Apertures are measured in f-stops because the f-number (f/stop) represents the ratio between the focal length of the lens and the diameter of the aperture opening. This standardized measurement ensures consistent exposure control across different lenses and cameras.

Why Use f-Stops?

The f-number (f/N) is calculated as:

f\text{-number} = \frac{\text{Focal Length}}{\text{Aperture Diameter}}

 

For example, on a 50mm lens:

• f/2 means the aperture diameter is 50mm ÷ 2 = 25mm.

• f/8 means the aperture diameter is 50mm ÷ 8 = 6.25mm.

 

This explains why:

• Lower f-numbers (e.g., f/1.4, f/2.8) → Larger aperture, more light.

• Higher f-numbers (e.g., f/11, f/16) → Smaller aperture, less light.

 

Why f-Stops Use a Non-Linear Sequence

The standard f-stop sequence follows a geometric progression, where each step halves or doubles the light:

f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22

Each step is based on the square root of 2 (√2 ≈ 1.41), ensuring that each stop lets in exactly half or twice the amount of light as the previous one.

 

ISO stands for International Organization for Standardization, which established the standard for measuring a camera sensor’s sensitivity to light.