How to use it
- Enter your current aperture and shutter speed.
- Choose whether to find a new shutter speed (for an aperture you pick) or a new aperture (for a shutter speed you pick).
- Read the matching value and the change in stops.
How it works
Exposure stays equal when N² ÷ t stays constant, with N the f-number and t the exposure time. So the new time is t₂ = t × (N₂ ÷ N)², and the new aperture is N₂ = N × √(t₂ ÷ t). The aperture change in stops is 2 × log2(N₂ ÷ N).
Worked example
f/8 at 1/125 s. Opening up to f/4 gathers four times the light, so the time must be four times shorter: 1/500 s. The aperture moved two stops wider, shown as −2.
Tips
- Each full stop of aperture is a factor of about 1.4 in the f-number (√2).
- For long exposures, very long times need reciprocity correction on film; digital sensors mostly do not.