Exposure Value (EV) Calculator - EV100, Equivalent Exposures and Star Trail Limits
Exposure Value Calculator
Aperture, shutter and ISO as one number, and equivalent exposures
EV and EV100
One number that stands for how much light the exposure lets in, adjusted to ISO 100 so scenes can be compared.
Equivalent Exposures
See the shutter speed for the same brightness at every full-stop aperture from f/1.4 to f/22.
Star Trail Limits
The 500 rule and the more careful NPF rule tell you how long you can expose the night sky before stars turn into streaks.
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What Exposure Value Means
Exposure value combines aperture and shutter speed into a single number: EV is the base-2 logarithm of the squared f-number divided by the shutter time. Every combination with the same EV lets in the same amount of light, so f/8 at 1/125 s, f/5.6 at 1/250 s and f/11 at 1/60 s are all the same exposure. Each step of 1 EV is a "stop", a doubling or halving of light. EV100 adjusts the value to ISO 100, so a bright sunny day is around EV100 15 whatever settings you used.
For night-sky photography, the Earth’s rotation makes stars drift across the frame, so there is a limit to how long you can expose before they blur. The 500 rule divides 500 by the full-frame equivalent focal length, a quick guide that is too generous for modern high-resolution sensors. The NPF rule also uses the aperture and the pixel size and is more accurate. Both are starting points: zoom in on a test shot to check.
Key Takeaways
- A stop is a doubling: Halving the shutter time, or closing the aperture by one full stop, lets in half the light.
- Sunny 16: In bright sun, f/16 with a shutter time of 1 over the ISO gives a good exposure, about EV100 15.
- Related calculators: See also the Depth of Field Calculator and the Print Size Calculator.