In optics, "aperture" refers to a group of tiny blades that are built into the lens and control how much light enters it. Though small in size, it plays a vital role in photographic expression. (Reported by: Ryosuke Takahashi)

 

What is aperture?

"Aperture" means "opening". We often use "aperture" to refer to the f-number (f-value/ f-stop/ aperture setting) on the camera. However,  it also refers to the opening in the lens ("aperture diaphragm") that is made up of a number of blades ("aperture blades"), and which allows light into the lens to reach the sensor. 

The aperture diaphragm mainly:

1. Regulates the amount of light that travels into the camera body
2. Controls the size of the area in focus (depth-of-field), or the amount of blurring in the background or foreground

 

The relationship between aperture opening & the f-number

The f-number is a value that indicates the size of the opening formed by the aperture blades. 
When you change the f-number on the camera, or when the camera adjusts the f-number, the size of the aperture diaphragm changes accordingly, which also changes the amount of light reaching the image sensor.

When the aperture diaphragm is “open”, it allows a large amount of light to enter.
When the aperture diaphragm is “closed”, the opening is narrowed, and less light can enter.

 

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."

More details in Camera Basics #1: Aperture

Tip: A smaller f-number is more effective for photographing at a dimly-lit location—it lets in more light.

 

The relationship between aperture and depth-of-field

Besides its function as a light control valve, the aperture can also be used to adjust the area that is in focus (depth-of-field).

When the aperture is small, the depth-of-field is large. This will bring all foreground and background subjects in focus

When the aperture is wide open (maximum aperture), the depth-of-field is shallow.
if your subject is in the foreground and you have set your focus on it, you can create an effect called bokeh (background blur). This isolates the subject from the background, so that the subject is sharp and the background is blurry. 

Tips:
- If there is something in front of your subject, you can also use maximum aperture to blur it out and create foreground bokeh.
- When depth-of-field is shallow, be careful where you establish focus—parts of the subject might end up in the out-of-focus zone. Check out this article on capturing facial expressions to learn more about where to focus for chest-up portraits. 

 

Large aperture: Shallow depth-of-field

Aperture-priority AE (f/1.8, 1/1000 sec, EV±0)

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.

 

Small aperture: Larger depth-of-field

Aperture-priority AE (f/11, 1/320 sec, EV±0)

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.

 

Effects of aperture on depth-of-field

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 has a "deep focus," while that with a small area in focus has a "shallow focus."

 

Diffraction and small apertures

One frequent piece of advice is to be careful of stopping down your aperture effectively. This is because when you use an aperture that is too narrow, diffraction occurs.

Diffraction happens when the aperture opening is so narrow that it causes light to bend when it enters, resulting in irregular reflection around the aperture blades. This irregular reflection causes your images to become less sharp.

For this reason, when you want to deep focus, an aperture value of f/8 to f/11 will usually be enough.

 

Effects of diffraction: f/8 v.s f/22

With the camera secured in place, I took several shots from the same position while changing the aperture value. The two photos below are enlargements of the area indicated by the red frame. Notice that the image taken at f/8 appears sharper than that taken at f/22.

Find out more about how Canon cameras and lenses combat diffraction and other lens aberrations here:
EOS-1D X Mark II – In-camera Lens Optimization for High Quality Images

 

What does the maximum aperture value of a lens really mean?

In layman terms, the maximum aperture value of a lens is the brightness when the aperture is fully open.

In very technical terms, the maximum aperture value of a lens is the inverse function of the effective diameter of the lens divided by the focal length.

In any case, it means that the larger the size of the aperture opening, the smaller the aperture value.

 

How to tell the maximum aperture on your lens

If the maximum aperture of the lens is f/3.5, this will be indicated as "1:3.5" on the lens.

On some zoom lenses the maximum aperture value varies with the focal length. 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.

 

For more on individual f-numbers and the scenes they are commonly used for, check out our Aperture-Priority Technique series:
#1: The Relationship Between Lens Aperture and Bokeh
#2: Create Background Bokeh for a Warm, Friendly Family Photo
#3: The Wonders of f/2.2 in Still-Life Photography
#4: Photographing Facial Expressions (f/2.8)
#5: Camera Settings for the Perfect Outdoor Portrait (f/4)
#6: A Useful Aperture Setting for Street Photography (f/5.6)
#7: Aperture Settings for Sharp Depictions of Nightscapes (f/8)
#8: The Ideal Aperture for Sharp Depictions of Natural Landscapes with Depth (f/11)
#9: Getting Sharp Depictions of Landscapes from Foreground to Background (f/16)

 

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