Exposure Calculator

Exposure value combines aperture, shutter speed and ISO into a single number.

How the Math Works

The Exposure Value (EV) calculation combines aperture (N), shutter speed (t), and ISO sensitivity into a single logarithmic value. The formula EV = log₂(N²/t) - log₂(ISO/100) first calculates the light-gathering capacity of the aperture and shutter combination using log₂(N²/t), which quantifies how much light reaches the sensor. The ISO adjustment term log₂(ISO/100) then normalizes this value to a standard reference point (ISO 100), allowing comparisons across different camera sensitivities. By subtracting the ISO component, the formula ensures that higher ISO settings (which amplify light) reduce the EV, reflecting their trade-off with image noise.

Practical Applications

To apply this calculation practically, photographers can determine optimal exposure settings for specific lighting conditions. For instance, if you have an aperture of f/4 (N=4), shutter speed of 1/125s (t=0.008s), and ISO 400, plugging into the formula gives EV = log₂(16/0.008) - log₂(4) ≈ 7.32 - 2 = 5.32. This EV value helps compare exposures across different scenarios, such as matching a fast shutter speed for sports photography or a wide aperture for portraits while maintaining consistent exposure.

Day-to-Day Use

In daily photography, this tool helps make quick, informed decisions when lighting changes unexpectedly. For example, moving from indoor to outdoor shooting requires adjusting settings to maintain exposure balance; the EV calculation reveals how much to compensate (e.g., switching from ISO 800 to ISO 200 to lower EV by 1 stop). It also aids creative choices, like achieving motion blur by balancing shutter speed with aperture, or maximizing depth of field without underexposure. Even casual users benefit from understanding exposure relationships, turning trial-and-error into precise adjustments for perfect shots.

Worked example

f/8, 1/125 s, ISO 100 gives about EV 13.

FAQ