How to calculate an equivalent exposure
- Enter the aperture, shutter speed and ISO that give a correct exposure, for example from your camera's meter.
- Choose what to solve for: usually Shutter.
- Enter the new aperture and ISO you want to use. The calculator shows the shutter speed that gives the same brightness.
- For long exposures, enter your metered shutter speed in the ND filter section and pick your filter.
Shutter speeds can be typed as fractions like 1/250 or as seconds like 0.5 or 30.
The exposure triangle in numbers
Exposure is proportional to shutter time × ISO ÷ f-number². Each "stop" doubles or halves the light: going from f/8 to f/5.6, from 1/250 to 1/125, or from ISO 100 to ISO 200 each adds one stop. If you open the aperture by two stops, you must shorten the shutter by two stops (or lower the ISO) to keep the same exposure. That is what this calculator does, without rounding to standard values, so you can also use it for film and manual lenses.
Exposure value (EV) combines aperture and shutter into one number: EV = log₂(N² ÷ t), adjusted to ISO 100. EV 15 is bright sun, which gives the "sunny 16" rule: at f/16, use a shutter of 1/ISO.
ND filters and long exposures
A neutral density filter blocks light evenly. Its strength is given as a factor (ND8), an optical density (0.9) or in stops (3). The new shutter time is the original multiplied by 2 to the power of the stops: a 10-stop ND1000 turns 1/60 s into about 17 seconds. When you stack filters, add their stops. Use the built-in timer for exposures longer than 30 seconds in Bulb mode.
How many stops is an ND1000 filter?
About 10 stops (2¹⁰ = 1024). It multiplies your exposure time by roughly a thousand.
Why does my long exposure look darker than calculated?
Film suffers reciprocity failure on long exposures and needs extra time. Digital sensors do not, but strong ND filters are often not exactly their rated strength; check the histogram and adjust.
What is EV 0?
EV 0 is the exposure given by f/1 at 1 second (ISO 100), roughly a dim street at night. Each EV step is one stop.