Aperture is a component inside the lens that controls the size of the opening. It is composed of several aperture blades, and is primarily used to adjust the amount of light entering the camera.

With all other settings unchanged, opening up the aperture allows more light into the camera, resulting in a brighter image. Stopping down the aperture has the opposite effect, producing a darker image.

More specifically, the working principle of aperture is very similar to the human pupil. To adapt to different lighting and visual environments, the human pupil constricts in bright sunlight to avoid discomfort and dilates in darkness to capture more light. The aperture functions in the same way, which is why people often say that a larger aperture delivers better low-light performance.

The aperture is formed by aperture blades, and the size of the aperture is controlled by adjusting the opening of these blades. Generally speaking, the more aperture blades a lens has, the higher the precision and the more perfectly circular the aperture opening can be. Most modern lenses typically have 5 to 9 aperture blades. (As shown in the image above, this lens has 7 aperture blades.)

Does the number of aperture blades affect actual shooting? As mentioned above, more blades create a rounder aperture, resulting in smoother, rounder bokeh. Conversely, fewer blades tend to produce polygonal bokeh (as shown in the top-right image below, the bokeh is heptagonal/7-sided). This is why high-end lenses often emphasize that they have 9 or more aperture blades.

Generally, we use the f‑number to represent aperture size, such as:f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22.

(Besides f/numbers, aperture may also be written as F 2.8, f 2.8, or 1:2.8 — all mean the same as f/2.8.)

The formula or origin of the f‑number is not important (those interested may refer to Section 5 [Supplementary Information]). However, you must remember that a smaller number means a larger aperture. Therefore, an aperture of f/2.8 is much larger than f/8.

How much larger? Twice? Three times? 2.85 times? None of these is correct—the right answer is 8 times. How do we know?

I recommend memorizing the common full f‑stops:f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22.

Why? Because each step to the right represents one full stop smaller, and the light intensity is halved.Since f/2.8 is 3 full stops larger than f/8, it allows 8 times more light than f/8.

Therefore, with all other settings and conditions identical, using f/4 will result in an image twice as dark as f/2.8 (half the light). To achieve the same exposure as f/2.8, you would need to double the shutter speed or double the ISO.

Similarly, if you want to reduce the brightness of an f/4 shot by one full stop using aperture alone, you would set it to f/5.6, not f/2.8 or f/8.