Focal-stop Number
光圈值（f-stop number） 是用于衡量镜头中 光圈 打开程度的指标。越大的光圈值，实际光圈越小。
常见的光圈值有：f/1.4，f/2，f/2.8，f/4，f5.6，f/8，f/11，f/16 和 f/22。
不同镜头光圈值的最大、最小值不同。
f 含义
f 代表焦距（focal length），也就是从镜头前面看到的叶片直径。光圈值 以分数（fraction）记录，也就是说 f/8 意味着 1/8。所以说光圈值越大，实际光圈越小。
光圈值 通过公式计算：光圈值（f） = 镜头焦距（mm）/叶片直径（mm）。
The F-stop number (or focal-stop number) is a reference to how open or how closed the aperture is in your camera lens.
In aperture and photography, the most common F-stop values follow a sequence like this:
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8,f/11, f/16, f/22
The maximum and minimum aperture number or F-stop value varies from lens to lens, and you’ll set the specific value on your lens/camera in order to determine the desired aperture that fits your goals.
Increasing/decreasing the F-Stop number coincides with opening up or closing (also called stopping) down your aperture.[1]
Why is your aperture written like that? What does something like “f/8” even mean? Actually, this is one of the most important parts about aperture: it’s written as a fraction.
You can think of an aperture of f/8 as the fraction 1/8 (one-eighth). An aperture of f/2 is equivalent to 1/2 (one-half). An aperture of f/16 is 1/16 (one-sixteenth). And so on.[2]
The “f” stands for “focal length”. When you substitute focal length into the fraction, you’re solving for the diameter of the aperture blades in your lens. (Or, more accurately, the diameter that the blades appear to be when you look through the front of the lens).
For example, say that you have an 80-200mm f/2.8 lens fully zoomed out to 80mm. If your f-stop is set to f/4, the diameter of the aperture blades in your lens will look exactly 20 millimeters across (80mm / 4), whereas at f/16, the diameter will be reduced to mere 5 millimeters (80mm / 16).[2:1]