F-stop calculator
How much brighter is f/2.8 than f/8? Photographers reach for an f-stop calculator whenever they change aperture and need to keep the exposure balanced — for example, opening up to blur a background, then compensating with shutter speed or ISO. Enter your starting aperture and your target aperture and this tool reports the difference in stops and the exact light-change factor between them.
How it works
The f-number is the ratio of focal length to aperture diameter, so light admitted scales with the inverse square of the f-number. The tool computes the stops between two apertures a and b as:
stops = 2 × log₂(a / b)
It then expresses the brightness change as a multiplier, 2^|stops|. A positive result means you are opening up (a smaller f-number, more light); a negative result means you are stopping down (less light). Each full stop doubles or halves the light.
Reference examples
Going from f/8 to f/4: stops = 2 × log₂(8 / 4) = 2 × log₂(2) = +2 stops. The light factor is 2² = 4×, so f/4 delivers four times as much light as f/8. To hold the same exposure you would shorten the shutter speed by two stops (for instance 1/60 s → 1/250 s) or drop ISO by two stops.
| Aperture change | Stops | Light factor |
|---|---|---|
| f/2.8 → f/2 | +1 | 2× more |
| f/4 → f/8 | −2 | 4× less |
| f/5.6 → f/2.8 | +2 | 4× more |
| f/16 → f/4 | +4 | 16× more |
The full f-stop series
Each full stop in the standard aperture sequence either halves or doubles the light passing through the lens. The series runs: f/1 → f/1.4 → f/2 → f/2.8 → f/4 → f/5.6 → f/8 → f/11 → f/16 → f/22 → f/32. Each step multiplies the f-number by √2 (approximately 1.41), which halves the light because area scales with diameter squared.
Most modern cameras also offer half-stop and third-stop increments (for example f/3.2 or f/3.5) for finer exposure control. The calculator handles non-standard apertures — just type the f-number directly.
Practical scenarios
Blurring the background — you want to open from f/8 to f/2.8 for a portrait. That is three stops more light. To keep the exposure correct you need to reduce shutter speed by three stops (for example 1/60 s → 1/500 s) or lower ISO by three stops (for example ISO 800 → ISO 100).
Flash photography — your flash-synced shutter speed is fixed at 1/200 s. To control depth of field you only have aperture and ISO to balance the ambient light against the flash power. Knowing how many stops of light each aperture step changes saves you from guessing.
Neutral density filters — an ND filter cuts light by a fixed number of stops, which lets you use a slower shutter or wider aperture in bright conditions. If your meter says f/8 at 1/1000 s and you attach a 6-stop ND filter, you need to open six stops of equivalent exposure — the calculator tells you which aperture or shutter combination achieves that.
Every calculation runs locally in your browser — nothing you type is uploaded.