera body and controls the background blur

Built into the lens, aperture is an important component that regulates the amount of light reaching the image sensor with the width of its opening. A large amount of light enters when it is open, while the intensity is reduced when the opening is narrowed. The range of the opening is referred to as the "aperture value," and the relationship between this value and the movement of the aperture is illustrated in the chart below.

Besides its function as a light control valve, the aperture can also be used to adjust the area that is in focus. When the aperture is wide open, it will isolate the foreground from the background making the foreground subject sharp and the background subject blurry. On the contrary, when the aperture is small, it will bring all foreground and background subjects in focus. The area in focus is known as the "depth of field."

Large Aperture

When you take a shot with the aperture fully open, the area that is in focus becomes narrower, and the background is significantly blurred as a result.

Aperture-priority AE (f/1.8, 1/1000 sec.)

Relationship between Aperture Opening & Aperture Value

The f-number is a value that indicates the size of the opening formed by the aperture blades. The narrower the opening, the larger the f-number. Adjusting this opening is referred to as "opening up the aperture" or "stopping down the aperture."

Small Aperture

By taking a shot with a small aperture, the result will be a sharp image that is in focus in both the foreground and background.

Aperture-priority AE (f/11, 1/320 sec.)

Be careful not to stop down the aperture excessively

Note that the image quality may deteriorate if the aperture is not used correctly. When you stop down the aperture too much, "diffraction" occurs, which results in irregular reflection around the aperture blades. This is caused by the excessively narrow opening for light to pass through. Generally, an aperture value of f/8 to f/11 will suffice when you want to produce a sharp image with a wide area in focus, such as in a landscape photo.

With the camera secured in place, I took several shots from the same position while changing the aperture value. The two photos on the right are the enlarged images of the area indicated by the red frame. Here, you can tell that the image taken at f/8 appears sharper than that taken at f/22.

At f/8.0

At f/22

What does the maximum aperture of a lens mean?

The maximum aperture of a lens is the inverse function of the effective diameter of the lens divided by the focal length. This value is generally used to indicate the brightness when the aperture is fully open, with a smaller value more effective for photographing at a dimly-lit location. Also, the maximum aperture value may vary with the focal length on some zoom lenses.

If the maximum aperture of the lens is f/3.5, this will be indicated as "1:3.5" on the lens. If it says "1:3.5-5.6" on a zoom lens, this means that the maximum aperture is f/3.5 at the wide-angle end, and f/5.6 at the telephoto end.

The area in focus changes with the aperture value








In this example, I set the focus to the lamp shade and took several shots with the aperture value varied. As illustrated, a wider area comes into focus and the bokeh effect in the background decreases as the aperture value increases. This area that is in focus is known as the "depth of field." An image with a large area in focus is described as having a "deep focus," while that with a small area in focus has a "shallow focus."

Ryosuke Takahashi

Born in Aichi in 1960, Takahashi started his freelance career in 1987 after working with an advertising photo studio and a publishing house. Photographing for major magazines, he has travelled to many parts of the world from his bases in Japan and China. Takahashi is a member of the Japan Professional Photographers Society (JPS).