How to use it
- Enter the aperture as an f-number, for example 8 for f/8.
- Choose the shutter speed as a fraction (1/125) or in seconds, and enter the ISO.
- Read the EV for those settings and the equivalent at ISO 100.
How it works
The exposure value is EV = log2(N² ÷ t), where N is the f-number and t the shutter time in seconds. Changing ISO does not change that number, so the ISO 100 value is EV₁₀₀ = EV − log2(ISO ÷ 100).
Worked example
f/8 at 1/125 s: N² = 64, and 64 ÷ (1/125) = 8000. log2(8000) is about 12.97, so the setting is EV 12.97. At ISO 100 the EV₁₀₀ is the same. Shot at ISO 400, the same settings give EV₁₀₀ 10.97: the scene they suit is two stops dimmer.
Tips
- Equal EV does not mean an equal picture: depth of field and motion blur differ even when the exposure is the same.
- Use the equivalent exposure calculator to trade aperture for shutter speed at a constant EV.