F-Number
Aperture f-number is focal length divided by aperture diameter.
Formula
Live Calculator
MembersVariables
Example
50 mm lens, 25 mm aperture: N = f/2
How it works
The f-number (or f-stop) tells you how large the lens opening is relative to the lens’s focal length. It is calculated as focal length divided by aperture diameter, so a smaller opening gives a larger f-number and less light reaches the sensor. This ratio matters because it standardizes aperture across different lenses: an 85 mm lens and a 35 mm lens with the same f-number admit light in a comparable proportion to their focal length.
When to use it
Use this formula when you need to find the aperture setting from lens measurements, or when comparing how bright two lens openings are. It is especially useful in photography, cinematography, and optical design when working out exposure or depth of field.
Worked example, step by step
Given: Focal length, f = 100 mm Aperture diameter, D = 20 mm Step 1: Write the formula N = f / D Step 2: Substitute the values N = 100 / 20 Step 3: Calculate N = 5 Final answer: The f-number is N = f/5, usually written as f/5.
Common mistakes to avoid
- Using the aperture opening size as if it were the f-number instead of dividing focal length by diameter
- Mixing up focal length and aperture diameter units, which makes the ratio meaningless
- Thinking a larger f-number means a larger opening, when it actually means a smaller opening
Did You Know?
Each full f-stop (f/2 → f/2.8 → f/4) halves the light by increasing N by a factor of √2.
Share this formula
More in Photography & Imaging
View allExposure Value (EV)
IntermediateCombines aperture and shutter speed into one exposure figure.
Depth of Field (approx.)
AdvancedRange of distances that appear acceptably sharp.
Hyperfocal Distance
AdvancedFocus distance that keeps everything from H/2 to infinity sharp.
Lens Magnification
IntermediateImage magnification from focal length and subject distance.
Angle of View
IntermediateField of view from sensor size and focal length.
Equivalent Focal Length
BasicFocal length on a crop sensor expressed in full-frame terms.