Mathematical notation for expressing the relative light transmission of a lens. Calculated as focal length divided by aperture diameter.
F-Stop / F-Number
The F-stop is a standardized mathematical notation that indicates the relative aperture (light-gathering capability) of a lens. Despite the name, the F-stop is not the actual aperture opening, but a ratio between the focal length of the lens and the diameter of the aperture opening.
Mathematical Basis
The F-number is calculated using the formula:
F-Number = Focal Length / Aperture Diameter
Example 1: 50mm Focal Length / 25mm Diameter = f/2.0
Example 2: 85mm Focal Length / 42.5mm Diameter = f/2.0
Example 3: 35mm Focal Length / 17.5mm Diameter = f/2.0This means that a 50mm f/2.0 and an 85mm f/2.0 theoretically let through the same amount of light, even though the physical diameter is different.
The F-Stop Sequence
The standard aperture sequence follows a mathematical progression:
| Aperture | Light (relative) | Typical Application |
|---|---|---|
| f/1.4 | 16x | Extreme Low-Light, Bokeh |
| f/2.0 | 8x | Available Light, Portrait |
| f/2.8 | 4x | Standard Film Production |
| f/4.0 | 2x | Daylight, Documentary |
| f/5.6 | 1x | Reference Value |
| f/8.0 | 0.5x | Maximum Sharpness |
| f/11 | 0.25x | Landscape, Deep Focus |
| f/16 | 0.125x | Extreme Depth of Field |
F-Stop vs. T-Stop
The most important difference for filmmakers:
- F-Stop: Theoretical value based on geometry
- T-Stop: Measured light transmission including glass losses
A lens with f/2.0 might actually be T/2.3 if 15% of the light is absorbed by glass elements.